React Interview Questions and Answers

The comments in React/JSX are similar to JavaScript Multiline comments but are wrapped in curly braces.

Single-line comments:

```jsx harmony
<div>
{/* Single-line comments(In vanilla JavaScript, the single-line comments are represented by double slash(//)) */}
{Welcome ${user}, let's play React}
</div>

    **Multi-line comments:**

    

You can use // and /* */ in JS logic, hooks, and functions.

Reconciliation is the process through which React updates the Browser DOM and makes React work faster. React use a diffing algorithm so that component updates are predictable and faster. React would first calculate the difference between the real DOM and the copy of DOM (Virtual DOM) when there's an update of components.
React stores a copy of Browser DOM which is called Virtual DOM. When we make changes or add data, React creates a new Virtual DOM and compares it with the previous one. This comparison is done by Diffing Algorithm.
Now React compares the Virtual DOM with Real DOM. It finds out the changed nodes and updates only the changed nodes in Real DOM leaving the rest nodes as it is. This process is called _Reconciliation_.

React uses className instead of class because of a JavaScript naming conflict with the class keyword.

1. class is a reserved keyword in JavaScript

In JavaScript, class is used to define ES6 classes:

        class Person {
          constructor(name) {
            this.name = name;
          }
        }
        

If you try to use class as a variable or property name, it will throw a syntax error. Since JSX is just JavaScript with XML-like syntax, using class directly in JSX would break the parser.

1. JSX Is JavaScript

When you write JSX like this:

        <div class="btn">Click</div>
        

It will be compiled to:

        React.createElement('div', { class: 'btn' }, 'Click');
        

But class is invalid in this object literal context (since it clashes with the JS keyword), hence React instead uses className.

        <div className="btn">Click</div>
        

which compiles to:

        React.createElement('div', { className: 'btn' }, 'Click');
        

React then translates className to class in the final HTML DOM.

1. Aligns with DOM APIs

In vanilla JavaScript, you interact with element classes using:

        element.className = 'my-class';
        

React follows this convention, staying consistent with the DOM API's property name rather than HTML’s attribute.

It's a common pattern or practice in React for a component to return multiple elements. _Fragments_ let you group a list of children without adding extra nodes to the DOM.
You need to use either <Fragment> or a shorter syntax having empty tag (<></>).

Below is the example of how to use fragment inside _Story_ component.

```jsx harmony
function Story({ title, description, date }) {
return (
<Fragment>
<h2>{title}</h2>
<p>{description}</p>
<p>{date}</p>
</Fragment>
);
}

    It is also possible to render list of fragments inside a loop with the mandatory **key** attribute supplied.

    

    Usually, you don't need to use `<Fragment>` until there is a need of _key_ attribute. The usage of shorter syntax looks like below.

    

Below are the list of reasons to prefer fragments over container DOM elements,

1. Fragments are a bit faster and use less memory by not creating an extra DOM node. This only has a real benefit on very large and deep trees.
2. Some CSS mechanisms like _Flexbox_ and _CSS Grid_ have a special parent-child relationships, and adding divs in the middle makes it hard to keep the desired layout.
3. The DOM Inspector is less cluttered.

If the behaviour of a component is independent of its state then it can be a stateless component. You can use either a function or a class for creating stateless components. But unless you need to use a lifecycle hook in your components, you should go for function components. There are a lot of benefits if you decide to use function components here; they are easy to write, understand, and test, a little faster, and you can avoid the this keyword altogether.

If the behaviour of a component is dependent on the _state_ of the component then it can be termed as stateful component. These _stateful components_ are either function components with hooks or _class components_.

Let's take an example of function stateful component which update the state based on click event,

    import React, {useState} from 'react';

    const App = (props) => {
    const [count, setCount] = useState(0);
    handleIncrement() {
      setCount(count+1);
    }

    return (
      <>
        <button onClick={handleIncrement}>Increment</button>
        <span>Counter: {count}</span>
      </>
      )
    }
    

<details><summary><b>See Class</b></summary>
<p>
The equivalent class stateful component with a state that gets initialized in the constructor.

```jsx harmony
class App extends Component {
constructor(props) {
super(props);
this.state = { count: 0 };
}

handleIncrement() {
setState({ count: this.state.count + 1 });
}

render() {
<>
<button onClick={() => this.handleIncrement}>Increment</button>
<span>Count: {count}</span>
</>;
}
}
```

</p>
</details>

When the application is running in _development mode_, React will automatically check all props that we set on components to make sure they have _correct type_. If the type is incorrect, React will generate warning messages in the console. It's disabled in _production mode_ due to performance impact. The mandatory props are defined with isRequired.

The set of predefined prop types:

1. PropTypes.number
2. PropTypes.string
3. PropTypes.array
4. PropTypes.object
5. PropTypes.func
6. PropTypes.node
7. PropTypes.element
8. PropTypes.bool
9. PropTypes.symbol
10. PropTypes.any

We can define propTypes for User component as below:

```jsx harmony
import React from "react";
import PropTypes from "prop-types";

class User extends React.Component {
static propTypes = {
name: PropTypes.string.isRequired,
age: PropTypes.number.isRequired,
};

render() {
return (
<>
<h1>{Welcome, ${this.props.name}}</h1>
<h2>{Age, ${this.props.age}}</h2>
</>
);
}
}

    **Note:** In React v15.5 _PropTypes_ were moved from `React.PropTypes` to `prop-types` library.
    
    **Modern Recommendation:** While PropTypes are still supported, **TypeScript** is now the industry standard for type checking in React applications. Consider using TypeScript for better type safety, IDE support, and compile-time error detection.

    _The Equivalent Functional Component_

    

function User({ name, age }) {
return (
<>
<h1>{Welcome, ${name}}</h1>
<h2>{Age, ${age}}</h2>
</>
);
}

User.propTypes = {
name: PropTypes.string.isRequired,
age: PropTypes.number.isRequired,
};

    
    _Modern TypeScript Version_

    

interface UserProps {
name: string;
age: number;
}

function User({ name, age }: UserProps) {
return (
<>
<h1>{Welcome, ${name}}</h1>
<h2>{Age, ${age}}</h2>
</>
);
}
```

Below are the list of main advantages of React,

1. Increases the application's performance with _Virtual DOM_.
2. JSX makes code easy to read and write.
3. It renders both on client and server side (_SSR_).
4. Easy to integrate with frameworks (Angular, Backbone) since it is only a view library.
5. Easy to write unit and integration tests with tools such as Jest.

Apart from the advantages, there are few limitations of React too,

1. React is just a view library, not a full framework.
2. There is a learning curve for beginners who are new to web development.
3. Integrating React into a traditional MVC framework requires some additional configuration.
4. The code complexity increases with inline templating and JSX.
5. Too many smaller components leading to over engineering or boilerplate.

Modern Recommendation (2026): TypeScript is the industry standard for type checking in React applications.

While PropTypes (React.PropTypes moved to prop-types package since React v15.5) are still available for runtime type checking, they have significant limitations:

• Only check types at runtime
• No compile-time errors
• Limited IDE autocomplete support
• No inference for complex types

TypeScript is now the recommended approach because it provides:

• Compile-time type checking
• Excellent IDE support with IntelliSense
• Type inference and generic types
• Better refactoring capabilities
• Growing ecosystem and community support

Note: Flow (Facebook's type checker) has seen declining adoption and is rarely used in new projects.

Example with TypeScript:

    interface UserProps {
      name: string;
      age: number;
      isActive?: boolean;
    }

    function User({ name, age, isActive = true }: UserProps) {
      return <div>{name} - {age}</div>;
    }
    

The react-dom package provides _DOM-specific methods_ that can be used at the top level of your app. Most of the components are not required to use this module. Some of the methods of this package are:

1. render()
2. hydrate()
3. unmountComponentAtNode()
4. findDOMNode()
5. createPortal()

The ReactDOMServer object enables you to render components to static markup (typically used on node server). This object is mainly used for _server-side rendering_ (SSR). The following methods can be used in both the server and browser environments:

1. renderToString()
2. renderToStaticMarkup()

For example, you generally run a Node-based web server like Express, Hapi, or Koa, and you call renderToString to render your root component to a string, which you then send as response.

    // using Express
    import { renderToString } from "react-dom/server";
    import MyPage from "./MyPage";

    app.get("/", (req, res) => {
      res.write(
        "<!DOCTYPE html><html><head><title>My Page</title></head><body>"
      );
      res.write('<div id="content">');
      res.write(renderToString(<MyPage />));
      res.write("</div></body></html>");
      res.end();
    });
    

The dangerouslySetInnerHTML attribute is React's replacement for using innerHTML in the browser DOM. Just like innerHTML, it is risky to use this attribute considering cross-site scripting (XSS) attacks. You just need to pass a __html object as key and HTML text as value.

In this example MyComponent uses dangerouslySetInnerHTML attribute for setting HTML markup:

```jsx harmony
function createMarkup() {
return { __html: "First &middot; Second" };
}

function MyComponent() {
return <div dangerouslySetInnerHTML={createMarkup()} />;
}
```

The style attribute accepts a JavaScript object with camelCased properties rather than a CSS string. This is consistent with the DOM style JavaScript property, is more efficient, and prevents XSS security holes.

```jsx harmony
const divStyle = {
color: "blue",
backgroundImage: "url(" + imgUrl + ")",
};

function HelloWorldComponent() {
return <div style={divStyle}>Hello World!</div>;
}
```

Style keys are camelCased in order to be consistent with accessing the properties on DOM nodes in JavaScript (e.g. node.style.backgroundImage).

Handling events in React elements has some syntactic differences:

1. React event handlers are named using camelCase, rather than lowercase.
2. With JSX you pass a function as the event handler, rather than a string.

Keys should be stable, predictable, and unique so that React can keep track of elements.

In the below code snippet each element's key will be based on ordering, rather than tied to the data that is being represented. This limits the optimizations that React can do and creates confusing bugs in the application.

```jsx harmony
{
todos.map((todo, index) => <Todo {...todo} key={index} />);
}

    If you use element data for unique key, assuming `todo.id` is unique to this list and stable, React would be able to reorder elements without needing to reevaluate them as much.

    

Note: If you don't specify key prop at all, React will use index as a key's value while iterating over an array of data.

In some cases you want to render different components depending on some state. JSX does not render false or undefined, so you can use conditional _short-circuiting_ to render a given part of your component only if a certain condition is true.

```jsx harmony
const MyComponent = ({ name, address }) => (
<div>
<h2>{name}</h2>
{address && <p>{address}</p>}
</div>
);

    If you need an `if-else` condition then use _ternary operator_.

    

When we _spread props_ we run into the risk of adding unknown HTML attributes, which is a bad practice. Instead we can use prop destructuring with ...rest operator, so it will add only required props.

For example,

```jsx harmony
const ComponentA = () => (
<ComponentB isDisplay={true} className={"componentStyle"} />
);

const ComponentB = ({ isDisplay, ...domProps }) => (
<div {...domProps}>{"ComponentB"}</div>
);
```

There are memoize libraries available which can be used on function components.

For example moize library can memoize the component in another component.

```jsx harmony
import moize from "moize";
import Component from "./components/Component"; // this module exports a non-memoized component

const MemoizedFoo = moize.react(Component);

const Consumer = () => {
<div>
{"I will memoize the following entry:"}
<MemoizedFoo />
</div>;
};

    **Update:** Since React v16.6.0, we have a `React.memo`. It provides a higher order component which memoizes component unless the props change. To use it, simply wrap the component using React.memo before you use it.

    

React is already equipped to handle rendering on Node servers. A special version of the DOM renderer is available, which follows the same pattern as on the client side.

```jsx harmony
import ReactDOMServer from "react-dom/server";
import App from "./App";

ReactDOMServer.renderToString(<App />);
```

This method will output the regular HTML as a string, which can be then placed inside a page body as part of the server response. On the client side, React detects the pre-rendered content and seamlessly picks up where it left off.

You should use Webpack's DefinePlugin method to set NODE_ENV to production, by which it strip out things like propType validation and extra warnings. Apart from this, if you minify the code, for example, Uglify's dead-code elimination to strip out development only code and comments, it will drastically reduce the size of your bundle.

A _switching component_ is a component that renders one of many components. We need to use object to map prop values to components.

For example, a switching component to display different pages based on page prop:

```jsx harmony
import HomePage from "./HomePage";
import AboutPage from "./AboutPage";
import ServicesPage from "./ServicesPage";
import ContactPage from "./ContactPage";

const PAGES = {
home: HomePage,
about: AboutPage,
services: ServicesPage,
contact: ContactPage,
};

const Page = (props) => {
const Handler = PAGES[props.page] || ContactPage;

return <Handler {...props} />;
};

// The keys of the PAGES object can be used in the prop types to catch dev-time errors.
Page.propTypes = {
page: PropTypes.oneOf(Object.keys(PAGES)).isRequired,
};
```

_Pointer Events_ provide a unified way of handling all input events. In the old days we had a mouse and respective event listeners to handle them but nowadays we have many devices which don't correlate to having a mouse, like phones with touch surface or pens. We need to remember that these events will only work in browsers that support the _Pointer Events_ specification.

The following event types are now available in _React DOM_:

1. onPointerDown
2. onPointerMove
3. onPointerUp
4. onPointerCancel
5. onGotPointerCapture
6. onLostPointerCapture
7. onPointerEnter
8. onPointerLeave
9. onPointerOver
10. onPointerOut

If you are rendering your component using JSX, the name of that component has to begin with a capital letter otherwise React will throw an error as an unrecognized tag. This convention is because only HTML elements and SVG tags can begin with a lowercase letter.

```jsx harmony
function SomeComponent {
// Code goes here
}

    You can define function component whose name starts with lowercase letter, but when it's imported it should have a capital letter. Here lowercase is fine:

    

export default myComponent;

    While when imported in another file it should start with capital letter:

    

Note: This question references React v16, which is outdated. The information below applies to React 16+, including current versions (React 18/19).

Yes. Starting with React 16, React no longer ignores unknown DOM attributes. If you write JSX with an attribute that React doesn't recognize, React will pass it through to the DOM.

For example, let's take a look at the below attribute:

```jsx harmony
<div mycustomattribute={"something"} />

    In React 15 and earlier, this would render an empty div:

    

    In React 16 and later (including React 18/19), any unknown attributes will end up in the DOM:

    

This is useful for supplying browser-specific non-standard attributes, trying new DOM APIs, and integrating with opinionated third-party libraries.

You can simply use Array.prototype.map with ES6 _arrow function_ syntax.

For example, the items array of objects is mapped into an array of components:

```jsx harmony
<tbody>
{items.map((item) => (
<SomeComponent key={item.id} name={item.name} />
))}
</tbody>

    But you can't iterate using `for` loop:

    

This is because JSX tags are transpiled into _function calls_, and you can't use statements inside expressions. This may change thanks to do expressions which are _stage 1 proposal_.

React (or JSX) doesn't support variable interpolation inside an attribute value. The below representation won't work:

```jsx harmony
<img className="image" src="images/{this.props.image}" />

    But you can put any JS expression inside curly braces as the entire attribute value. So the below expression works:

    

    Using _template strings_ will also work:

    

If you want to pass an array of objects to a component with a particular shape then use React.PropTypes.shape() as an argument to React.PropTypes.arrayOf().

    ReactComponent.propTypes = {
      arrayWithShape: React.PropTypes.arrayOf(
        React.PropTypes.shape({
          color: React.PropTypes.string.isRequired,
          fontSize: React.PropTypes.number.isRequired,
        })
      ).isRequired,
    };
    

You shouldn't use curly braces inside quotes because it is going to be evaluated as a string.

```jsx harmony
<div className="btn-panel {this.props.visible ? 'show' : 'hidden'}">

    Instead you need to move curly braces outside (don't forget to include spaces between class names):

    

    _Template strings_ will also work:

    

The react package contains React.createElement(), React.Component, React.Children, and other helpers related to elements and component classes. You can think of these as the isomorphic or universal helpers that you need to build components. The react-dom package contains ReactDOM.render(), and in react-dom/server we have _server-side rendering_ support with ReactDOMServer.renderToString() and ReactDOMServer.renderToStaticMarkup().

> Modern React Note (React 18/19): ReactDOM.render was deprecated in React 18 and removed in React 19. In modern applications, always use createRoot from react-dom/client:
>

> import { createRoot } from 'react-dom/client';
> const root = createRoot(document.getElementById('root'));
> root.render(<App />);
> 

The React team worked on extracting all DOM-related features into a separate library called _ReactDOM_. React v0.14 is the first release in which the libraries are split. By looking at some of the packages, react-native, react-art, react-canvas, and react-three, it has become clear that the beauty and essence of React has nothing to do with browsers or the DOM.

To build more environments that React can render to, React team planned to split the main React package into two: react and react-dom. This paves the way to writing components that can be shared between the web version of React and React Native.

If you try to render a <label> element bound to a text input using the standard for attribute, then it produces HTML missing that attribute and prints a warning to the console.

```jsx harmony
<label for={'user'}>{'User'}</label>
<input type={'text'} id={'user'} />

    Since `for` is a reserved keyword in JavaScript, use `htmlFor` instead.

    

You can use _spread operator_ in regular React:

```jsx harmony
<button style={{ ...styles.panel.button, ...styles.panel.submitButton }}>
{"Submit"}
</button>

    If you're using React Native then you can use the array notation:

    

You can use the useState hook to manage the width and height state variables, and the useEffect hook to add and remove the resize event listener. The [] dependency array passed to useEffect ensures that the effect only runs once (on mount) and not on every re-render.

    import React, { useState, useEffect } from "react";
    function WindowDimensions() {
      const [dimensions, setDimensions] = useState({
        width: window.innerWidth,
        height: window.innerHeight,
      });

      useEffect(() => {
        function handleResize() {
          setDimensions({
            width: window.innerWidth,
            height: window.innerHeight,
          });
        }
        window.addEventListener("resize", handleResize);
        return () => window.removeEventListener("resize", handleResize);
      }, []);

      return (
        <span>
          {dimensions.width} x {dimensions.height}
        </span>
      );
    }
    

<details>
<summary><h4>Using Class Component</h4></summary>

You can listen to the resize event in componentDidMount() and then update the dimensions (width and height). You should remove the listener in componentWillUnmount() method.

    class WindowDimensions extends React.Component {
      constructor(props) {
        super(props);
        this.updateDimensions = this.updateDimensions.bind(this);
      }

      componentWillMount() {
        this.updateDimensions();
      }

      componentDidMount() {
        window.addEventListener("resize", this.updateDimensions);
      }

      componentWillUnmount() {
        window.removeEventListener("resize", this.updateDimensions);
      }

      updateDimensions() {
        this.setState({
          width: window.innerWidth,
          height: window.innerHeight,
        });
      }

      render() {
        return (
          <span>
            {this.state.width} x {this.state.height}
          </span>
        );
      }
    }
    

</details>

> Modern React Note: componentWillMount, componentWillReceiveProps, and componentWillUpdate are legacy lifecycles deprecated since React 16.3 and removed in modern React. In modern functional React, use the useEffect Hook or derive state values directly during render.

We can use <pre> tag so that the formatting of the JSON.stringify() is retained:

```jsx harmony
const data = { name: "John", age: 42 };

function User {
return <pre>{JSON.stringify(data, null, 2)}</pre>;
}

const container = createRoot(document.getElementById("container"));

container.render(<User />);

      <details><summary><b>See Class</b></summary>
      <p>

    

class User extends React.Component {
render() {
return <pre>{JSON.stringify(data, null, 2)}</pre>;
}
}

React.render(<User />, document.getElementById("container"));
```

</p>
</details>

The React philosophy is that props should be _immutable_(read only) and _top-down_. This means that a parent can send any prop values to a child, but the child can't modify received props.

You need to use useEffect hook to set focus on input field during page load time for functional component.

```jsx harmony
import React, { useEffect, useRef } from "react";

const App = () => {
const inputElRef = useRef(null);

useEffect(() => {
inputElRef.current.focus();
}, []);

return (
<div>
<input defaultValue={"Won't focus"} />
<input ref={inputElRef} defaultValue={"Will focus"} />
</div>
);
};

ReactDOM.render(<App />, document.getElementById("app"));

      <details><summary><b>See Class</b></summary>
      <p>
      You can do it by creating _ref_ for `input` element and using it in `componentDidMount()`:

    

render() {
return (
<div>
<input defaultValue={"Won't focus"} />
<input
ref={(input) => (this.nameInput = input)}
defaultValue={"Will focus"}
/>
</div>
);
}
}

ReactDOM.render(<App />, document.getElementById("app"));

      </p>
      </details>

> **Modern React Note (React 18/19):** `ReactDOM.render` was deprecated in React 18 and removed in React 19. In modern applications, always use `createRoot` from `react-dom/client`:
> 

You can use React.version to get the version.

```jsx harmony
const REACT_VERSION = React.version;

ReactDOM.render(
<div>{React version: ${REACT_VERSION}}</div>,
document.getElementById("app")
);

> **Modern React Note (React 18/19):** `ReactDOM.render` was deprecated in React 18 and removed in React 19. In modern applications, always use `createRoot` from `react-dom/client`:
> 

Add a listener on the history object to record each page view:

    history.listen(function (location) {
      window.ga("set", "page", location.pathname + location.search);
      window.ga("send", "pageview", location.pathname + location.search);
    });
    

React _does not_ apply _vendor prefixes_ automatically. You need to add vendor prefixes manually.

```jsx harmony
<div
style={{
transform: "rotate(90deg)",
WebkitTransform: "rotate(90deg)", // note the capital 'W' here
msTransform: "rotate(90deg)", // 'ms' is the only lowercase vendor prefix
}}
/>
```

You should use default for exporting the components

```jsx harmony
import User from "user";

export default function MyProfile {
return <User type="customer">//...</User>;
}

    <details><summary><b>See Class</b></summary>
    <p>
     

export default class MyProfile extends React.Component {
render() {
return <User type="customer">//...</User>;
}
}

    </p>
    </details>

    With the export specifier, the MyProfile is going to be the member and exported to this module and the same can be imported without mentioning the name in other components.
    

The component names should start with an uppercase letter but there are few exceptions to this convention. The lowercase tag names with a dot (property accessors) are still considered as valid component names.
For example, the below tag can be compiled to a valid component,

```jsx harmony
render() {
return (
<obj.component/> // React.createElement(obj.component)
)
}
```

Yes, you can use async/await in plain React, as long as your JavaScript environment supports ES2017+. Nowadays most modern browsers and build tools support ES2017+ version. If you're using Create React App, Next.js, Remix, or any modern React setup, async/await is supported out of the box through Babel.

### Example Usage

    import { useEffect, useState } from 'react';

    function UserProfile() {
      const [user, setUser] = useState(null);

      useEffect(() => {
        const fetchUser = async () => {
          const response = await fetch('/api/user');
          const data = await response.json();
          setUser(data);
        };

        fetchUser();
      }, []);

      return user ? <div>Hello, {user.name}</div> : <div>Loading...</div>;
    }
    

But If you're not using a bundler like Webpack or Babel, you will need _Babel_ and transform-async-to-generator plugin. However, React Native ships with Babel and a set of transforms.

There are two common practices for React project file structure.

1. Grouping by features or routes:

One common way to structure projects is locate CSS, JS, and tests together, grouped by feature or route.

        common/
        ├─ Avatar.js
        ├─ Avatar.css
        ├─ APIUtils.js
        └─ APIUtils.test.js
        feed/
        ├─ index.js
        ├─ Feed.js
        ├─ Feed.css
        ├─ FeedStory.js
        ├─ FeedStory.test.js
        └─ FeedAPI.js
        profile/
        ├─ index.js
        ├─ Profile.js
        ├─ ProfileHeader.js
        ├─ ProfileHeader.css
        └─ ProfileAPI.js
        

1. Grouping by file type:

Another popular way to structure projects is to group similar files together.

        api/
        ├─ APIUtils.js
        ├─ APIUtils.test.js
        ├─ ProfileAPI.js
        └─ UserAPI.js
        components/
        ├─ Avatar.js
        ├─ Avatar.css
        ├─ Feed.js
        ├─ Feed.css
        ├─ FeedStory.js
        ├─ FeedStory.test.js
        ├─ Profile.js
        ├─ ProfileHeader.js
        └─ ProfileHeader.css
        

In modern React application development, animating components smoothly requires specialized libraries that handle component mount/unmount lifecycles and GPU-accelerated transitions:

### 1. Framer Motion (Industry Standard)
The most widely used and modern animation library for React today. It provides declarative syntax, gesture support (drag, hover, tap), and layout animations:

import { motion } from 'framer-motion';

export const FadeCard = () => (
  <motion.div
    initial={{ opacity: 0, y: 20 }}
    animate={{ opacity: 1, y: 0 }}
    transition={{ duration: 0.4 }}
  >
    Animated Content
  </motion.div>
);

### 2. React Spring
A physics-based animation library that models motion on real-world spring mechanics (mass, tension, friction) rather than fixed curve durations, making animations feel organic and interruptible.

### 3. GSAP (GreenSock Animation Platform)
The gold standard for high-performance complex timelines, SVG animations, and scroll-triggered choreographies (ScrollTrigger), fully compatible with React via the @gsap/react hook.

### 4. React Transition Group & React Motion (Legacy)
Historically used in earlier React versions:

• React Transition Group: Exposes lifecycle transitions (CSSTransition, TransitionGroup) to apply CSS classes as elements enter and exit the DOM.
• React Motion: An early physics-based motion library (largely superseded by React Spring and Framer Motion).

It is recommended to avoid hard coding style values in components. Any values that are likely to be used across different UI components should be extracted into their own modules.

For example, these styles could be extracted into a separate component:

    export const colors = {
      white,
      black,
      blue,
    };

    export const space = [0, 8, 16, 32, 64];
    

And then imported individually in other components:

    import { space, colors } from "./styles";
    

ESLint is a popular JavaScript linter. There are plugins available that analyse specific code styles. One of the most common for React is an npm package called eslint-plugin-react. By default, it will check a number of best practices, with rules checking things from keys in iterators to a complete set of prop types.

Another popular plugin is eslint-plugin-jsx-a11y, which will help fix common issues with accessibility. As JSX offers slightly different syntax to regular HTML, issues with alt text and tabindex, for example, will not be picked up by regular plugins.

## React Router

React Router is a powerful routing library built on top of React that helps you add new screens and flows to your application incredibly quickly, all while keeping the URL in sync with what's being displayed on the page.

React Router is a wrapper around the history library which handles interaction with the browser's window.history with its browser and hash histories. It also provides memory history which is useful for environments that don't have global history, such as mobile app development (React Native) and unit testing with Node.

React Router v6 provides below 4 <Router> components:

1. <BrowserRouter>:Uses the HTML5 history API for standard web apps.
2. <HashRouter>:Uses hash-based routing for static servers.
3. <MemoryRouter>:Uses in-memory routing for testing and non-browser environments.
4. <StaticRouter>:Provides static routing for server-side rendering (SSR).

The above components will create _browser_, _hash_, _memory_ and _static_ history instances. React Router v6 makes the properties and methods of the history instance associated with your router available through the context in the router object.

A history instance has two methods for navigation purpose.

1. push()
2. replace()

If you think of the history as an array of visited locations, push() will add a new location to the array and replace() will replace the current location in the array with the new one.

There are three different ways to achieve programmatic routing/navigation within components.

1. Using the withRouter() higher-order function:

The withRouter() higher-order function will inject the history object as a prop of the component. This object provides push() and replace() methods to avoid the usage of context.

```jsx harmony
import { withRouter } from "react-router-dom"; // this also works with 'react-router-native'

const Button = withRouter(({ history }) => (
<button
type="button"
onClick={() => {
history.push("/new-location");
}}
>
{"Click Me!"}
</button>
));

    2.  **Using `<Route>` component and render props pattern:**

        The `<Route>` component passes the same props as `withRouter()`, so you will be able to access the history methods through the history prop.

        

const Button = () => (
<Route
render={({ history }) => (
<button
type="button"
onClick={() => {
history.push("/new-location");
}}
>
{"Click Me!"}
</button>
)}
/>
);

    3.  **Using context:**

        This option is not recommended and treated as unstable API.

        

Button.contextTypes = {
history: React.PropTypes.shape({
push: React.PropTypes.func.isRequired,
}),
};
```

The ability to parse query strings was taken out of React Router v4 because there have been user requests over the years to support different implementation. So the decision has been given to users to choose the implementation they like. The recommended approach is to use query strings library.

    const queryString = require("query-string");
    const parsed = queryString.parse(props.location.search);
    

You can also use URLSearchParams if you want something native:

    const params = new URLSearchParams(props.location.search);
    const foo = params.get("name");
    

You should use a _polyfill_ for IE11.

You have to wrap your Route's in a <Switch> block because <Switch> is unique in that it renders a route exclusively.

At first you need to add Switch to your imports:

    import { Switch, Router, Route } from "react-router";
    

Then define the routes within <Switch> block:

```jsx harmony
<Router>
<Switch>
<Route {/* ... */} />
<Route {/* ... */} />
</Switch>
</Router>
```

While navigating you can pass props to the history object:

    this.props.history.push({
      pathname: "/template",
      search: "?name=sudheer",
      state: { detail: response.data },
    });
    

The search property is used to pass query params in push() method.

A <Switch> renders the first child <Route> that matches. A <Route> with no path always matches. So you just need to simply drop path attribute as below

```jsx harmony
<Switch>
<Route exact path="/" component={Home} />
<Route path="/user" component={User} />
<Route component={NotFound} />
</Switch>
```

Below are the list of steps to get history object on React Router v4,

1. Create a module that exports a history object and import this module across the project.

For example, create history.js file:

        import { createBrowserHistory } from "history";

        export default createBrowserHistory({
          /* pass a configuration object here if needed */
        });
        

1. You should use the <Router> component instead of built-in routers. Import the above history.js inside index.js file:

```jsx harmony
import { Router } from "react-router-dom";
import history from "./history";
import App from "./App";

ReactDOM.render(
<Router history={history}>
<App />
</Router>,
holder
);

    3.  You can also use push method of `history` object similar to built-in history object:

        

history.push("/go-here");

> **Modern React Note (React 18/19):** `ReactDOM.render` was deprecated in React 18 and removed in React 19. In modern applications, always use `createRoot` from `react-dom/client`:
> 

The react-router package provides <Redirect> component in React Router. Rendering a <Redirect> will navigate to a new location. Like server-side redirects, the new location will override the current location in the history stack.

    import { Redirect } from "react-router";

    export default function Login {
        if (this.state.isLoggedIn === true) {
          return <Redirect to="/your/redirect/page" />;
        } else {
          return <div>{"Login Please"}</div>;
        }
    }
    

<details><summary><b>See Class</b></summary>
<p>

    import React, { Component } from "react";
    import { Redirect } from "react-router";

    export default class LoginComponent extends Component {
      render() {
        if (this.state.isLoggedIn === true) {
          return <Redirect to="/your/redirect/page" />;
        } else {
          return <div>{"Login Please"}</div>;
        }
      }
    }
    

</p>
</details>

## React Internationalization

The _React Intl_ library makes internationalization in React straightforward, with off-the-shelf components and an API that can handle everything from formatting strings, dates, and numbers, to pluralization. React Intl is part of _FormatJS_ which provides bindings to React via its components and API.

Below are the main features of React Intl,

1. Display numbers with separators.
2. Display dates and times correctly.
3. Display dates relative to "now".
4. Pluralize labels in strings.
5. Support for 150+ languages.
6. Runs in the browser and Node.
7. Built on standards.

The library provides two ways to format strings, numbers, and dates:

1. Using react components:

```jsx harmony
<FormattedMessage
id={"account"}
defaultMessage={"The amount is less than minimum balance."}
/>

    2.  **Using an API:**

        

formatMessage(messages.accountMessage);
```

The <Formatted... /> components from react-intl return elements, not plain text, so they can't be used for placeholders, alt text, etc. In that case, you should use lower level API formatMessage(). You can inject the intl object into your component using injectIntl() higher-order component and then format the message using formatMessage() available on that object.

```jsx harmony
import React from "react";
import { injectIntl, intlShape } from "react-intl";

const MyComponent = ({ intl }) => {
const placeholder = intl.formatMessage({ id: "messageId" });
return <input placeholder={placeholder} />;
};

MyComponent.propTypes = {
intl: intlShape.isRequired,
};

export default injectIntl(MyComponent);
```

You can get the current locale in any component of your application using injectIntl():

```jsx harmony
import { injectIntl, intlShape } from "react-intl";

const MyComponent = ({ intl }) => (
<div>{The current locale is ${intl.locale}}</div>
);

MyComponent.propTypes = {
intl: intlShape.isRequired,
};

export default injectIntl(MyComponent);
```

The injectIntl() higher-order component will give you access to the formatDate() method via the props in your component. The method is used internally by instances of FormattedDate and it returns the string representation of the formatted date.

```jsx harmony
import { injectIntl, intlShape } from "react-intl";

const stringDate = this.props.intl.formatDate(date, {
year: "numeric",
month: "numeric",
day: "numeric",
});

const MyComponent = ({ intl }) => (
<div>{The formatted date is ${stringDate}}</div>
);

MyComponent.propTypes = {
intl: intlShape.isRequired,
};

export default injectIntl(MyComponent);
```

## React Testing

_Shallow rendering_ is useful for writing unit test cases in React. It lets you render a component _one level deep_ and assert facts about what its render method returns, without worrying about the behavior of child components, which are not instantiated or rendered.

For example, if you have the following component:

    function MyComponent() {
      return (
        <div>
          <span className={"heading"}>{"Title"}</span>
          <span className={"description"}>{"Description"}</span>
        </div>
      );
    }
    

Then you can assert as follows:

```jsx harmony
import ShallowRenderer from "react-test-renderer/shallow";

// in your test
const renderer = new ShallowRenderer();
renderer.render(<MyComponent />);

const result = renderer.getRenderOutput();

expect(result.type).toBe("div");
expect(result.props.children).toEqual([
<span className={"heading"}>{"Title"}</span>,
<span className={"description"}>{"Description"}</span>,
]);
```

This package provides a renderer that can be used to render components to pure JavaScript objects, without depending on the DOM or a native mobile environment. This package makes it easy to grab a snapshot of the platform view hierarchy (similar to a DOM tree) rendered by a ReactDOM or React Native without using a browser or jsdom.

```jsx harmony
import TestRenderer from "react-test-renderer";

const Link = ({ page, children }) => <a href={page}>{children}</a>;

const testRenderer = TestRenderer.create(
<Link page={"https://www.facebook.com/"}>{"Facebook"}</Link>
);

console.log(testRenderer.toJSON());
// {
// type: 'a',
// props: { href: 'https://www.facebook.com/' },
// children: [ 'Facebook' ]
// }
```

_ReactTestUtils_ are provided in the with-addons package and allow you to perform actions against a simulated DOM for the purpose of unit testing.

_Jest_ is a JavaScript unit testing framework created by Facebook based on Jasmine and provides automated mock creation and a jsdom environment. It's often used for testing components.

There are couple of advantages compared to Jasmine:

• Automatically finds tests to execute in your source code.
• Automatically mocks dependencies when running your tests.
• Allows you to test asynchronous code synchronously.
• Runs your tests with a fake DOM implementation (via jsdom) so that your tests can be run on the command line.
• Runs tests in parallel processes so that they finish sooner.

Let's write a test for a function that adds two numbers in sum.js file:

     const sum = (a, b) => a + b;

     export default sum;
     

Create a file named sum.test.js which contains actual test:

     import sum from "./sum";

     test("adds 1 + 2 to equal 3", () => {
       expect(sum(1, 2)).toBe(3);
     });
     

And then add the following section to your package.json:

     {
       "scripts": {
         "test": "jest"
       }
     }
     

Finally, run yarn test or npm test and Jest will print a result:

     $ yarn test
     PASS ./sum.test.js
     ✓ adds 1 + 2 to equal 3 (2ms)
     

## React Redux

Flux is an application architecture (not a framework or library) designed by Facebook to manage data flow in React applications. It was created as an alternative to the traditional MVC (Model-View-Controller) pattern, and it emphasizes a unidirectional data flow to make state changes more predictable and easier to debug.

Flux complements React by organizing the way data moves through your application, especially in large-scale or complex projects.

#### Core Concepts of Flux

Flux operates using four key components, each with a specific responsibility:

• Actions
• Plain JavaScript objects or functions that describe _what happened_ (e.g., user interactions or API responses).
• Example: { type: 'ADD_TODO', payload: 'Buy milk' }
• Dispatcher
• A central hub that receives actions and dispatches them to the appropriate stores.
• There is only one dispatcher in a Flux application.
• Stores
• Hold the application state and business logic.
• Respond to actions from the dispatcher and update themselves accordingly.
• They emit change events that views can listen to.
• Views (React Components)
• Subscribe to stores and re-render when the data changes.
• They can also trigger new actions (e.g., on user input).

The workflow between dispatcher, stores and views components with distinct inputs and outputs as follows:


Redux is a predictable state container for JavaScript applications, most commonly used with React. It helps you manage and centralize your application’s state in a single source of truth, enabling easier debugging, testing, and maintenance—especially in large or complex applications. Redux core is tiny library(about 2.5kB gzipped) and has no dependencies.

Redux follows three fundamental principles:

1. Single source of truth: The state of your whole application is stored in an object tree within a single store. The single state tree makes it easier to keep track of changes over time and debug or inspect the application.

      const store = createStore(reducer);
      

1. State is read-only: The only way to change the state is to emit an action, an object describing what happened. This ensures that neither the views nor the network callbacks will ever write directly to the state.

      const action = { type: 'INCREMENT' };
      store.dispatch(action);
      

1. Changes are made with pure functions(Reducers): To specify how the state tree is transformed by actions, you write reducers. Reducers are just pure functions that take the previous state and an action as parameters, and return the next state.

      function counter(state = 0, action) {
        switch (action.type) {
          case 'INCREMENT':
            return state + 1;
          case 'DECREMENT':
            return state - 1;
          default:
            return state;
        }
      }
      

While Redux offers a powerful and predictable state management solution, it comes with a few trade-offs when compared to Flux. These include:

1. Immutability is essential

Redux enforces a strict immutability model for state updates, which differs from Flux’s more relaxed approach. This means you must avoid mutating state directly. Many Redux-related libraries assume immutability, so your team must be disciplined in writing pure update logic. You can use tools like redux-immutable-state-invariant, Immer, or Immutable.js to help enforce this practice, especially during development.

1. Careful selection of complementary packages

Redux is more minimal by design and provides extension points such as middleware and store enhancers. This has led to a large ecosystem, but it also means you must thoughtfully choose and configure additional packages for features like undo/redo, persistence, or form handling—something Flux explicitly leaves out but may be simpler to manage in smaller setups.

1. Limited static type integration

While Flux has mature support for static type checking with tools like Flow, Redux’s type integration is less seamless. Although TypeScript is commonly used with Redux now, early Flow support was limited, and more boilerplate was required for static type safety. This may affect teams that rely heavily on type systems for large codebases.

mapStateToProps() is a utility which helps your component get updated state (which is updated by some other components):

     const mapStateToProps = (state) => {
       return {
         todos: getVisibleTodos(state.todos, state.visibilityFilter),
       };
     };
     

mapDispatchToProps() is a utility which will help your component to fire an action event (dispatching action which may cause change of application state):

     const mapDispatchToProps = (dispatch) => {
       return {
         onTodoClick: (id) => {
           dispatch(toggleTodo(id));
         },
       };
     };
     

It is recommended to always use the “object shorthand” form for the mapDispatchToProps.

Redux wraps it in another function that looks like (…args) => dispatch(onTodoClick(…args)), and pass that wrapper function as a prop to your component.

     const mapDispatchToProps = {
       onTodoClick,
     };
     

Dispatching an action within a reducer is an anti-pattern. Your reducer should be _without side effects_, simply digesting the action payload and returning a new state object. Adding listeners and dispatching actions within the reducer can lead to chained actions and other side effects.

You just need to export the store from the module where it created with createStore(). Also, it shouldn't pollute the global window object.

     store = createStore(myReducer);

     export default store;
     

1. DOM manipulation is very expensive which causes applications to behave slow and inefficient.
2. Due to circular dependencies, a complicated model was created around models and views.
3. Lot of data changes happens for collaborative applications(like Google Docs).
4. No way to do undo (travel back in time) easily without adding so much extra code.

These libraries are very different for very different purposes, but there are some vague similarities.

Redux is a tool for managing state throughout the application. It is usually used as an architecture for UIs. Think of it as an alternative to (half of) Angular. RxJS is a reactive programming library. It is usually used as a tool to accomplish asynchronous tasks in JavaScript. Think of it as an alternative to Promises. Redux uses the Reactive paradigm because the Store is reactive. The Store observes actions from a distance, and changes itself. RxJS also uses the Reactive paradigm, but instead of being an architecture, it gives you basic building blocks, Observables, to accomplish this pattern.

You need to write a _root reducer_ in your application which delegate handling the action to the reducer generated by combineReducers().

For example, let us take rootReducer() to return the initial state after USER_LOGOUT action. As we know, reducers are supposed to return the initial state when they are called with undefined as the first argument, no matter the action.

     const appReducer = combineReducers({
       /* your app's top-level reducers */
     });

     const rootReducer = (state, action) => {
       if (action.type === "USER_LOGOUT") {
         state = undefined;
       }

       return appReducer(state, action);
     };
     

In case of using redux-persist, you may also need to clean your storage. redux-persist keeps a copy of your state in a storage engine. First, you need to import the appropriate storage engine and then, to parse the state before setting it to undefined and clean each storage state key.

     const appReducer = combineReducers({
       /* your app's top-level reducers */
     });

     const rootReducer = (state, action) => {
       if (action.type === "USER_LOGOUT") {
         Object.keys(state).forEach((key) => {
           storage.removeItem(`persist:${key}`);
         });

         state = undefined;
       }

       return appReducer(state, action);
     };
     

Reducers always return the accumulation of the state (based on all previous and current actions). Therefore, they act as a reducer of state. Each time a Redux reducer is called, the state and action are passed as parameters. This state is then reduced (or accumulated) based on the action, and then the next state is returned. You could _reduce_ a collection of actions and an initial state (of the store) on which to perform these actions to get the resulting final state.

You can use redux-thunk middleware which allows you to define async actions.

Let's take an example of fetching specific account as an AJAX call using _fetch API_:

     export function fetchAccount(id) {
       return (dispatch) => {
         dispatch(setLoadingAccountState()); // Show a loading spinner
         fetch(`/account/${id}`, (response) => {
           dispatch(doneFetchingAccount()); // Hide loading spinner
           if (response.status === 200) {
             dispatch(setAccount(response.json)); // Use a normal function to set the received state
           } else {
             dispatch(someError);
           }
         });
       };
     }

     function setAccount(data) {
       return { type: "SET_Account", data: data };
     }
     

No. In professional state architecture, keeping 100% of application state in a global Redux store is an anti-pattern. You should carefully separate global server/client cache state from local transient UI state:

### What belongs in Redux / Global State:

1. Authenticated User Session: User profile, roles, permissions, auth tokens.
2. Global Application Preferences: Active theme (dark/light), locale/language, currency.
3. Shared Business Entities: Shopping cart items, notifications, cross-page workflow data.
4. Server Cache (via RTK Query): Cached API payloads and background synchronization.

### What belongs in Local Component State (useState / useReducer):

1. Form Input State: Field keystrokes, validation touched states before submission.
2. UI Toggles: Dropdown open/closed states, modal visibility, accordions.
3. Transient Visual State: Hover tooltips, tab indices, loading spinner states within a specific card.

### Guiding Architectural Rule:
*If no other component in the tree cares about this state when the current component unmounts, keep it local.* Storing transient keystrokes in Redux causes unnecessary store dispatches, serializations, and re-renders.

The best way to access your store in a component is to use the connect() function, that creates a new component that wraps around your existing one. This pattern is called _Higher-Order Components_, and is generally the preferred way of extending a component's functionality in React. This allows you to map state and action creators to your component, and have them passed in automatically as your store updates.

Let's take an example of <FilterLink> component using connect:

     import { connect } from "react-redux";
     import { setVisibilityFilter } from "../actions";
     import Link from "../components/Link";

     const mapStateToProps = (state, ownProps) => ({
       active: ownProps.filter === state.visibilityFilter,
     });

     const mapDispatchToProps = (dispatch, ownProps) => ({
       onClick: () => dispatch(setVisibilityFilter(ownProps.filter)),
     });

     const FilterLink = connect(mapStateToProps, mapDispatchToProps)(Link);

     export default FilterLink;
     

Due to it having quite a few performance optimizations and generally being less likely to cause bugs, the Redux developers almost always recommend using connect() over accessing the store directly (using context API).

     function MyComponent {
       someMethod() {
         doSomethingWith(this.context.store);
       }
     }
     

Component is a class or function component that describes the presentational part of your application.

Container is an informal term for a component that is connected to a Redux store. Containers _subscribe_ to Redux state updates and _dispatch_ actions, and they usually don't render DOM elements; they delegate rendering to presentational child components.

Constants allows you to easily find all usages of that specific functionality across the project when you use an IDE. It also prevents you from introducing silly bugs caused by typos – in which case, you will get a ReferenceError immediately.

Normally we will save them in a single file (constants.js or actionTypes.js).

     export const ADD_TODO = "ADD_TODO";
     export const DELETE_TODO = "DELETE_TODO";
     export const EDIT_TODO = "EDIT_TODO";
     export const COMPLETE_TODO = "COMPLETE_TODO";
     export const COMPLETE_ALL = "COMPLETE_ALL";
     export const CLEAR_COMPLETED = "CLEAR_COMPLETED";
     

In Redux, you use them in two places:

1. During action creation:

Let's take actions.js:

        import { ADD_TODO } from "./actionTypes";

        export function addTodo(text) {
          return { type: ADD_TODO, text };
        }
        

1. In reducers:

Let's create reducer.js:

        import { ADD_TODO } from "./actionTypes";

        export default (state = [], action) => {
          switch (action.type) {
            case ADD_TODO:
              return [
                ...state,
                {
                  text: action.text,
                  completed: false,
                },
              ];
            default:
              return state;
          }
        };
        

There are a few ways of binding _action creators_ to dispatch() in mapDispatchToProps().

Below are the possible options:

     const mapDispatchToProps = (dispatch) => ({
       action: () => dispatch(action()),
     });
     

     const mapDispatchToProps = (dispatch) => ({
       action: bindActionCreators(action, dispatch),
     });
     

     const mapDispatchToProps = { action };
     

The third option is just a shorthand for the first one.

If the ownProps parameter is specified, React Redux will pass the props that were passed to the component into your _connect_ functions. So, if you use a connected component:

```jsx harmony
import ConnectedComponent from "./containers/ConnectedComponent";

<ConnectedComponent user={"john"} />;

     The `ownProps` inside your `mapStateToProps()` and `mapDispatchToProps()` functions will be an object:

     

You can use this object to decide what to return from those functions.

Most of the applications has several top-level directories as below:

1. Components: Used for _dumb_ components unaware of Redux.
2. Containers: Used for _smart_ components connected to Redux.
3. Actions: Used for all action creators, where file names correspond to part of the app.
4. Reducers: Used for all reducers, where files name correspond to state key.
5. Store: Used for store initialization.

This structure works well for small and medium size apps.

redux-saga is a library that aims to make side effects (asynchronous things like data fetching and impure things like accessing the browser cache) in React/Redux applications easier and better.

It is available in NPM:

     $ npm install --save redux-saga
     

_Saga_ is like a separate thread in your application, that's solely responsible for side effects. redux-saga is a redux _middleware_, which means this thread can be started, paused and cancelled from the main application with normal Redux actions, it has access to the full Redux application state and it can dispatch Redux actions as well.

Both call() and put() are effect creator functions. call() function is used to create effect description, which instructs middleware to call the promise. put() function creates an effect, which instructs middleware to dispatch an action to the store.

Let's take example of how these effects work for fetching particular user data.

     function* fetchUserSaga(action) {
       // `call` function accepts rest arguments, which will be passed to `api.fetchUser` function.
       // Instructing middleware to call promise, it resolved value will be assigned to `userData` variable
       const userData = yield call(api.fetchUser, action.userId);

       // Instructing middleware to dispatch corresponding action.
       yield put({
         type: "FETCH_USER_SUCCESS",
         userData,
       });
     }
     

_Redux Thunk_ middleware allows you to write action creators that return a function instead of an action. The thunk can be used to delay the dispatch of an action, or to dispatch only if a certain condition is met. The inner function receives the store methods dispatch() and getState() as parameters.

Both _Redux Thunk_ and _Redux Saga_ take care of dealing with side effects. In most of the scenarios, Thunk uses _Promises_ to deal with them, whereas Saga uses _Generators_. Thunk is simple to use and Promises are familiar to many developers, Sagas/Generators are more powerful but you will need to learn them. But both middleware can coexist, so you can start with Thunks and introduce Sagas when/if you need them.

_Redux DevTools_ is a live-editing time travel environment for Redux with hot reloading, action replay, and customizable UI. If you don't want to bother with installing Redux DevTools and integrating it into your project, consider using Redux DevTools Extension for Chrome and Firefox.

Some of the main features of Redux DevTools are below,

1. Lets you inspect every state and action payload.
2. Lets you go back in time by _cancelling_ actions.
3. If you change the reducer code, each _staged_ action will be re-evaluated.
4. If the reducers throw, you will see during which action this happened, and what the error was.
5. With persistState() store enhancer, you can persist debug sessions across page reloads.

_Selectors_ are functions that take Redux state as an argument and return some data to pass to the component.

For example, to get user details from the state:

     const getUserData = (state) => state.user.data;
     

These selectors have two main benefits,

1. The selector can compute derived data, allowing Redux to store the minimal possible state
2. The selector is not recomputed unless one of its arguments changes

_Redux Form_ works with React and Redux to enable a form in React to use Redux to store all of its state. Redux Form can be used with raw HTML5 inputs, but it also works very well with common UI frameworks like Material UI, React Widgets and React Bootstrap.

Some of the main features of Redux Form are:

1. Field values persistence via Redux store.
2. Validation (sync/async) and submission.
3. Formatting, parsing and normalization of field values.

You can use applyMiddleware().

For example, you can add redux-thunk and logger passing them as arguments to applyMiddleware():

     import { createStore, applyMiddleware } from "redux";
     const createStoreWithMiddleware = applyMiddleware(
       ReduxThunk,
       logger
     )(createStore);
     

You need to pass initial state as second argument to createStore:

     const rootReducer = combineReducers({
       todos: todos,
       visibilityFilter: visibilityFilter,
     });

     const initialState = {
       todos: [{ id: 123, name: "example", completed: false }],
     };

     const store = createStore(rootReducer, initialState);
     

Relay is similar to Redux in that they both use a single store. The main difference is that relay only manages state originated from the server, and all access to the state is used via _GraphQL_ queries (for reading data) and mutations (for changing data). Relay caches the data for you and optimizes data fetching for you, by fetching only changed data and nothing more.

_Actions_ are plain JavaScript objects or payloads of information that send data from your application to your store. They are the only source of information for the store. Actions must have a type property that indicates the type of action being performed.

For example, let's take an action which represents adding a new todo item:

     {
       type: ADD_TODO,
       text: 'Add todo item'
     }
     

## React Native

React is a JavaScript library, supporting both front end web and being run on the server, for building user interfaces and web applications.

React Native is a mobile framework that compiles to native app components, allowing you to build native mobile applications (iOS, Android, and Windows) in JavaScript that allows you to use React to build your components, and implements React under the hood.

React Native can be tested only in mobile simulators like iOS and Android. You can run the app in your mobile using expo app (https://expo.io) Where it syncs using QR code, your mobile and computer should be in same wireless network.

You can use console.log, console.warn, etc. As of React Native v0.29 you can simply run the following to see logs in the console:

     $ react-native log-ios
     $ react-native log-android
     

Follow the below steps to debug React Native app:

1. Run your application in the iOS simulator.
2. Press Command + D and a webpage should open up at http://localhost:8081/debugger-ui.
3. Enable _Pause On Caught Exceptions_ for a better debugging experience.
4. Press Command + Option + I to open the Chrome Developer tools, or open it via View -> Developer -> Developer Tools.
5. You should now be able to debug as you normally would.

## React supported libraries & Integration

_Reselect_ is a selector library (for Redux) which uses _memoization_ concept. It was originally written to compute derived data from Redux-like applications state, but it can't be tied to any architecture or library.

Reselect keeps a copy of the last inputs/outputs of the last call, and recomputes the result only if one of the inputs changes. If the same inputs are provided twice in a row, Reselect returns the cached output. It's memoization and cache are fully customizable.

_Flow_ is a _static type checker_ designed to find type errors in JavaScript. Flow types can express much more fine-grained distinctions than traditional type systems. For example, Flow helps you catch errors involving null, unlike most type systems.

Flow is a _static analysis tool_ (static checker) which uses a superset of the language, allowing you to add type annotations to all of your code and catch an entire class of bugs at compile time.

PropTypes is a _basic type checker_ (runtime checker) which has been patched onto React. It can't check anything other than the types of the props being passed to a given component. If you want more flexible typechecking for your entire project Flow/TypeScript are appropriate choices.

The below steps followed to include Font Awesome in React:

1. Install font-awesome:

        $ npm install --save font-awesome
        

1. Import font-awesome in your index.js file:

        import "font-awesome/css/font-awesome.min.css";
        

1. Add Font Awesome classes in className:

        function MyComponent {
          return <div><i className={'fa fa-spinner'} /></div>
        }
        

_React Developer Tools_ let you inspect the component hierarchy, including component props and state. It exists both as a browser extension (for Chrome and Firefox), and as a standalone app (works with other environments including Safari, IE, and React Native).

The official extensions available for different browsers or environments.

1. Chrome extension
2. Firefox extension
3. Standalone app (Safari, React Native, etc)

If you opened a local HTML file in your browser (file://...) then you must first open _Chrome Extensions_ and check Allow access to file URLs.

You need to follow below steps to use Polymer in React,

1. Create a Polymer element:

```jsx harmony
<link
rel="import"
href="../../bower_components/polymer/polymer.html"
/>;
Polymer({
is: "calendar-element",
ready: function () {
this.textContent = "I am a calendar";
},
});

     2. Create the Polymer component HTML tag by importing it in a HTML document, e.g. import it in the `index.html` of your React application:

        

     3. Use that element in the JSX file:

        

React has the following advantages over Vue.js:

1. Gives more flexibility in large apps developing.
2. Easier to test.
3. Suitable for mobile apps creating.
4. More information and solutions available.

Note: The above list of advantages are purely opinionated and it vary based on the professional experience. But they are helpful as base parameters.

Let's see the difference between React and Angular in a table format.

| React | Angular |
| ------------------------------------------------------------------------------------------- | ---------------------------------------------------------------------------------------------------------------------------------- |
| React is a library and has only the View layer | Angular is a framework and has complete MVC functionality |
| React handles rendering on the server side | AngularJS renders only on the client side but Angular 2 and above renders on the server side |
| React uses JSX that looks like HTML in JS which can be confusing | Angular follows the template approach for HTML, which makes code shorter and easy to understand |
| React Native, which is a React type to build mobile applications are faster and more stable | Ionic, Angular's mobile native app is relatively less stable and slower |
| In React, data flows only in one way and hence debugging is easy | In Angular, data flows both way i.e it has two-way data binding between children and parent and hence debugging is often difficult |

Note: The above list of differences are purely opinionated and it vary based on the professional experience. But they are helpful as base parameters.

When the page loads, _React DevTools_ sets a global named __REACT_DEVTOOLS_GLOBAL_HOOK__, then React communicates with that hook during initialization. If the website is not using React or if React fails to communicate with DevTools then it won't show up the tab.

styled-components is a JavaScript library for styling React applications. It removes the mapping between styles and components, and lets you write actual CSS augmented with JavaScript.

Lets create <Title> and <Wrapper> components with specific styles for each.

     import React from "react";
     import styled from "styled-components";

     // Create a <Title> component that renders an <h1> which is centered, red and sized at 1.5em
     const Title = styled.h1`
       font-size: 1.5em;
       text-align: center;
       color: palevioletred;
     `;

     // Create a <Wrapper> component that renders a <section> with some padding and a papayawhip background
     const Wrapper = styled.section`
       padding: 4em;
       background: papayawhip;
     `;
     

These two variables, Title and Wrapper, are now components that you can render just like any other react component.

```jsx harmony
<Wrapper>
<Title>{"Lets start first styled component!"}</Title>
</Wrapper>
```

Relay is a JavaScript framework for providing a data layer and client-server communication to web applications using the React view layer.

## Miscellaneous

Let's see the main features of Reselect library,

1. Selectors can compute derived data, allowing Redux to store the minimal possible state.
2. Selectors are efficient. A selector is not recomputed unless one of its arguments changes.
3. Selectors are composable. They can be used as input to other selectors.

Let's take calculations and different amounts of a shipment order with the simplified usage of Reselect:

     import { createSelector } from "reselect";

     const shopItemsSelector = (state) => state.shop.items;
     const taxPercentSelector = (state) => state.shop.taxPercent;

     const subtotalSelector = createSelector(shopItemsSelector, (items) =>
       items.reduce((acc, item) => acc + item.value, 0)
     );

     const taxSelector = createSelector(
       subtotalSelector,
       taxPercentSelector,
       (subtotal, taxPercent) => subtotal * (taxPercent / 100)
     );

     export const totalSelector = createSelector(
       subtotalSelector,
       taxSelector,
       (subtotal, tax) => ({ total: subtotal + tax })
     );

     let exampleState = {
       shop: {
         taxPercent: 8,
         items: [
           { name: "apple", value: 1.2 },
           { name: "orange", value: 0.95 },
         ],
       },
     };

     console.log(subtotalSelector(exampleState)); // 2.15
     console.log(taxSelector(exampleState)); // 0.172
     console.log(totalSelector(exampleState)); // { total: 2.322 }
     

Redux can be used as a data store for any UI layer. The most common usage is with React and React Native, but there are bindings available for Angular, Angular 2, Vue, Mithril, and more. Redux simply provides a subscription mechanism which can be used by any other code.

Redux is originally written in ES6 and transpiled for production into ES5 with Webpack and Babel. You should be able to use it regardless of your JavaScript build process. Redux also offers a UMD build that can be used directly without any build process at all.

You need to add enableReinitialize : true setting.

     const InitializeFromStateForm = reduxForm({
       form: "initializeFromState",
       enableReinitialize: true,
     })(UserEdit);
     

If your initialValues prop gets updated, your form will update too.

You can use oneOfType() method of PropTypes.

For example, the height property can be defined with either string or number type as below:

     Component.propTypes = {
       size: PropTypes.oneOfType([PropTypes.string, PropTypes.number]),
     };
     

You can import SVG directly as component instead of loading it as a file. This feature is available with react-scripts@2.0.0 and higher.

```jsx harmony
import { ReactComponent as Logo } from "./logo.svg";

const App = () => (
<div>
{/* Logo is an actual react component */}
<Logo />
</div>
);
```

Note: Don't forget about the curly braces in the import.

The concept of render hijacking is the ability to control what a component will output from another component. It means that you decorate your component by wrapping it into a Higher-Order component. By wrapping, you can inject additional props or make other changes, which can cause changing logic of rendering. It does not actually enable hijacking, but by using HOC you make your component behave differently.

We can pass numbers as props to React component using curly braces {} where as strings in double quotes "" or single quotes ''

     import React from "react";

     const ChildComponent = ({ name, age }) => {
       return (
         <>
           My Name is {name} and Age is {age}
         </>
       );
     };

     const ParentComponent = () => {
       return (
         <>
           <ChildComponent name="Chetan" age={30} />
         </>
       );
     };

     export default ParentComponent;
     

It is up to the developer's decision, i.e., it is developer's job to determine what kinds of state make up your application, and where each piece of state should live. Some users prefer to keep every single piece of data in Redux, to maintain a fully serializable and controlled version of their application at all times. Others prefer to keep non-critical or UI state, such as “is this dropdown currently open”, inside a component's internal state.

Below are the rules of thumb to determine what kind of data should be put into Redux

1. Do other parts of the application care about this data?
2. Do you need to be able to create further derived data based on this original data?
3. Is the same data being used to drive multiple components?
4. Is there value to you in being able to restore this state to a given point in time (ie, time travel debugging)?
5. Do you want to cache the data (i.e, use what's in state if it's already there instead of re-requesting it)?

React creates a service worker for you without any configuration by default. The service worker is a web API that helps you cache your assets and other files so that when the user is offline or on a slow network, he/she can still see results on the screen, as such, it helps you build a better user experience, that's what you should know about service worker for now. It's all about adding offline capabilities to your site.

     import React from "react";
     import ReactDOM from "react-dom";
     import App from "./App";
     import registerServiceWorker from "./registerServiceWorker";

     ReactDOM.render(<App />, document.getElementById("root"));
     registerServiceWorker();
     

> Modern React Note (React 18/19): ReactDOM.render was deprecated in React 18 and removed in React 19. In modern applications, always use createRoot from react-dom/client:
>

> import { createRoot } from 'react-dom/client';
> const root = createRoot(document.getElementById('root'));
> root.render(<App />);
> 

Class components can be restricted from re-rendering when their input props are the same using PureComponent or shouldComponentUpdate. Now you can do the same with function components by wrapping them in React.memo.

     const MyComponent = React.memo(function MyComponent(props) {
       /* only rerenders if props change */
     });
     

You can compare the current value of the state with an existing state value and decide whether to rerender the page or not. If the values are the same then you need to return null to stop re-rendering otherwise return the latest state value.

For example, the user profile information is conditionally rendered as follows,

     getUserProfile = (user) => {
       const latestAddress = user.address;
       this.setState((state) => {
         if (state.address === latestAddress) {
           return null;
         } else {
           return { title: latestAddress };
         }
       });
     };
     

Note: This question references React 16. These features remain valid in current React versions (18/19).

Arrays: Starting with React 16, you can return multiple sibling elements without a wrapping element by returning an array or using Fragments.

Array approach:

     const ReactJSDevs = () => {
       return [
         <li key="1">John</li>,
         <li key="2">Jackie</li>,
         <li key="3">Jordan</li>,
       ];
     };
     

Fragment approach (preferred):

     const ReactJSDevs = () => {
       return (
         <>
           <li>John</li>
           <li>Jackie</li>
           <li>Jordan</li>
         </>
       );
     };
     

You can also merge this array of items in another array component.

     const JSDevs = () => {
       return (
         <ul>
           <li>Brad</li>
           <li>Brodge</li>
           <ReactJSDevs />
           <li>Brandon</li>
         </ul>
       );
     };
     

Strings and Numbers: You can also return string and number type from the render method.

     render() {
      return 'Welcome to ReactJS questions';
     }
     // Number
     render() {
      return 2018;
     }
     

The biggest conceptual difference is the data flow itself:

     Flux:

     View
      ↓
     Action
      ↓
     Dispatcher
      ↓
     Multiple Stores
      ↓
     View
     

versus:

     Redux:

     View
      ↓
     Action
      ↓
     Middleware
      ↓
     Reducer
      ↓
     Single Store
      ↓
     View
     

Flux routes every action through a single Dispatcher, which broadcasts it to multiple stores, each holding its own slice of state and its own change logic. Redux replaces the dispatcher with middleware (for side effects) and pure reducer functions (for state updates), all funneling into a single store as the one source of truth.

Below are the major differences between Flux and Redux

| Flux | Redux |
| ---------------------------------------------- | ------------------------------------------ |
| State is mutable | State is immutable |
| The Store contains both state and change logic | The Store and change logic are separate |
| There are multiple stores exist | There is only one store exist |
| All the stores are disconnected and flat | Single store with hierarchical reducers |
| It has a singleton dispatcher | There is no concept of dispatcher |
| React components subscribe to the store | Container components uses connect function |

Below are the main benefits of React Router V4 module,

1. In React Router v4(version 4), the API is completely about components. A router can be visualized as a single component(<BrowserRouter>) which wraps specific child router components(<Route>).
2. You don't need to manually set history. The router module will take care history by wrapping routes with <BrowserRouter> component.
3. The application size is reduced by adding only the specific router module(Web, core, or native)

Below are the cases in which error boundaries don't work,

1. Inside Event handlers
2. Asynchronous code using setTimeout or requestAnimationFrame callbacks
3. During Server side rendering
4. When errors thrown in the error boundary code itself

Note: This behavior was introduced in React 16 and continues in React 18/19.

In React 16+, errors that are not caught by any error boundary will result in unmounting of the whole React component tree. The reason behind this decision is that it is worse to leave corrupted UI in place than to completely remove it. For example, it is worse for a payments app to display a wrong amount than to render nothing.

Best Practice: Always wrap your application or critical sections in error boundaries to prevent complete unmounting and provide a better user experience.

     <ErrorBoundary fallback={<ErrorPage />}>
       <App />
     </ErrorBoundary>
     

The granularity of error boundaries usage is up to the developer based on project needs. You can follow either of these approaches,

1. You can wrap top-level route components to display a generic error message for the entire application.
2. You can also wrap individual components in an error boundary to protect them from crashing the rest of the application.

The _defaultProps_ can be defined as a property on the component to set the default values for the props. These default props are used when props not supplied(i.e., undefined props), but not for null or 0 as props. That means, If you provide null value then it remains null value. It's the same behavior with 0 as well.

For example, let us create color default prop for the button component,

     function MyButton {
       // ...
     }

     MyButton.defaultProps = {
       color: "red",
     };
     

If props.color is not provided then it will set the default value to 'red'. i.e, Whenever you try to access the color prop it uses the default value

     function MyButton() {
       return <MyButton />; // props.color will contain red value
     }
     

The displayName string is used in debugging messages. Usually, you don’t need to set it explicitly because it’s inferred from the name of the function or class that defines the component. You might want to set it explicitly if you want to display a different name for debugging purposes or when you create a higher-order component.

For example, To ease debugging, choose a display name that communicates that it’s the result of a withSubscription HOC.

     function withSubscription(WrappedComponent) {
       class WithSubscription extends React.Component {
         /* ... */
       }
       WithSubscription.displayName = `WithSubscription(${getDisplayName(
         WrappedComponent
       )})`;
       return WithSubscription;
     }
     function getDisplayName(WrappedComponent) {
       return (
         WrappedComponent.displayName || WrappedComponent.name || "Component"
       );
     }
     

React supports all popular browsers, including Internet Explorer 9 and above, although some polyfills are required for older browsers such as IE 9 and IE 10. If you use es5-shim and es5-sham polyfill then it even support old browsers that doesn't support ES5 methods.

Code-Splitting is a feature supported by bundlers like Webpack and Browserify which can create multiple bundles that can be dynamically loaded at runtime. The react project supports code splitting via dynamic import() feature.

For example, in the below code snippets, it will make moduleA.js and all its unique dependencies as a separate chunk that only loads after the user clicks the 'Load' button.

moduleA.js

     const moduleA = "Hello";

     export { moduleA };
     

App.js

     export default function App {
       function handleClick() {
         import("./moduleA")
           .then(({ moduleA }) => {
             // Use moduleA
           })
           .catch((err) => {
             // Handle failure
           });
       };

      return (
        <div>
          <button onClick={this.handleClick}>Load</button>
        </div>
      );
     }
     

<details><summary><b>See Class</b></summary>
<p>

    import React, { Component } from "react";

     class App extends Component {
       handleClick = () => {
         import("./moduleA")
           .then(({ moduleA }) => {
             // Use moduleA
           })
           .catch((err) => {
             // Handle failure
           });
       };

       render() {
         return (
           <div>
             <button onClick={this.handleClick}>Load</button>
           </div>
         );
       }
     }

     export default App;
  

</p>
</details>

The Fragments declared with the explicit <React.Fragment> syntax may have keys. The general use case is mapping a collection to an array of fragments as below,

     function Glossary(props) {
       return (
         <dl>
           {props.items.map((item) => (
             // Without the `key`, React will fire a key warning
             <React.Fragment key={item.id}>
               <dt>{item.term}</dt>
               <dd>{item.description}</dd>
             </React.Fragment>
           ))}
         </dl>
       );
     }
     

Note: key is the only attribute that can be passed to Fragment. In the future, there might be a support for additional attributes, such as event handlers.

As of React 16, both standard or custom DOM attributes are fully supported. Since React components often take both custom and DOM-related props, React uses the camelCase convention just like the DOM APIs.

Let us take few props with respect to standard HTML attributes,

     <div tabIndex="-1" />      // Just like node.tabIndex DOM API
     <div className="Button" /> // Just like node.className DOM API
     <input readOnly={true} />  // Just like node.readOnly DOM API
     

These props work similarly to the corresponding HTML attributes, with the exception of the special cases. It also support all SVG attributes.

If you pass no value for a prop, it defaults to true. This behavior is available so that it matches the behavior of HTML.

For example, below expressions are equivalent,

     <MyInput autocomplete />

     <MyInput autocomplete={true} />
     

Note: It is not recommended to use this approach because it can be confused with the ES6 object shorthand (example, {name} which is short for {name: name})

Next.js is a popular and lightweight framework for static and server‑rendered applications built with React. It also provides styling and routing solutions. Below are the major features provided by NextJS,

1. Server-rendered by default
2. Automatic code splitting for faster page loads
3. Simple client-side routing (page based)
4. Webpack-based dev environment which supports (HMR)
5. Able to implement with Express or any other Node.js HTTP server
6. Customizable with your own Babel and Webpack configurations

You can pass event handlers and other functions as props to child components. The functions can be passed to child component as below,

     function Button({ onClick }) {
       return <button onClick={onClick}>Download</button>;
     }

     export default function downloadExcel() {
       function handleClick() {
         alert("Downloaded");
       }

       return <Button onClick={handleClick}></Button>;
     }
     

If you use an event handler such as onClick or onScroll and want to prevent the callback from being fired too quickly, then you can limit the rate at which callback is executed. This can be achieved in the below possible ways,

1. Throttling: Changes based on a time based frequency. For example, it can be used using \_.throttle lodash function
2. Debouncing: Publish changes after a period of inactivity. For example, it can be used using \_.debounce lodash function
3. RequestAnimationFrame throttling: Changes based on requestAnimationFrame. For example, it can be used using raf-schd lodash function

React DOM escapes any values embedded in JSX before rendering them. Thus it ensures that you can never inject anything that’s not explicitly written in your application. Everything is converted to a string before being rendered.

For example, you can embed user input as below,

     const name = response.potentiallyMaliciousInput;
     const element = <h1>{name}</h1>;
     

This way you can prevent XSS(Cross-site-scripting) attacks in the application.

You can update UI(represented by rendered element) by passing the newly created element to ReactDOM's render method.

For example, lets take a ticking clock example, where it updates the time by calling render method multiple times,

     function tick() {
       const element = (
         <div>
           <h1>Hello, world!</h1>
           <h2>It is {new Date().toLocaleTimeString()}.</h2>
         </div>
       );
       ReactDOM.render(element, document.getElementById("root"));
     }

     setInterval(tick, 1000);
     

> Modern React Note (React 18/19): ReactDOM.render was deprecated in React 18 and removed in React 19. In modern applications, always use createRoot from react-dom/client:
>

> import { createRoot } from 'react-dom/client';
> const root = createRoot(document.getElementById('root'));
> root.render(<App />);
> 

When you declare a component as a function or a class, it must never modify its own props.

Let us take a below capital function,

     function capital(amount, interest) {
       return amount + interest;
     }
     

The above function is called “pure” because it does not attempt to change their inputs, and always return the same result for the same inputs. Hence, React has a single rule saying "All React components must act like pure functions with respect to their props."

There are three conditions to make sure, it is safe use the index as a key.

1. The list and items are static– they are not computed and do not change
2. The items in the list have no ids
3. The list is never reordered or filtered.

The keys used within arrays should be unique among their siblings but they don’t need to be globally unique. i.e, You can use the same keys with two different arrays.

For example, the below Book component uses two arrays with different arrays,

     function Book(props) {
       const index = (
         <ul>
           {props.pages.map((page) => (
             <li key={page.id}>{page.title}</li>
           ))}
         </ul>
       );
       const content = props.pages.map((page) => (
         <div key={page.id}>
           <h3>{page.title}</h3>
           <p>{page.content}</p>
           <p>{page.pageNumber}</p>
         </div>
       ));
       return (
         <div>
           {index}
           <hr />
           {content}
         </div>
       );
     }
     

Formik is a form library for react which provides solutions such as validation, keeping track of the visited fields, and handling form submission.

In detail, You can categorize them as follows,

1. Getting values in and out of form state
2. Validation and error messages
3. Handling form submission

It is used to create a scalable, performant, form helper with a minimal API to solve annoying stuff.

Below are the main reasons to recommend formik over redux form library,

1. The form state is inherently short-term and local, so tracking it in Redux (or any kind of Flux library) is unnecessary.
2. Redux-Form calls your entire top-level Redux reducer multiple times ON EVERY SINGLE KEYSTROKE. This way it increases input latency for large apps.
3. Redux-Form is 22.5 kB minified gzipped whereas Formik is 12.7 kB

In React, it is recommended to use composition over inheritance to reuse code between components. Both Props and composition give you all the flexibility you need to customize a component’s look and behavior explicitly and safely.
Whereas, If you want to reuse non-UI functionality between components, it is suggested to extract it into a separate JavaScript module. Later components import it and use that function, object, or class, without extending it.

Yes, you can use web components in a react application. Even though many developers won't use this combination, it may require especially if you are using third-party UI components that are written using Web Components.

For example, let us use Vaadin date picker web component as below,

     import "./App.css";
     import "@vaadin/vaadin-date-picker";
     export default function App() {
       return (
         <div className="App">
           <vaadin-date-picker label="When were you born?"></vaadin-date-picker>
         </div>
       );
     }
     

You can achieve code-splitting in your app using dynamic import.

Let's take an example of addition,

1. Normal Import

     import { add } from "./math";
     console.log(add(10, 20));
     

1. Dynamic Import

     import("./math").then((math) => {
       console.log(math.add(10, 20));
     });
     

With the release of React 18, React.lazy and Suspense are now available for server-side rendering. However, prior to React 18, it was recommended to use Loadable Components for code-splitting in a server-side rendered app because React.lazy and Suspense were not available for server-side rendering. Loadable Components lets you render a dynamic import as a regular component. For example, you can use Loadable Components to load the OtherComponent in a separate bundle like this:

     import loadable from "@loadable/component";

     const OtherComponent = loadable(() => import("./OtherComponent"));

     function MyComponent() {
       return (
         <div>
           <OtherComponent />
         </div>
       );
     }
     

Now OtherComponent will be loaded in a separated bundle
Loadable Components provides additional benefits beyond just code-splitting, such as automatic code reloading, error handling, and preloading. By using Loadable Components, you can ensure that your application loads quickly and efficiently, providing a better user experience for your users.

React Suspense is a built-in feature that lets you delay rendering part of the UI until asynchronous work (such as loading a component or fetching data) is ready. While waiting, React shows a fallback UI (loading text, skeleton, spinner, etc.) defined by the nearest <Suspense> boundary.

Common use cases:

1. Code splitting with React.lazy()

        import { Suspense, lazy } from "react";

        const OtherComponent = lazy(() => import("./OtherComponent"));

        function MyComponent() {
          return (
            <Suspense fallback={<div>Loading component...</div>}>
              <OtherComponent />
            </Suspense>
          );
        }
        

The fallback is shown until OtherComponent is downloaded and rendered.

1. Data fetching that can suspend

        import { Suspense, use } from "react";

        function UserProfile() {
          const user = use(fetchUser()); // suspends until promise resolves
          return <div>{user.name}</div>;
        }

        function App() {
          return (
            <Suspense fallback={<div>Loading user...</div>}>
              <UserProfile />
            </Suspense>
          );
        }
        

1. Nested Suspense for progressive loading

Nested boundaries let important content render first while slower sections keep loading.

        import { Suspense, lazy } from "react";

        const ProductDetails = lazy(() => import("./ProductDetails"));
        const ProductReviews = lazy(() => import("./ProductReviews"));

        function ProductPage() {
          return (
            <Suspense fallback={<PageSkeleton />}>
              <ProductDetails />

              <Suspense fallback={<ReviewsSkeleton />}>
                <ProductReviews />
              </Suspense>
            </Suspense>
          );
        }
        

Here, ProductDetails can appear first, and ProductReviews can continue loading with its own fallback.

Tip: Keep Suspense boundaries close to slow UI regions so users see meaningful content sooner.

One of the best place to do code splitting is with routes. The entire page is going to re-render at once so users are unlikely to interact with other elements in the page at the same time. Due to this, the user experience won't be disturbed.

Let us take an example of route based website using libraries like React Router with React.lazy,

     import { BrowserRouter as Router, Route, Switch } from "react-router-dom";
     import React, { Suspense, lazy } from "react";

     const Home = lazy(() => import("./routes/Home"));
     const About = lazy(() => import("./routes/About"));

     const App = () => (
       <Router>
         <Suspense fallback={<div>Loading...</div>}>
           <Switch>
             <Route exact path="/" component={Home} />
             <Route path="/about" component={About} />
           </Switch>
         </Suspense>
       </Router>
     );
     

In the above code, the code splitting will happen at each route level.

React needs to use algorithms to find out how to efficiently update the UI to match the most recent tree. The diffing algorithms is generating the minimum number of operations to transform one tree into another. However, the algorithms have a complexity in the order of O(n³) where n is the number of elements in the tree.

In this case, displaying 1000 elements would require in the order of one billion comparisons. This is far too expensive. Instead, React implements a heuristic O(n) algorithm based on two assumptions:

1. Two elements of different types will produce different trees.
2. The developer can hint at which child elements may be stable across different renders with a key prop.

When diffing two trees, React first compares the two root elements. The behavior is different depending on the types of the root elements. It covers the below rules during reconciliation algorithm,

1. Elements Of Different Types:

Whenever the root elements have different types, React will tear down the old tree and build the new tree from scratch. For example, elements <a> to <img>, or from <Article> to <Comment> of different types lead a full rebuild.

1. DOM Elements Of The Same Type:

When comparing two React DOM elements of the same type, React looks at the attributes of both, keeps the same underlying DOM node, and only updates the changed attributes. Lets take an example with same DOM elements except className attribute,

        <div className="show" title="ReactJS" />

        <div className="hide" title="ReactJS" />
        

1. Component Elements Of The Same Type:

When a component updates, the instance stays the same, so that state is maintained across renders. React updates the props of the underlying component instance to match the new element, and calls componentWillReceiveProps() and componentWillUpdate() on the underlying instance. After that, the render() method is called and the diff algorithm recurses on the previous result and the new result.

1. Recursing On Children:

when recursing on the children of a DOM node, React just iterates over both lists of children at the same time and generates a mutation whenever there’s a difference. For example, when adding an element at the end of the children, converting between these two trees works well.

        <ul>
          <li>first</li>
          <li>second</li>
        </ul>

        <ul>
          <li>first</li>
          <li>second</li>
          <li>third</li>
        </ul>

        

1. Handling keys:

React supports a key attribute. When children have keys, React uses the key to match children in the original tree with children in the subsequent tree. For example, adding a key can make the tree conversion efficient,

     <ul>
       <li key="2015">Duke</li>
       <li key="2016">Villanova</li>
     </ul>

     <ul>
       <li key="2014">Connecticut</li>
       <li key="2015">Duke</li>
       <li key="2016">Villanova</li>
     </ul>
     

> Modern React Note: componentWillMount, componentWillReceiveProps, and componentWillUpdate are legacy lifecycles deprecated since React 16.3 and removed in modern React. In modern functional React, use the useEffect Hook or derive state values directly during render.

There are few use cases to go for refs,

1. Managing focus, text selection, or media playback.
2. Triggering imperative animations.
3. Integrating with third-party DOM libraries.

Even though the pattern named render props, you don’t have to use a prop named render to use this pattern. i.e, Any prop that is a function that a component uses to know what to render is technically a “render prop”. Lets take an example with the children prop for render props,

     <Mouse
       children={(mouse) => (
         <p>
           The mouse position is {mouse.x}, {mouse.y}
         </p>
       )}
     />
     

Actually children prop doesn’t need to be named in the list of “attributes” in JSX element. Instead, you can keep it directly inside element,

     <Mouse>
       {(mouse) => (
         <p>
           The mouse position is {mouse.x}, {mouse.y}
         </p>
       )}
     </Mouse>
     

While using this above technique(without any name), explicitly state that children should be a function in your propTypes.

     Mouse.propTypes = {
       children: PropTypes.func.isRequired,
     };
     

Windowing is a technique that only renders a small subset of your rows at any given time, and can dramatically reduce the time it takes to re-render the components as well as the number of DOM nodes created. If your application renders long lists of data then this technique is recommended. Both react-window and react-virtualized are popular windowing libraries which provides several reusable components for displaying lists, grids, and tabular data.

The falsy values such as false, null, undefined, and true are valid children but they don't render anything. If you still want to display them then you need to convert it to string. Let's take an example on how to convert to a string,

     <div>My JavaScript variable is {String(myVariable)}.</div>
     

Even though the tech stack varies from developer to developer, the most popular stack is used in react boilerplate project code. It mainly uses Redux and redux-saga for state management and asynchronous side-effects, react-router for routing purpose, styled-components for styling react components, axios for invoking REST api, and other supported stack such as webpack, reselect, ESNext, Babel.
You can clone the project https://github.com/react-boilerplate/react-boilerplate and start working on any new react project.

Below are the main differences between Real DOM and Virtual DOM,

| Real DOM | Virtual DOM |
| ------------------------------------ | ------------------------------------ |
| Updates are slow | Updates are fast |
| DOM manipulation is very expensive. | DOM manipulation is very easy |
| You can update HTML directly. | You Can’t directly update HTML |
| It causes too much of memory wastage | There is no memory wastage |
| Creates a new DOM if element updates | It updates the JSX if element update |

Bootstrap can be added to your React app in a three possible ways,

1. Using the Bootstrap CDN:

This is the easiest way to add bootstrap. Add both bootstrap CSS and JS resources in a head tag.

1. Bootstrap as Dependency:

If you are using a build tool or a module bundler such as Webpack, then this is the preferred option for adding Bootstrap to your React application

        npm install bootstrap
        

1. React Bootstrap Package:

In this case, you can add Bootstrap to our React app is by using a package that has rebuilt Bootstrap components to work particularly as React components. Below packages are popular in this category,

1. react-bootstrap
2. reactstrap

Below are the top 10 websites using React as their front-end framework,

1. Facebook
2. Uber
3. Instagram
4. WhatsApp
5. Khan Academy
6. Airbnb
7. Dropbox
8. Flipboard
9. Netflix
10. PayPal

React does not have any opinion about how styles are defined but if you are a beginner then good starting point is to define your styles in a separate \*.css file as usual and refer to them using className. This functionality is not part of React but came from third-party libraries. But If you want to try a different approach(CSS-In-JS) then styled-components library is a good option.

When we declare a state variable with useState, it returns a pair — an array with two items. The first item is the current value, and the second is a function that updates the value. Using [0] and [1] to access them is a bit confusing because they have a specific meaning. This is why we use array destructuring instead.

For example, the array index access would look as follows:

     var userStateVariable = useState("userProfile"); // Returns an array pair
     var user = userStateVariable[0]; // Access first item
     var setUser = userStateVariable[1]; // Access second item
     

Whereas with array destructuring the variables can be accessed as follows:

     const [user, setUser] = useState("userProfile");
     

Web Components often expose an imperative API to implement its functions. You will need to use a ref to interact with the DOM node directly if you want to access imperative API of a web component. But if you are using third-party Web Components, the best solution is to write a React component that behaves as a wrapper for your Web Component.

Formik is a small react form library that helps you with the three major problems,

1. Getting values in and out of form state
2. Validation and error messages
3. Handling form submission

When managing asynchronous side effects (such as API calls, debouncing, and web sockets) in Redux, several middleware libraries have evolved:

### 1. Redux Toolkit Query (RTK Query) - Modern Recommendation
The current standard provided out of the box with Redux Toolkit (@reduxjs/toolkit). RTK Query eliminates the need to hand-craft thunks, reducers, and loading/error states for server data fetching, providing automatic caching, invalidation, and polling.

### 2. Redux Thunk
The most common and lightweight approach. A thunk is an action creator that returns a function (dispatch, getState) instead of an action object, allowing asynchronous promises to be awaited before dispatching resolved actions:

export const fetchUser = (id) => async (dispatch) => {
  dispatch({ type: 'user/pending' });
  try {
    const res = await api.getUser(id);
    dispatch({ type: 'user/fulfilled', payload: res.data });
  } catch (err) {
    dispatch({ type: 'user/rejected', error: err.message });
  }
};

### 3. Redux Saga
Uses ES6 Generator functions (yield takeEvery, yield call, yield put) to orchestrate complex asynchronous workflows. Ideal for applications requiring cancellation, debouncing, concurrent worker pools, or complex background synchronization.

### 4. Redux Observable
Built on RxJS Observables, treating actions as streams. Highly powerful for real-time finance feeds, live chat, or complex event-driven pipelines requiring RxJS operators like switchMap, debounceTime, and mergeMap.

No, browsers can't understand JSX code. You need a transpiler to convert your JSX to regular Javascript that browsers can understand. The most widely used transpiler right now is Babel.

React implements one-way reactive data flow using props which reduce boilerplate and is easier to understand than traditional two-way data binding.

MobX is a simple, scalable and battle tested state management solution for applying functional reactive programming (TFRP). For ReactJS application, you need to install below packages,

     npm install mobx --save
     npm install mobx-react --save
     

Below are the main differences between Redux and MobX,

| Topic | Redux | MobX |
| ------------- | ------------------------------------------------------------- | ---------------------------------------------------------------------- |
| Definition | It is a javascript library for managing the application state | It is a library for reactively managing the state of your applications |
| Programming | It is mainly written in ES6 | It is written in JavaScript(ES5) |
| Data Store | There is only one large store exist for data storage | There is more than one store for storage |
| Usage | Mainly used for large and complex applications | Used for simple applications |
| Performance | Need to be improved | Provides better performance |
| How it stores | Uses JS Object to store | Uses observable to store the data |

No, you don’t have to learn es2015/es6 to learn react. But you may find many resources or React ecosystem uses ES6 extensively. Let's see some of the frequently used ES6 features,

1. Destructuring: To get props and use them in a component

        // in es 5
        var someData = this.props.someData;
        var dispatch = this.props.dispatch;

        // in es6
        const { someData, dispatch } = this.props;
        

1. Spread operator: Helps in passing props down into a component

        // in es 5
        <SomeComponent someData={this.props.someData} dispatch={this.props.dispatch} />

        // in es6
        <SomeComponent {...this.props} />
        

1. Arrow functions: Makes compact syntax

        // es 5
        var users = usersList.map(function (user) {
          return <li>{user.name}</li>;
        });
        // es 6
        const users = usersList.map((user) => <li>{user.name}</li>);
        

Imagine a simple UI component, such as a "Like" button. When you tap it, it turns blue if it was previously grey, and grey if it was previously blue.

The imperative way of doing this would be:

     if (user.likes()) {
       if (hasBlue()) {
         removeBlue();
         addGrey();
       } else {
         removeGrey();
         addBlue();
       }
     }
     

Basically, you have to check what is currently on the screen and handle all the changes necessary to redraw it with the current state, including undoing the changes from the previous state. You can imagine how complex this could be in a real-world scenario.

In contrast, the declarative approach would be:

     if (this.state.liked) {
       return <blueLike />;
     } else {
       return <greyLike />;
     }
     

Because the declarative approach separates concerns, this part of it only needs to handle how the UI should look in a specific state, and is therefore much simpler to understand.

Below are some of the benefits of using TypeScript with ReactJS,

1. It is possible to use latest JavaScript features
2. Use of interfaces for complex type definitions
3. IDEs such as VS Code was made for TypeScript
4. Avoid bugs with the ease of readability and Validation

There are three major benefits of new JSX transform,

1. It is possible to use JSX without importing React packages
2. The compiled output might improve the bundle size in a small amount
3. The future improvements provides the flexibility to reduce the number of concepts to learn React.

The new JSX transform doesn’t require React to be in scope. i.e, You don't need to import React package for simple scenarios.

Let's take an example to look at the main differences between the old and the new transform,

Old Transform:

     import React from "react";

     function App() {
       return <h1>Good morning!!</h1>;
     }
     

Now JSX transform convert the above code into regular JavaScript as below,

     import React from "react";

     function App() {
       return React.createElement("h1", null, "Good morning!!");
     }
     

New Transform:

The new JSX transform doesn't require any React imports

     function App() {
       return <h1>Good morning!!</h1>;
     }
     

Under the hood JSX transform compiles to below code

     import { jsx as _jsx } from "react/jsx-runtime";

     function App() {
       return _jsx("h1", { children: "Good morning!!" });
     }
     

Note: You still need to import React to use Hooks.

Prop Drilling is the process by which you pass data from one component of the React Component tree to another by going through other components that do not need the data but only help in passing it around.

A wrapper in React is a component that wraps or surrounds another component or group of components. It can be used for a variety of purposes such as adding additional functionality, styling, or layout to the wrapped components.

For example, consider a simple component that displays a message:

     const Message = ({ text }) => {
       return <p>{text}</p>;
     };
     

We can create a wrapper component that will add a border to the message component:

     const MessageWrapper = (props) => {
       return (
         <div style={{ border: "1px solid black" }}>
           <Message {...props} />
         </div>
       );
     };
     

Now we can use the MessageWrapper component instead of the Message component and the message will be displayed with a border:

     <MessageWrapper text="Hello World" />
     

Wrapper component can also accept its own props and pass them down to the wrapped component, for example, we can create a wrapper component that will add a title to the message component:

     const MessageWrapperWithTitle = ({ title, ...props }) => {
       return (
         <div>
           <h3>{title}</h3>
           <Message {...props} />
         </div>
       );
     };
     

Now we can use the MessageWrapperWithTitle component and pass title props:

     <MessageWrapperWithTitle title="My Message" text="Hello World" />
     

This way, the wrapper component can add additional functionality, styling, or layout to the wrapped component while keeping the wrapped component simple and reusable.

There are two different ways to create components in ReactJS. The main differences are listed down as below,

## 1. Syntax:

The class components uses ES6 classes to create the components. It uses render function to display the HTML content in the webpage.

The syntax for class component looks like as below.

     class App extends React.Component {
       render() {
         return <h1>This is a class component</h1>;
       }
     }
     

Note: The Pascal Case is the recommended approach to provide naming to a component.

Functional component has been improved over the years with some added features like Hooks. Here is a syntax for functional component.

     function App() {
       return (
         <div className="App">
           <h1>Hello, I'm a function component</h1>
         </div>
       );
     }
     

## 2. State:

State contains information or data about a component which may change over time.

In class component, you can update the state when a user interacts with it or server updates the data using the setState() method. The initial state is going to be assigned in the Constructor() method using the this.state object and it is possible to assign different data types such as string, boolean, numbers, etc.

A simple example showing how we use the setState() and constructor():

     class App extends Component {
       constructor() {
         super();
         this.state = {
           message: "This is a class component",
         };
       }
       updateMessage() {
         this.setState({
           message: "Updating the class component",
         });
       }
       render() {
         return (
           <>
             <h1>{this.state.message}</h1>
             <button
               onClick={() => {
                 this.updateMessage();
               }}>
               Click!!
             </button>
           </>
         );
       }
     }

     

You didn't use state in functional components because it was only supported in class components. But over the years hooks have been implemented in functional components which enables to use state too.

The useState() hook can used to implement state in functional components. It returns an array with two items: the first item is current state and the next one is a function (setState) that updates the value of the current state.

Let's see an example to demonstrate the state in functional components,

     function App() {
       const [message, setMessage] = useState("This is a functional component");
       const updateMessage = () => {
         setMessage("Updating the functional component");
       };
       return (
         <div className="App">
           <h1>{message} </h1>
           <button onClick={updateMessage}>Click me!!</button>
         </div>
       );
     }
     

## 3. Props:

Props are referred to as "properties". The props are passed into React component just like arguments passed to a function. In other words, they are similar to HTML attributes.

The props are accessible in child class component using this.props as shown in below example,

     class Child extends React.Component {
       render() {
         return (
           <h1>
             {" "}
             This is a functional component and component name is {
               this.props.name
             }{" "}
           </h1>
         );
       }
     }

     class Parent extends React.Component {
       render() {
         return (
           <div className="Parent">
             <Child name="First child component" />
             <Child name="Second child component" />
           </div>
         );
       }
     }
     

Props in functional components are similar to that of the class components but the difference is the absence of 'this' keyword.

     function Child(props) {
       return (
         <h1>
           This is a child component and the component name is{props.name}
         </h1>
       );
     }

     function Parent() {
       return (
         <div className="Parent">
           <Child name="First child component" />
           <Child name="Second child component" />
         </div>
       );
     }
     

React.StrictMode is a useful component for highlighting potential problems in an application. Just like <Fragment>, <StrictMode> does not render any extra DOM elements. It activates additional checks and warnings for its descendants. These checks apply for _development mode_ only.

```jsx harmony
import { StrictMode } from "react";

function App() {
return (
<div>
<Header />
<StrictMode>
<div>
<ComponentOne />
<ComponentTwo />
</div>
</StrictMode>
<Header />
</div>
);
}
```

In the example above, the _strict mode_ checks apply to <ComponentOne> and <ComponentTwo> components only. i.e., Part of the application only.

Note: Frameworks such as NextJS has this flag enabled by default.

The <StrictMode> will be helpful in the below cases,

1. To find the bugs caused by impure rendering where the components will re-render twice.
2. To find the bugs caused by missing cleanup of effects where the components will re-run effects one more extra time.
3. Identifying components with unsafe lifecycle methods.
4. Warning about legacy string ref API usage.
5. Detecting unexpected side effects.
6. Detecting legacy context API.
7. Warning about deprecated findDOMNode usage

StrictMode renders components twice in development mode(not production) in order to detect any problems with your code and warn you about those problems. This is used to detect accidental side effects in the render phase. If you used create-react-app development tool then it automatically enables StrictMode by default.

     const root = createRoot(document.getElementById("root"));
     root.render(
       <StrictMode>
         <App />
       </StrictMode>
     );
     

If you want to disable this behavior then you can simply remove strict mode.

     const root = createRoot(document.getElementById("root"));
     root.render(<App />);
     

To detect side effects the following functions are invoked twice:

1. Function component bodies, excluding the code inside event handlers.
2. Functions passed to useState, useMemo, or useReducer (any other Hook)
3. Class component's constructor, render, and shouldComponentUpdate methods
4. Class component static getDerivedStateFromProps method
5. State updater functions

> Modern React Note: The official React documentation no longer recommends create-react-app. For modern applications, Vite is the standard SPA build tool, or production frameworks like Next.js, Remix, or Astro.

The below 3 rules needs to be followed while using JSX in a react application.

1. Return a single root element:

If you are returning multiple elements from a component, wrap them in a single parent element. Otherwise you will receive the below error in your browser console.

html Adjacent JSX elements must be wrapped in an enclosing tag.

1. All the tags needs to be closed:

Unlike HTML, all tags needs to closed explicitly with in JSX. This rule applies for self-closing tags(like hr, br and img tags) as well.

1. Use camelCase naming:

It is suggested to use camelCase naming for attributes in JSX. For example, the common attributes of HTML elements such as class, tabindex will be used as className and tabIndex.
Note: There is an exception for aria-* and data-* attributes which should be lower cased all the time.

Behind the scenes, JSX is transformed into plain javascript objects. It is not possible to return two or more objects from a function without wrapping into an array. This is the reason you can't simply return two or more JSX tags from a function without
wrapping them into a single parent tag or a Fragment.

The preexisting variables outside of the function scope including state, props and context leads to a mutation and they result in unpredictable bugs during the rendering stage. The below points should be taken care while working with arrays variables.

1. You need to take copy of the original array and perform array operations on it for the rendering purpose. This is called local mutation.
2. Avoid triggering mutation methods such as push, pop, sort and reverse methods on original array. It is safe to use filter, map and slice method because they create a new array.

The onClickCapture React event is helpful to catch all the events of child elements irrespective of event propagation logic or even if the events propagation stopped. This is useful if you need to log every click events for analytics purpose.

For example, the below code triggers the click event of parent first followed by second level child eventhough leaf child button elements stops the propagation.

     <div onClickCapture={() => alert("parent")}>
       <div onClickCapture={() => alert("child")}>
         <button onClick={(e) => e.stopPropagation()} />
         <button onClick={(e) => e.stopPropagation()} />
       </div>
     </div>
     

The event propagation for the above code snippet happens in the following order:

1. It travels downwards in the DOM tree by calling all onClickCapture event handlers.
2. It executes onClick event handler on the target element.
3. It travels upwards in the DOM tree by call all onClick event handlers above to it.

React updates UI in three steps,

1. Triggering or initiating a render: The component is going to triggered for render in two ways.

1. Initial render: When the app starts, you can trigger the initial render by calling creatRoot with the target DOM node followed by invoking component's render method. For example, the following code snippet renders App component on root DOM node.

        import { createRoot } from "react-dom/client";

        const root = createRoot(document.getElementById("root"));
        root.render(<App />);
        

1. Re-render when the state updated: When you update the component state using the state setter function, the componen't state automatically queues for a render.

1. Rendering components: After triggering a render, React will call your components to display them on the screen. React will call the root component for initial render and call the function component whose state update triggered the render. This is a recursive process for all nested components of the target component.

1. Commit changes to DOM: After calling components, React will modify the DOM for initial render using appendChild() DOM API and apply minimal necessary DOM updates for re-renders based on differences between rerenders.

React prevents component from re-rendering for each and every state update by grouping multiple state updates within an event handler. This strategy improves the application performance and this process known as batching. The older version of React only supported batching for browser events whereas React18 supported for asynchronous actions, timeouts and intervals along with native events. This improved version of batching is called automatic batching.

Let's demonstrate this automatic batching feature with a below example.

      import { useState } from "react";

      export default function BatchingState() {
        const [count, setCount] = useState(0);
        const [message, setMessage] = useState("batching");

        console.log("Application Rendered");

        const handleAsyncFetch = () => {
          fetch("https://jsonplaceholder.typicode.com/users/1").then(() => {
            // Automatic Batching re-render only once
            setCount(count + 1);
            setMessage("users fetched");
          });
        };

        return (
          <>
            <h1>{count}</h1>
            <button onClick={handleAsyncFetch}>Click Me!</button>
          </>
        );
      }
      

The preceding code updated two state variables with in an event handler. However, React will perform automatic batching feature and the component will be re-rendered only once for better performance.

Yes, it is possible to prevent automatic batching default behavior. There might be cases where you need to re-render your component after each state update or updating one state depends on another state variable. Considering this situation, React introduced flushSync method from react-dom API for the usecases where you need to flush state updates to DOM immediately.

The usage of flushSync method within an onClick event handler will be looking like as below,

      import { flushSync } from "react-dom";

      const handleClick = () => {
        flushSync(() => {
          setClicked(!clicked); //Component will create a re-render here
        });

        setCount(count + 1); // Component will create a re-render again here
      };
      

In the above click handler, React will update DOM at first using flushSync and second time updates DOM because of the counter setter function by avoiding automatic batching.

React hydration is used to add client-side JavaScript interactivity to pre-rendered static HTML generated by the server. It is used only for server-side rendering(SSR) to enhance the initial rendering time and make it SEO friendly application. This hydration acts as a bridge to reduce the gap between server side and client-side rendering.

After the page loaded with generated static HTML, React will add application state and interactivity by attaching all event handlers for the respective elements. Let's demonstrate this with an example.

Consider that React DOM API(using renderToString) generated HTML for <App> component which contains <button> element to increment the counter.

      import {useState} from 'react';
      import { renderToString } from 'react-dom/server';

      export default function App() {
        const [count, setCount] = React.useState(0);

        return (
          <h1>Counter</h1>
          <button onClick={() => setCount(prevCount => prevCount + 1)}>
            {count} times
          </button>
          );
      }

      const html = renderToString(<App />);
      

The above code generates the below HTML with a header text and button component without any interactivity.

      <h1>Counter</h1>
      <button>
        <!-- -->0<!-- -->
        times
      </button>
      

At this stage hydrateRoot API can be used to perform hydration by attaching onClick event handler.

      import { hydrateRoot } from "react-dom/client";
      import App from "./App.js";

      hydrateRoot(document.getElementById("root"), <App />);
      

After this step, you are able to run React application on server-side and hydrating the javascript bundle on client-side for smooth user experience and SEO purposes.

You cannot update the objects which exists in the state directly. Instead, you should create a fresh new object (or copy from the existing object) and update the latest state using the newly created object. Eventhough JavaScript objects are mutable, you need to treat objects inside state as read-only while updating the state.

Let's see this comparison with an example. The issue with regular object mutation approach can be described by updating the user details fields of Profile component. The properties of Profile component such as firstName, lastName and age details mutated in an event handler as shown below.

      import { useState } from "react";

      export default function Profile() {
        const [user, setUser] = useState({
          firstName: "John",
          lastName: "Abraham",
          age: 30,
        });

        function handleFirstNameChange(e) {
          user.firstName = e.target.value;
        }

        function handleLastNameChange(e) {
          user.lastName = e.target.value;
        }

        function handleAgeChange(e) {
          user.age = e.target.value;
        }

        return (
          <>
            <label>
              First name:
              <input value={user.firstName} onChange={handleFirstNameChange} />
            </label>
            <label>
              Last name:
              <input value={user.lastName} onChange={handleLastNameChange} />
            </label>
            <label>
              Age:
              <input value={user.age} onChange={handleAgeChange} />
            </label>
            <p>
              Profile:
              {person.firstName} {person.lastName} ({person.age})
            </p>
          </>
        );
      }
      

Once you run the application with above user profile component, you can observe that user profile details won't be update upon entering the input fields.
This issue can be fixed by creating a new copy of object which includes existing properties through spread syntax(...obj) and add changed values in a single event handler itself as shown below.

      handleProfileChange(e) {
        setUser({
        ...user,
          [e.target.name]: e.target.value
        });
      }
      

The above event handler is concise instead of maintaining separate event handler for each field. Now, UI displays the updated field values as expected without an issue.

You cannot simply use spread syntax for all kinds of objects inside state. Because spread syntax is shallow and it copies properties for one level deep only. If the object has nested object structure, UI doesn't work as expected with regular JavaScript nested property mutation. Let's demonstrate this behavior with an example of User object which has address nested object inside of it.

      const user = {
        name: "John",
        age: 32,
        address: {
          country: "Singapore",
          postalCode: 440004,
        },
      };
      

If you try to update the country nested field in a regular javascript fashion(as shown below) then user profile screen won't be updated with latest value.

      user.address.country = "Germany";
      

This issue can be fixed by flattening all the fields into a top-level object or create a new object for each nested object and point it to it's parent object. In this example, first you need to create copy of address object and update it with the latest value. Later, the address object should be linked to parent user object something like below.

      setUser({
        ...user,
        address: {
          ...user.address,
          country: "Germany",
        },
      });
      

This approach is bit verbose and not easy for deep hierarchical state updates. But this workaround can be used for few levels of nested objects without much hassle.

Eventhough arrays in JavaScript are mutable in nature, you need to treat them as immutable while storing them in a state. That means, similar to objects, the arrays cannot be updated directly inside state. Instead, you need to create a copy of the existing array and then set the state to use newly copied array.

To ensure that arrays are not mutated, the mutation operations like direct direct assignment(arr[1]='one'), push, pop, shift, unshift, splice etc methods should be avoided on original array. Instead, you can create a copy of existing array with help of array operations such as filter, map, slice, spread syntax etc.

For example, the below push operation doesn't add the new todo to the total todo's list in an event handler.

      onClick = {
        todos.push({
          id: id+1,
          name: name
        })
      }
      

This issue is fixed by replacing push operation with spread syntax where it will create a new array and the UI updated with new todo.

      onClick = {
        [
          ...todos,
          { id: id+1, name: name }
        ]
      }
      

Immer library enforces the immutability of state based on copy-on-write mechanism. It uses JavaScript proxy to keep track of updates to immutable states. Immer has 3 main states as below,

1. Current state: It refers to actual state
2. Draft state: All new changes will be applied to this state. In this state, draft is just a proxy of the current state.
3. Next state: It is formed after all mutations applied to the draft state

Immer can be used by following below instructions,

1. Install the dependency using npm install use-immer command
2. Replace useState hook with useImmer hook by importing at the top
3. The setter function of useImmer hook can be used to update the state.

For example, the mutation syntax of immer library simplifies the nested address object of user state as follows,

      import { useImmer } from "use-immer";
      const [user, setUser] = useImmer({
        name: "John",
        age: 32,
        address: {
          country: "Singapore",
          postalCode: 440004,
        },
      });

      //Update user details upon any event
      setUser((draft) => {
        draft.address.country = "Germany";
      });
      

The preceding code enables you to update nested objects with a conceise mutation syntax.

In React and modern JavaScript state management, immutability (preventing direct mutation of state objects and arrays) is a core foundational architectural requirement.

### Why Direct State Mutation Must Be Prevented:

1. Reliable Change Detection (Shallow Equality):

React components and hooks (useState, useMemo, PureComponent, React.memo) detect state changes using shallow comparison (Object.is(prev, next) or ===). If you mutate an existing object in place (state.count++), its memory reference remains identical, causing React to miss the update and fail to re-render the UI.

1. Predictable Component Re-rendering:

When a new object reference is created on every update (setState(prev => ({ ...prev, count: prev.count + 1 }))), React's reconciliation engine instantly knows the subtree needs to update.

1. Time-Travel Debugging and State History:

Libraries like Redux and React DevTools rely on state snapshots. If state is mutated in place, previous snapshots are overwritten, rendering undo/redo features and Redux DevTools time-travel debugging impossible.

1. Race Condition Prevention:

In concurrent React (Fiber architecture with time-slicing), asynchronous renders may pause and resume. Mutating state concurrently across interrupted renders leads to subtle tearing bugs and unpredictable DOM state.

The below table represent preferred and non-preferred array operations for updating the component state.

| Action | Preferred | Non-preferred |
| --------- | -------------------- | -------------------------- |
| Adding | concat, [...arr] | push, unshift |
| Removing | filter, slice | pop, shift, splice |
| Replacing | map | splice, arr[i] = someValue |
| sorting | copying to new array | reverse, sort |

If you use Immer library then you can able to use all array methods without any problem.

Technically it is possible to write nested function components but it is not suggested to write nested function definitions. Because it leads to unexpected bugs and performance issues.

Keys are primarily used for rendering list items but they are not just for list items. You can also use them React to distinguish components. By default, React uses order of the components in

There are two guidelines to be taken care while writing reducers in your code.

1. Reducers must be pure without mutating the state. That means, same input always returns the same output. These reducers run during rendering time similar to state updater functions. So these functions should not send any requests, schedule time outs and any other side effects.

1. Each action should describe a single user interaction even though there are multiple changes applied to data. For example, if you "reset" registration form which has many user input fields managed by a reducer, it is suggested to send one "reset" action instead of creating separate action for each fields. The proper ordering of actions should reflect the user interactions in the browser and it helps a lot for debugging purpose.

ReactJS is a powerful JavaScript library for building user interfaces. While it appears simple on the surface, React performs a lot of complex operations behind the scenes to efficiently update the UI. Here's an overview of how it works internally:

#### 1. Virtual DOM & Component Rendering

React doesn't manipulate the real DOM directly. Instead, it uses a Virtual DOM — a lightweight JavaScript representation of the UI.

When a component renders (e.g., <App />):

• React executes the component function (e.g., App()).
• Hooks like useState are registered and tracked in order.
• React builds a Virtual DOM tree from the returned JSX.
• This virtual DOM is a plain JS object that describes the desired UI.

This process ensures fast and efficient rendering before React decides how to update the real DOM.

#### 2. React Fiber Architecture

React’s core engine is called Fiber, introduced in React 16. Fiber is a reimplementation of the React reconciliation algorithm with the following capabilities:

• Breaks rendering work into units of work (fiber nodes).
• Enables interruptible rendering (important for responsiveness).
• Supports priority scheduling and concurrent rendering.

Each Fiber node represents a component and stores:

• The component type (function/class).
• Props, state, and effects.
• Links to parent, child, and sibling fibers.

#### 3. Reconciliation (Diffing Algorithm)

When state or props change:

• React re-executes the component to produce a new virtual DOM.
• It compares the new virtual DOM to the previous one using an efficient diffing algorithm.
• React determines the minimal set of DOM changes required.

This process is known as reconciliation.

#### 4. Commit Phase (Real DOM Updates)

Once reconciliation is done:

• React enters the commit phase.
• It applies calculated changes to the real DOM.
• It also runs side effects like useEffect or useLayoutEffect.

This is the only time React interacts directly with the browser DOM.

#### 5. Hooks and State Management

With Hooks (like useState, useEffect):

• React keeps an internal list of hooks per component.
• Hooks are identified by their order in the function.
• When state updates occur, React re-renders the component and re-runs the hooks in the same order.

#### 6. React Scheduler

React uses an internal Scheduler to control how updates are prioritized:

• Urgent tasks like clicks and inputs are processed immediately.
• Non-urgent tasks (like data fetching) can be delayed or paused.
• This improves responsiveness and allows for time slicing in Concurrent Mode.

There are notable differences between useState and useReducer hooks.

| Feature | useState | useReducer |
|-----------------------|--------------------------------------|---------------------------------------|
| State complexity | Simple (one variable or flat object) | Complex, multi-part or deeply nested |
| Update style | Direct (e.g. setState(x)) | Through actions (e.g. dispatch({})) |
| Update logic | In component | In reducer function |
| Reusability & testing | Less reusable | Highly reusable & testable |

Next.js provides two different routing systems: the Page Router (traditional) and the App Router (introduced in Next.js 13). The App Router is built on React Server Components and offers more powerful features for modern web applications.

Here are the main differences between them:

| Feature | Page Router | App Router |
|---------|-------------|------------|
| Directory | Uses pages/ directory | Uses app/ directory |
| Routing | File-based routing with files like pages/about.js | File-based routing with folders and special files like app/about/page.js |
| Components | All components are Client Components by default | All components are Server Components by default |
| Layouts | Custom _app.js and _document.js for shared layouts | Native nested layouts using layout.js files |
| Data Fetching | Uses getServerSideProps, getStaticProps, and getInitialProps | Uses async/await in Server Components with native fetch |
| Loading States | Manual implementation required | Built-in loading.js for streaming and suspense |
| Error Handling | Custom _error.js page | Built-in error.js for error boundaries at any level |
| Streaming | Limited support | Built-in support for streaming with Suspense |
| Server Actions | Not available | Native support for server-side mutations |
| Metadata | Using Head component from next/head | Native Metadata API with metadata object or generateMetadata function |
| Rendering | SSR, SSG, ISR, and CSR | SSR, SSG, ISR, CSR plus React Server Components |

Example of Page Router structure:

     pages/
     ├── index.js          // Home page (/)
     ├── about.js          // About page (/about)
     ├── _app.js           // Custom App component
     ├── _document.js      // Custom Document
     └── posts/
         └── [id].js       // Dynamic route (/posts/:id)
     

Example of App Router structure:

     app/
     ├── page.js           // Home page (/)
     ├── layout.js         // Root layout
     ├── loading.js        // Loading UI
     ├── error.js          // Error UI
     ├── about/
     │   └── page.js       // About page (/about)
     └── posts/
         └── [id]/
             └── page.js   // Dynamic route (/posts/:id)
     

Note: The App Router is recommended for new Next.js applications as it provides better performance, simpler data fetching patterns, and improved developer experience with React Server Components.

An updater function is a form of setState where you pass a function instead of a direct value. This function receives the previous state as an argument and returns the next state.

The updater function expression looks like below,

     setCount(prevCount => prevCount + 1); // Safe and predictable
     

Here, prevCount => prevCount + 1 is the updater function.

In the React community, there's often a recommendation to use updater functions when updating state that depends on its previous value. This helps prevent unexpected behaviors that can arise from working with outdated or "stale" state.

While using an updater function is a good habit, it's not always necessary. In most cases, React batches updates and ensures that the state is up-to-date at the beginning of the event handler, so you typically don’t run into stale state issues during a single synchronous event.
However, if you’re doing multiple updates to the same state variable within a single handler, using the updater form ensures that each update correctly uses the latest state value, rather than a potentially outdated one.

Example: Multiple Updates in One Handler

     function handleCount() {
        setCounter(a => a + 1);
        setCounter(a => a + 1);
        setCounter(a => a + 1);
     }
     

In this example, a => a + 1 is an updater function. React queues these updater functions and applies them sequentially, each using the most recent state value. As a result, the counter will correctly increment by 3.

In many cases, such as setting state based on user input or assigning static values, you don’t need the updater function:

     setName('Sudheer');
     

Yes, useState can take a function as an initial value, and this is a useful feature in React called lazy initialization. This function is also known as initializer function.

When you call useState(initialValue), you normally pass in a value directly:

     const [count, setCount] = useState(0);  // initial value is 0
     

But if calculating that initial value is expensive or involves logic, you can pass a function that returns the value:

        const [count, setCount] = useState(() => {
        // This function only runs once — when the component first renders
        return expensiveComputation();
        });
     

This function avoids doing heavy computation on every render. If you don't use this function form and invokes it directly, the function will run everytime the component renders and impact the performance.
For example, the below usage is not recommended.

     const [count, setCount] = useState(expensiveComputation());
     

The useState hook accepts different types of values.

• Primitives: number, string, boolean
• Arrays
• Objects
• Functions
• null or undefined

But you needs to be cautious with reference types (objects/arrays) because React compares old and new values by reference, so direct mutations won't trigger a re-render.
For example, the correct and wrong ways of state updates as shown below,

     user.name = "Sudheer"; //wrong way
     setUser(prev => ({ ...prev, name: 'Sudheer' })); //correct way
     

As per rules of React Hooks, hooks must be called unconditionally. For example, if you conditionally call it:

     if (someCondition) { 
        const [state, setState] = useState(0); 
     }
     

React will throw a runtime error because it relies on the order of Hook calls, and conditional logic breaks that order.

The useState hook is synchronous, but state updates are asynchronous. When you call useState(), it runs synchronously and returns the state variable and setter function as tuple.

     const [count, setCount] = useState(0);
     

This happens immediately during rendering.
However, the state update function (setState) is asynchronous in the sense that it doesn't update the state immediately.
React batches updates and applies them before the next render. You won’t see the updated value immediately after calling setState.

Example:

     const [count, setCount] = useState(0);
    
     function handleClick() {
       setCount(count + 1);
       console.log(count); // ❗️Still logs the old value
     }
     

The > console.log(count) prints the old value, because the update hasn’t happened yet.

To see the updated state value, you can use useEffect() hook. It runs after the component has re-rendered.  By the time useEffect runs:

• The component has been updated.
• The state contains the new value.

        import React, { useState, useEffect } from 'react';

        function Counter() {
        const [count, setCount] = useState(0);
        
        const handleClick = () => {
        setCount(count + 1);
        console.log('Clicked count (old):', count); // Old value
        };
        
        useEffect(() => {
        console.log('Updated count:', count); // New value
        }, [count]); // Only runs when `count` changes
        
        return <button onClick={handleClick}>Count: {count}</button>;
        }
        

React’s hooks, including useState, rely on some internal machinery that keeps track of state per component and per hook call during rendering. Here's a simplified explanation of the internal mechanics:

#### 1. Hook List / Linked List

• React maintains a linked list or array of "hook states" for each component.
• When a component renders, React keeps track of which hook it is currently processing via a cursor/index.
• Each call to useState() corresponds to one "slot" in this list.

#### 2. State Storage

• Each slot stores:
• The current state value.
• A queue of pending state updates.

#### 3. Initial Render

• When the component first renders, React:
• Creates a new slot for useState with the initial state (e.g., 0).
• Returns [state, updaterFunction].

#### 4. Updater Function

• The updater function (setCount) is a closure that, when called:
• Enqueues a state update to React's internal queue.
• Schedules a re-render of the component.

#### 5. Re-render and State Update

• On the next render:
• React processes all queued updates for each hook slot.
• Updates the stored state value accordingly.
• Returns the new state to the component.

#### 6. Important: Hook Order

• Hooks must be called in the same order on every render so React can match hook calls to their internal slots.
• That’s why you can’t call hooks conditionally.

The pseudocode for internal implementation of useState looks like below,

        let hookIndex = 0;
        const hooks = [];
        
        function useState(initialValue) {
            const currentIndex = hookIndex;
        
            if (!hooks[currentIndex]) {
                // First render: initialize state
                hooks[currentIndex] = {
                    state: initialValue,
                    queue: [],
                };
            }
        
            const hook = hooks[currentIndex];
        
            // Process queued updates
            hook.queue.forEach(update => {
                hook.state = update(hook.state);
            });
            hook.queue = [];
        
            // Define updater function
            function setState(action) {
                // action can be new state or function(state) => new state
                hook.queue.push(typeof action === 'function' ? action : () => action);
                scheduleRender(); // triggers React re-render
            }
        
            hookIndex++;
            return [hook.state, setState];
        }
        

The useReducer hook is a React hook used to manage complex state logic inside functional components. It is conceptually similar to Redux. i.e, Instead of directly updating state like with useState, you dispatch an action to a reducer function, and the reducer returns the new state.

The useReducer hook takes three arguments:

        const [state, dispatch] = useReducer(reducer, initialState, initFunction);
        

• reducer: A function (state, action) => newState that handles how state should change based on the action.
• initialState: The starting state.
• dispatch: A function you call to trigger an update by passing an action.

The useReducer hook is used when:

• The state is complex, such as nested structures or multiple related values.
• State updates depend on the previous state and logic.
• You want to separate state update logic from UI code to make it cleaner and testable.
• You’re managing features like:
• Forms
• Wizards / Multi-step flows
• Undo/Redo functionality
• Shopping cart logic
• Toggle & conditional UI logic

The useReducer hooks works similarly to Redux, where:

• You define a reducer function to handle state transitions.
• You dispatch actions to update the state.

Counter Example with Increment, Decrement, and Reset:

1. Reducer function:

Define a counter reducer function that takes the current state and an action object with a type, and returns a new state based on that type.

         function counterReducer(state, action) {
            switch (action.type) {
              case 'increment':
                return { count: state.count + 1 };
              case 'decrement':
                return { count: state.count - 1 };
              case 'reset':
                return { count: 0 };
              default:
                return state;
            }
          }
         

1. Using useReducer:

Invoke useReducer with above reducer function along with initial state. Thereafter, you can attach dispatch actions for respective button handlers.

      import React, { useReducer } from 'react';

        function Counter() {
          const initialState = { count: 0 };
          const [state, dispatch] = useReducer(counterReducer, initialState);

          return (
            <div style={{ textAlign: 'center' }}>
              <h2>Count: {state.count}</h2>
              <button onClick={() => dispatch({ type: 'increment' })}>Increment</button>
              <button onClick={() => dispatch({ type: 'decrement' })}>Decrement</button>
              <button onClick={() => dispatch({ type: 'reset' })}>Reset</button>
            </div>
          );
        }

      export default Counter;
      

Once the new state has been returned, React re-renders the component with the updated state.count.

Yes, you can dispatch multiple actions in a row using useReducer but not directly in one call. You'd have to call dispatch multiple times or create a composite action in your reducer that performs multiple updates based on the action type.

Example: Dispatching Multiple Actions
You can define a custom function with dispatching actions one by one.

     function handleMultipleActions(dispatch) {
       dispatch({ type: 'increment' });
       dispatch({ type: 'increment' });
       dispatch({ type: 'reset' });
     }
     

After that, you need to invoke it through event handler

     <button onClick={() => handleMultipleActions(dispatch)}>
       Run Multiple Actions
     </button>
     

Note: You can also define a custom action type If you want multiple state changes to be handled in one reducer call.

     case 'increment_twice':
       return { count: state.count + 2 };
     

Then dispatch

     dispatch({ type: 'increment_twice' });
     

The dispatch function returned by useReducer is not asynchronous — it is a synchronous function call. When you call dispatch(action), React synchronously invokes your reducer with the current state and the action, computes the new state, and schedules a re-render. However, the state variable does not update immediately within the same render cycle. The updated state is only available in the next render.

This behavior is similar to useState's setState — React batches state updates for performance optimization, meaning the component does not re-render immediately after each dispatch call. Instead, React processes all dispatched actions and re-renders once with the final state.

#### Key Points

1. dispatch is synchronous: The reducer runs immediately when dispatch is called.
2. State update is not immediate in the current render: The state variable still holds the old value until the next render.
3. React batches updates: Multiple dispatch calls within the same event handler result in a single re-render.
4. Reducer is a pure function: It computes the new state without side effects.

#### Example demonstrating that state does not update immediately

     import React, { useReducer } from 'react';

     function reducer(state, action) {
       switch (action.type) {
         case 'increment':
           return { count: state.count + 1 };
         default:
           return state;
       }
     }

     function Counter() {
       const [state, dispatch] = useReducer(reducer, { count: 0 });

       const handleClick = () => {
         dispatch({ type: 'increment' });
         console.log(state.count); // Still logs the OLD value (e.g., 0), not 1
         dispatch({ type: 'increment' });
         console.log(state.count); // Still logs the OLD value (e.g., 0), not 2
       };

       // After re-render, state.count will be 2 (both dispatches are processed)
       return (
         <div>
           <p>Count: {state.count}</p>
           <button onClick={handleClick}>Increment Twice</button>
         </div>
       );
     }
     

In the above example, even though dispatch is called twice, state.count still reflects the previous value inside the event handler. React batches both dispatches and re-renders the component once with count: 2.

#### How to read updated state after dispatch

If you need the updated value right after dispatching, you have several options:

1. Use useEffect to react to state changes:

        useEffect(() => {
          console.log('Updated count:', state.count);
        }, [state.count]);
        

1. Compute the next state manually:

        const handleClick = () => {
          const nextState = reducer(state, { type: 'increment' });
          console.log('Next state will be:', nextState.count);
          dispatch({ type: 'increment' });
        };
        

1. Use useRef to track the latest state:

        const stateRef = useRef(state);
        useEffect(() => {
          stateRef.current = state;
        }, [state]);
        

Note: This behavior is by design in React. The dispatch function itself has a stable identity (it doesn't change between re-renders), which makes it safe to omit from useEffect dependency arrays.

You need to use useLayoutEffect when your effect must run before the browser paints, such as:

• Reading layout measurements (e.g., element size, scroll position)
• Synchronously applying DOM styles to prevent visual flicker
• Animating layout or transitions
• Integrating with third-party libraries that require DOM manipulation

If there's no visual or layout dependency, prefer useEffect — it's more performance-friendly.

      useLayoutEffect(() => {
        const width = divRef.current.offsetWidth;
        if (width < 400) {
          divRef.current.style.background = 'blue'; // prevents flicker
        }
      }, []);
      

The useLayoutEffect hook does not run on the server, because there is no DOM. React issues a warning in server environments like Next.js if useLayoutEffect is used directly.

This can be mitigated using a conditional polyfill:

      const useIsomorphicLayoutEffect =
        typeof window !== 'undefined' ? useLayoutEffect : useEffect;
      

i.e, Use useIsomorphicLayoutEffect in components that render both on client and server.

Using useLayoutEffect for logic unrelated to layout or visual DOM changes (such as logging, data fetching, or analytics) is not recommended. It can lead to performance issues or even unexpected behavior.

Example: Anti-pattern

      useLayoutEffect(() => {
        console.log("Tracking analytics");
        fetch('/log-page-view');
      }, []);
      

The above usage delays the paint of the UI just to send a network request, which could (and should) be done after paint using useEffect.

The useLayoutEffect can cause layout thrashing when you repeatedly read and write to the DOM in ways that force the browser to recalculate layout multiple times per frame. This is because useLayoutEffect runs _before the browser paints_, these reflows happen _synchronously_, blocking rendering and degrading performance.

Example:

      function ThrashingComponent() {
        const ref = useRef();

        useLayoutEffect(() => {
          const height = ref.current.offsetHeight; //Read
          ref.current.style.height = height + 20 + 'px'; //Write
          const newHeight = ref.current.offsetHeight; //Read again — forces reflow
        }, []);

        return <div ref={ref}>Hello</div>;
      }
      

In the above code, each read/write cycle triggers synchronous reflows, blocking the main thread and delays UI rendering.

This issue can be avoided by batching your DOM reads and writes and prevent unnecessary reads after writes.

The useRef hook is commonly used in React to directly reference and interact with DOM elements — like focusing an input, scrolling to a section, or controlling media elements.

When you assign a ref to a DOM element using useRef, React gives you access to the underlying DOM node via the .current property of the ref object.

Example: Focus an input

        import React, { useRef } from 'react';
        
        function FocusInput() {
          const inputRef = useRef(null); // create the ref
        
          const handleFocus = () => {
            inputRef.current.focus(); // access DOM element and focus it
          };
        
          return (
            <div>
              <input type="text" ref={inputRef} />
              <button onClick={handleFocus}>Focus the input</button>
            </div>
          );
        }
        

Note: The DOM reference is only available after the component has mounted — typically accessed in useEffect or event handlers.

Yes, you can use useRef to persist values across renders in React. Unlike useState, changing .current does not cause re-renders, but the value is preserved across renders.

Example:

        function Timer() {
          const renderCount = useRef(0);
          useEffect(() => {
            renderCount.current++;
            console.log("Render count:", renderCount.current);
          });
        
          return <div>Check console for render count.</div>;
        }
        

Yes, useRef is a common pattern when you want to compare current and previous props or state without causing re-renders.

Example: Storing previous state value

        import { useEffect, useRef, useState } from 'react';
        
        function PreviousValueExample() {
          const [count, setCount] = useState(0);
          const prevCountRef = useRef();
        
          useEffect(() => {
            prevCountRef.current = count;
          }, [count]);
        
          const prevCount = prevCountRef.current;
        
          return (
            <div>
              <p>Current: {count}</p>
              <p>Previous: {prevCount}</p>
              <button onClick={() => setCount(c => c + 1)}>Increment</button>
            </div>
          );
        }
        

Yes, you can access a ref in the render method, but what you get from it depends on how you're using the ref and when in the component lifecycle you're rendering.

For example, when using ref to access a DOM node (e.g., divRef.current), it's not immediately available on the first render.

        const divRef = useRef(null);
        
        console.log(divRef.current); // ❌ null on initial render
        return <div ref={divRef}>Hello</div>;
        

The useImperativeHandler hook will be used in below cases:

• You want to expose imperative methods from a child component
• Custom input controls exposing focus, clear, or validate methods
• Modal components exposing open() and close() methods
• Scroll containers exposing scrollToTop() or scrollToBottom() methods
• You want to hide internal implementation but provide controlled external access.
• You're building reusable component libraries (e.g., inputs, modals, form controls).

React 18, released in March 2022, introduced several groundbreaking features focused on performance and user experience:

#### 1. Automatic Batching
Batch multiple state updates together (even in async code) to reduce re-renders.

     // Before React 18: Only batched in event handlers
     // After React 18: Batched everywhere
     setTimeout(() => {
       setCount(c => c + 1);
       setFlag(f => !f);
       // Only 1 re-render in React 18!
     }, 1000);
     

#### 2. Concurrent Features

• useTransition: Mark updates as non-urgent
• useDeferredValue: Defer expensive re-renders
• Suspense on Server: SSR with Suspense support

#### 3. New createRoot API

     // Old way (React 17)
     ReactDOM.render(<App />, document.getElementById('root'));

     // New way (React 18)
     import { createRoot } from 'react-dom/client';
     const root = createRoot(document.getElementById('root'));
     root.render(<App />);
     

#### 4. Streaming SSR with Suspense
Stream HTML from server and hydrate components as they arrive.

#### 5. New Hooks

• useId: Generate unique IDs for accessibility
• useSyncExternalStore: Subscribe to external stores
• useInsertionEffect: For CSS-in-JS libraries

#### 6. Strict Mode Improvements
Double-invokes effects in development to catch bugs.

React 19 (released 2024) brings major improvements for full-stack React applications:

#### 1. React Compiler (formerly React Forget)
Automatic memoization - no more manual useMemo, useCallback, or React.memo needed!

     // Before: Manual optimization
     const memoizedValue = useMemo(() => expensiveCalc(a, b), [a, b]);
     
     // React 19: Compiler does it automatically
     const value = expensiveCalc(a, b); // Automatically optimized!
     

#### 2. Server Actions
Call server functions directly from components:

     async function createPost(formData) {
       'use server'
       const post = await db.posts.create({
         title: formData.get('title')
       });
       revalidatePath('/posts');
       return post;
     }

     function NewPost() {
       return (
         <form action={createPost}>
           <input name="title" />
           <button type="submit">Create</button>
         </form>
       );
     }
     

#### 3. Actions & Form Actions
Automatic handling of pending states, errors, and optimistic updates:

     function Form() {
       const [state, formAction] = useFormState(serverAction, initialState);
       const { pending } = useFormStatus();
       
       return (
         <form action={formAction}>
           <input disabled={pending} />
           <button disabled={pending}>
             {pending ? 'Submitting...' : 'Submit'}
           </button>
         </form>
       );
     }
     

#### 4. use() Hook
Read resources (Promises, Context) inside render:

     function User({ userPromise }) {
       const user = use(userPromise); // Suspends until resolved
       return <div>{user.name}</div>;
     }
     

#### 5. useOptimistic Hook
Implement optimistic UI updates:

     function TodoList({ todos }) {
       const [optimisticTodos, addOptimisticTodo] = useOptimistic(
         todos,
         (state, newTodo) => [...state, { ...newTodo, pending: true }]
       );

       async function createTodo(title) {
         addOptimisticTodo({ id: Date.now(), title });
         await saveTodo(title);
       }

       return optimisticTodos.map(todo => (
         <Todo key={todo.id} {...todo} />
       ));
     }
     

#### 6. Document Metadata
Built-in support for <title>, <meta>, etc.:

     function BlogPost({ post }) {
       return (
         <>
           <title>{post.title}</title>
           <meta name="description" content={post.excerpt} />
           <article>{post.content}</article>
         </>
       );
     }
     

#### 7. Asset Loading APIs
Preload resources for better performance:

     import { preload, preinit } from 'react-dom';

     preload('/font.woff2', { as: 'font' });
     preinit('/script.js', { as: 'script' });
     

Server Actions allow you to call server-side functions directly from client components without writing API endpoints.

#### Basic Server Action

     // app/actions.js
     'use server'

     export async function createPost(formData) {
       const title = formData.get('title');
       const content = formData.get('content');
       
       const post = await db.posts.create({
         title,
         content,
         userId: await getCurrentUser()
       });
       
       revalidatePath('/posts');
       redirect(`/posts/${post.id}`);
     }
     

#### Using in Forms

     // app/new-post.jsx
     import { createPost } from './actions';

     export default function NewPost() {
       return (
         <form action={createPost}>
           <input name="title" required />
           <textarea name="content" required />
           <button type="submit">Create Post</button>
         </form>
       );
     }
     

#### With useFormState for Loading States

     'use client'
     import { useFormState } from 'react-dom';
     import { createPost } from './actions';

     export default function NewPost() {
       const [state, formAction] = useFormState(createPost, { message: '' });
       
       return (
         <form action={formAction}>
           <input name="title" required />
           <textarea name="content" required />
           <button type="submit">Create Post</button>
           {state.message && <p>{state.message}</p>}
         </form>
       );
     }
     

#### Progressive Enhancement
Server Actions work even if JavaScript is disabled!

     // This form works without JavaScript
     <form action={serverAction}>
       <input name="email" type="email" />
       <button>Subscribe</button>
     </form>
     

#### Security

• Automatically CSRF protected
• Always run on server (never exposed to client)
• Can use server-only packages safely

The React Compiler (formerly known as React Forget) automatically optimizes your components by adding memoization where needed - eliminating the need for manual useMemo, useCallback, and React.memo.

#### Before React Compiler

     function TodoList({ todos, filter }) {
       // Manual optimization needed
       const filteredTodos = useMemo(() => {
         return todos.filter(todo => todo.status === filter);
       }, [todos, filter]);

       const handleToggle = useCallback((id) => {
         toggleTodo(id);
       }, [toggleTodo]);

       return (
         <div>
           {filteredTodos.map(todo => (
             <TodoItem 
               key={todo.id} 
               todo={todo} 
               onToggle={handleToggle} 
             />
           ))}
         </div>
       );
     }

     // Need to wrap in React.memo
     export default React.memo(TodoList);
     

#### With React Compiler

     function TodoList({ todos, filter }) {
       // Compiler automatically optimizes this!
       const filteredTodos = todos.filter(todo => todo.status === filter);

       const handleToggle = (id) => {
         toggleTodo(id);
       };

       return (
         <div>
           {filteredTodos.map(todo => (
             <TodoItem 
               key={todo.id} 
               todo={todo} 
               onToggle={handleToggle} 
             />
           ))}
         </div>
       );
     }

     // No React.memo needed!
     export default TodoList;
     

#### How It Works

1. Analyzes code during build time
2. Identifies expensive calculations and renders
3. Automatically inserts memoization where beneficial
4. Preserves React semantics - your code still behaves correctly

#### Benefits

• ✅ Simpler code - no manual optimization
• ✅ Better performance by default
• ✅ Fewer bugs from incorrect dependencies
• ✅ Easier to maintain and read
• ✅ Works with existing code

#### Enabling React Compiler

     // next.config.js (Next.js)
     module.exports = {
       experimental: {
         reactCompiler: true
       }
     }

     // vite.config.js (Vite)
     import { defineConfig } from 'vite'
     import react from '@vitejs/plugin-react'

     export default defineConfig({
       plugins: [
         react({
           babel: {
             plugins: [['babel-plugin-react-compiler']]
           }
         })
       ]
     })
     

#### When to Still Use Manual Optimization

     // For external libraries without Compiler support
     import { expensiveLibFunction } from 'old-library';

     function MyComponent() {
       // May still need manual memoization here
       const result = useMemo(() => expensiveLibFunction(), []);
       return <div>{result}</div>;
     }
     

#### Compatibility

• Works with React 18.3+ and React 19
• Compatible with TypeScript
• Works with all React hooks
• Supports Server Components and Client Components

Streaming SSR sends HTML to the browser in chunks as it's generated, rather than waiting for the entire page. React 18+ dramatically improves this with Suspense integration.

#### Traditional SSR (Pre-React 18)

     Server: Wait for ALL data → Generate ALL HTML → Send to client
     Client: Receive HTML → Download ALL JS → Hydrate ALL components
     Problem: Slow components block entire page!
     

#### Streaming SSR (React 18+)

     Server: Send HTML as it's ready, wrap slow parts in <Suspense>
     Client: Render immediately, hydrate progressively
     Benefit: User sees content faster!
     

#### Basic Example

     import { Suspense } from 'react';

     export default function Page() {
       return (
         <html>
           <body>
             {/* Sent immediately */}
             <header>
               <h1>My App</h1>
             </header>

             {/* Sent immediately with fallback */}
             <Suspense fallback={<div>Loading comments...</div>}>
               <Comments /> {/* Streamed when ready */}
             </Suspense>

             {/* Also streamed separately */}
             <Suspense fallback={<div>Loading recommendations...</div>}>
               <Recommendations /> {/* Streamed when ready */}
             </Suspense>

             <footer>© 2026</footer>
           </body>
         </html>
       );
     }
     

#### Server Component with Data Fetching

     // This is a Server Component (async!)
     async function Comments() {
       const comments = await db.comments.findMany();
       
       return (
         <ul>
           {comments.map(comment => (
             <li key={comment.id}>{comment.text}</li>
           ))}
         </ul>
       );
     }
     

#### How It Works

1. Server starts sending HTML immediately
2. When it hits <Suspense>, it sends the fallback
3. Continues streaming rest of the page
4. When data is ready, sends the actual component
5. Client replaces fallback with real content
6. Hydration happens independently per component

#### Selective Hydration

     function App() {
       return (
         <div>
           <header>Header</header> {/* Hydrates first */}
           
           <Suspense fallback={<Spinner />}>
             <HeavyComponent /> {/* Hydrates when user interacts */}
           </Suspense>
           
           <Suspense fallback={<Spinner />}>
             <Comments /> {/* Hydrates independently */}
           </Suspense>
         </div>
       );
     }
     

#### Benefits

• Faster TTFB (Time to First Byte): User sees content sooner
• Better UX: Progressive loading instead of blank screen
• Prioritized hydration: Interactive elements hydrate first
• Resilient: Slow components don't block fast ones

#### Next.js App Router Example

     // app/page.tsx
     import { Suspense } from 'react';
     import ProductList from './ProductList';
     import Reviews from './Reviews';

     export default function ProductPage() {
       return (
         <div>
           <h1>Product Page</h1>
           
           {/* Streams product list first */}
           <Suspense fallback={<ProductSkeleton />}>
             <ProductList />
           </Suspense>

           {/* Reviews stream separately */}
           <Suspense fallback={<ReviewSkeleton />}>
             <Reviews />
           </Suspense>
         </div>
       );
     }

     // These are async Server Components
     async function ProductList() {
       const products = await fetchProducts(); // Doesn't block Reviews
       return <div>{/* render products */}</div>;
     }

     async function Reviews() {
       const reviews = await fetchReviews(); // Doesn't block ProductList
       return <div>{/* render reviews */}</div>;
     }
     

#### Key Requirements

• Use React 18+ with createRoot and hydrateRoot
• Wrap slow components in <Suspense>
• Use frameworks supporting streaming (Next.js, Remix, etc.)
• Server must support streaming responses

renderToPipeableStream is the React 18+ Node.js streaming SSR API from react-dom/server. It streams HTML in chunks, so users can see content earlier and React can coordinate Suspense boundaries during server rendering.

renderToString is the older API that generates the entire HTML string first and sends it only after rendering is fully complete.

#### Key Differences

| Aspect | renderToString | renderToPipeableStream |
| --- | --- | --- |
| Rendering model | All-at-once | Streaming/chunked |
| Suspense on server | Limited fallback behavior | First-class Suspense streaming |
| Time to first byte | Usually slower for large pages | Faster, can flush shell early |
| UX for slow data | Entire page waits | Progressive reveal with fallbacks |
| Best for | Small/simple SSR pages | Modern SSR apps with async data |

#### renderToString example

     import express from "express";
     import { renderToString } from "react-dom/server";
     import App from "./App";

     const app = express();

     app.get("/", (req, res) => {
       const html = renderToString(<App />);
       res.send(`<!doctype html><html><body><div id="root">${html}</div></body></html>`);
     });
     

#### renderToPipeableStream example

     import express from "express";
     import { renderToPipeableStream } from "react-dom/server";
     import App from "./App";

     const app = express();

     app.get("/", (req, res) => {
       const { pipe } = renderToPipeableStream(<App />, {
         onShellReady() {
           res.setHeader("Content-Type", "text/html");
           res.write("<!doctype html><html><body><div id=\"root\">");
           pipe(res);
         },
         onAllReady() {
           res.end("</div></body></html>");
         },
         onError(error) {
           console.error(error);
         },
       });
     });
     

In practice, prefer renderToPipeableStream for React 18+ SSR because it improves perceived performance and works naturally with Suspense-driven progressive loading.

Redux Toolkit (RTK) is the official, opinionated way to write Redux logic today. It's built on top of plain Redux and re-exports its APIs, but wraps them with utilities that remove most of the boilerplate and footguns of hand-written Redux.

| Problem with plain Redux | How Redux Toolkit fixes it |
| --- | --- |
| Verbose store setup (combineReducers, manual middleware wiring) | configureStore() sets up the store, redux-thunk, and Redux DevTools automatically |
| Hand-written action types/creators and switch-based reducers | createSlice() generates action types, action creators, and a reducer from a single object |
| Reducers must return new state immutably (easy to mutate by mistake) | Uses Immer internally, so you can write "mutating" logic that's safely converted to immutable updates |
| Async logic needs extra middleware setup (thunk/saga boilerplate) | createAsyncThunk() standardizes async action creators with pending/fulfilled/rejected states |
| Repeated normalization code for lists/entities | createEntityAdapter() provides prebuilt reducers/selectors for normalized state |
| No built-in data-fetching/caching layer | RTK Query (built on top of RTK) handles caching, refetching, and loading/error states for API calls |

#### Example: plain Redux vs. Redux Toolkit

Plain Redux:

     const INCREMENT = "counter/increment";
     const increment = () => ({ type: INCREMENT });

     function counterReducer(state = { value: 0 }, action) {
       switch (action.type) {
         case INCREMENT:
           return { ...state, value: state.value + 1 }; // must copy manually
         default:
           return state;
       }
     }
     

Redux Toolkit:

     import { createSlice, configureStore } from "@reduxjs/toolkit";

     const counterSlice = createSlice({
       name: "counter",
       initialState: { value: 0 },
       reducers: {
         increment: (state) => {
           state.value += 1; // "mutation" is safe — Immer produces new state
         },
       },
     });

     export const { increment } = counterSlice.actions;

     const store = configureStore({ reducer: { counter: counterSlice.reducer } });
     

In short, Redux Toolkit is recommended because it enforces best practices by default (immutability, DevTools, sensible middleware), drastically cuts boilerplate via createSlice/createAsyncThunk, and is what the official Redux docs now recommend for any new Redux code.

Client state is data that lives entirely in the browser and is owned by the UI — it doesn't need to be synchronized with a backend. Server state is data that actually lives on a remote server/database; the client only holds a cached copy of it, which can go stale at any time.

| Aspect | Client state | Server state |
| --- | --- | --- |
| Ownership | Owned entirely by the UI | Owned by the backend/database; UI only has a cached copy |
| Examples | Form input values, modal open/closed, selected tab, theme, filters before submit | Fetched user profile, product list, comments, any REST/GraphQL response |
| Persistence | Usually reset on refresh (unless explicitly persisted) | Persists on the server regardless of client refreshes |
| Staleness | Never "stale" — it's the source of truth itself | Can become stale/out of date; needs refetching, caching, and invalidation |
| Sharing | Typically local to one component/feature | Often shared across many components/screens |
| Async concerns | None — synchronous updates via setState/reducers | Requires handling loading, error, retries, pagination, race conditions |
| Typical tools | useState, useReducer, Context, Redux/Zustand | React Query / TanStack Query, RTK Query, SWR, Apollo Client |

#### Why the distinction matters

Treating server state like client state (e.g., dumping a fetched API response into useState or a Redux slice and manually managing loading/error flags) reinvents caching, deduplication, background refetching, and invalidation by hand. Dedicated server-state libraries solve these problems out of the box:

     // Server state handled manually — lots of boilerplate to get right
     function Users() {
       const [users, setUsers] = useState([]);
       const [loading, setLoading] = useState(true);
       const [error, setError] = useState(null);

       useEffect(() => {
         fetch("/api/users")
           .then((res) => res.json())
           .then(setUsers)
           .catch(setError)
           .finally(() => setLoading(false));
       }, []);
       // ...
     }
     

     // Server state via React Query — caching, refetching, and status handled for you
     function Users() {
       const { data: users, isLoading, error } = useQuery({
         queryKey: ["users"],
         queryFn: () => fetch("/api/users").then((res) => res.json()),
       });
       // ...
     }
     

In short, use local state/Context/Redux for client state (UI-only concerns), and a data-fetching library like React Query, RTK Query, or SWR for server state (anything that mirrors data owned by a backend).

By default, useSelector re-renders a component whenever the selected value's reference changes between renders (it uses strict === comparison). Most unnecessary re-renders come from selectors that return a *new* object/array/function on every call, so React-Redux thinks the data changed even though it didn't. Ways to prevent this:

1. Select the smallest piece of state you need instead of the whole slice:

         // BAD: re-renders whenever ANY field in `user` changes
         const user = useSelector((state) => state.user);

         // GOOD: only re-renders when `name` changes
         const name = useSelector((state) => state.user.name);
         

1. Memoize derived/computed selectors with createSelector (see next question) so a new array/object is only created when its actual inputs change.
2. Use a custom equality function (e.g., shallowEqual from react-redux) when you must select multiple fields as an object:

         import { shallowEqual, useSelector } from "react-redux";

         const { name, email } = useSelector(
           (state) => ({ name: state.user.name, email: state.user.email }),
           shallowEqual
         );
         

1. Split one large connected component into smaller ones, each with its own narrow useSelector, so a state change only re-renders the component that actually cares about it.
2. Wrap presentational children in React.memo so they skip re-rendering when their own props haven't changed, even if their parent re-renders.

createSelector (from Reselect, which Redux Toolkit re-exports) creates memoized selectors. It takes one or more "input selectors" and a "result function", and only recomputes the result when the *outputs* of the input selectors change — otherwise it returns the previously cached value.

     import { createSelector } from "@reduxjs/toolkit";

     const selectItems = (state) => state.cart.items;

     // Recomputes total only when `items` reference actually changes
     const selectTotal = createSelector([selectItems], (items) =>
       items.reduce((sum, item) => sum + item.price * item.qty, 0)
     );

     function Cart() {
       const total = useSelector(selectTotal);
       // ...
     }
     

Without memoization, computing total inline inside useSelector would return a brand-new number/object on every dispatch, which is fine for primitives but becomes a real problem when deriving arrays/objects (e.g., filtering or sorting a list), since a new reference on every render defeats React.memo/useSelector's equality check and causes unnecessary re-renders.

Normalized state stores data the way a relational database would: each type of entity is kept in a lookup table keyed by ID, instead of being nested/duplicated inside arrays. Redux Toolkit's createEntityAdapter generates this shape for you.

     // Un-normalized — posts are duplicated wherever they appear
     {
       posts: [
         { id: 1, title: "Post 1", author: { id: 1, name: "Sudheer" } },
         { id: 2, title: "Post 2", author: { id: 1, name: "Sudheer" } },
       ];
     }

     // Normalized — each entity stored once, referenced by id
     {
       posts: {
         ids: [1, 2],
         entities: {
           1: { id: 1, title: "Post 1", authorId: 1 },
           2: { id: 2, title: "Post 2", authorId: 1 },
         },
       },
       users: {
         ids: [1],
         entities: { 1: { id: 1, name: "Sudheer" } },
       },
     }
     

     import { createEntityAdapter, createSlice } from "@reduxjs/toolkit";

     const postsAdapter = createEntityAdapter();

     const postsSlice = createSlice({
       name: "posts",
       initialState: postsAdapter.getInitialState(),
       reducers: {
         postAdded: postsAdapter.addOne,
         postUpdated: postsAdapter.updateOne,
       },
     });
     

1. O(1) lookups by ID instead of scanning/find()-ing through an array every time a component needs one item.
2. Updates are localized: changing one entity only replaces that entity's entry in the entities map, so components subscribed to *other* entities don't see a changed reference and don't re-render.
3. No duplicate data to keep in sync: with nested/duplicated data, updating a single author's name might require updating it in every post that embeds it; normalized state updates it once.
4. Cheaper equality checks: since each entity is a small, independent object, memoized selectors (createSelector) and useSelector reference comparisons stay cheap and precise instead of comparing/deep-cloning large nested trees.
5. Simpler reducers: adding/removing/updating one item is a straightforward map operation, rather than mapping/filtering over deeply nested arrays.

Middleware provides a way to extend Redux with custom logic that runs between dispatching an action and the moment it reaches the reducer. It's the standard extension point for side effects (logging, crash reporting, async calls, analytics) since reducers themselves must stay pure and synchronous.

     import { configureStore } from "@reduxjs/toolkit";

     const logger = (store) => (next) => (action) => {
       console.log("dispatching", action);
       const result = next(action);
       console.log("next state", store.getState());
       return result;
     };

     const store = configureStore({
       reducer: rootReducer,
       middleware: (getDefaultMiddleware) => getDefaultMiddleware().concat(logger),
     });
     

Common middleware includes redux-thunk (dispatch functions for async logic), redux-saga (generator-based side effects), redux-logger (action/state logging), and RTK's built-in serializable/immutable-state check middleware.

Middleware is written as three nested (curried) functions:

     const exampleMiddleware = (store) => (next) => (action) => {
       // store  => { getState, dispatch }
       // next   => calls the next middleware in the chain (or the reducer if last)
       // action => the action currently being dispatched
       return next(action);
     };
     

• store gives access to getState() and dispatch().
• next passes the action along to the next middleware (or the root reducer, if this is the last middleware).
• action is the action object being processed.

Middleware must call next(action) (or dispatch(...) for a different action) to keep the chain moving — forgetting to call next silently swallows every action.

1. store.dispatch(action) is called with a plain action object ({ type, payload }).
2. The action passes through the middleware chain in order (e.g., logger → thunk → your custom middleware); async/thunk middleware may intercept it here (e.g., a thunk function is invoked instead of being forwarded, or a Promise/generator is handled) or asynchronous side-effects can trigger further dispatches later.
3. Once every middleware calls next(action), the action reaches the root reducer.
4. The root reducer (usually built with combineReducers or RTK's configureStore) forwards the action to every slice reducer, each returning either its unchanged state or a new state object.
5. The store replaces its internal state tree with the combined result and marks the update complete.
6. The store notifies all subscribers (React-Redux's Provider/useSelector internals) that state changed.
7. Each connected component's selector is re-run and compared (via === or a custom equality function) against its previous result; components only re-render if their selected slice actually changed.

The recommended approach (and what Redux Toolkit is designed around) is a "feature folder" / "ducks" structure rather than splitting by technical type (actions/, reducers/, constants/):

     src/
       app/
         store.js            # configureStore() + root reducer wiring
       features/
         posts/
           postsSlice.js     # createSlice: reducer + actions + thunks
           postsSelectors.js # createSelector-based selectors
           PostList.jsx       # components using useSelector/useDispatch
         users/
           usersSlice.js
           usersSelectors.js
     

Guidelines:

1. Colocate a feature's slice, selectors, thunks, and components in one folder instead of scattering related code across parallel actions/reducers/constants folders.
2. One slice per domain concept (posts, users, cart), created with createSlice, combined once in store.js.
3. Keep components decoupled from the store shape by reading data only through selectors, never state.posts.entities directly in a component.
4. Normalize related/nested data (see Q336) so features can reference each other by ID instead of duplicating data.
5. Use RTK Query (or a dedicated API slice) for server state, and keep hand-written slices for genuine client/app state.

Since each createSlice only manages its own piece of state, a few patterns let one slice react to actions or data from another:

1. extraReducers: listen for another slice's action type without importing its reducer.

         const notificationsSlice = createSlice({
           name: "notifications",
           initialState: [],
           reducers: {},
           extraReducers: (builder) => {
             builder.addCase(postsSlice.actions.postAdded, (state, action) => {
               state.push({ message: `New post: ${action.payload.title}` });
             });
           },
         });
         

1. Thunks that touch multiple slices: a single createAsyncThunk/thunk can dispatch actions belonging to several slices, or read getState() to combine data before deciding what to dispatch.
2. Selectors that combine slices: a createSelector can take input selectors from different slices (e.g., posts + users) and join them together for the UI, without the slices knowing about each other.
3. Listener middleware / sagas: createListenerMiddleware (RTK) or redux-saga's takeEvery can watch for an action dispatched by one slice and, in response, dispatch actions belonging to a completely different slice.

In all cases, slices stay decoupled — they communicate only through dispatched actions or shared selectors, never by directly importing and mutating each other's reducer state.

Redux (and Redux Toolkit's default middleware) expects state and actions to contain only plain, serializable data: strings, numbers, booleans, null, plain objects, and arrays. Values like class instances, Promises, functions, Map/Set, Symbol, or DOM elements are not serializable and shouldn't be put in the store or dispatched inside an action.

     // BAD: Date and a function are not serializable
     dispatch({
       type: "user/loaded",
       payload: { createdAt: new Date(), onDone: () => {} },
     });

     // GOOD: store a serializable representation instead
     dispatch({
       type: "user/loaded",
       payload: { createdAt: new Date().toISOString() },
     });
     

This matters because:

• Redux DevTools rely on serializing actions/state for time-travel debugging and persisting/replaying action logs.
• redux-persist needs to serialize state to localStorage/AsyncStorage.
• Non-serializable values can hide subtle bugs (e.g., mutable class instances bypassing Redux's change-detection).

Redux Toolkit's configureStore includes a serializableCheck middleware (dev-only) that warns in the console whenever a non-serializable value is dispatched or stored.

A race condition happens when multiple async requests are in flight and they resolve out of order — e.g., a user types quickly in a search box, an older request for "re" resolves *after* the newer request for "react", overwriting the correct results with stale ones.

1. createAsyncThunk + request tracking: store the latest requestId and ignore results from stale requests.

         const fetchResults = createAsyncThunk("search/fetch", async (query) => {
           const res = await fetch(`/api/search?q=${query}`);
           return res.json();
         });

         builder.addCase(fetchResults.fulfilled, (state, action) => {
           // Only apply the result if it belongs to the most recent request
           if (action.meta.requestId === state.currentRequestId) {
             state.results = action.payload;
           }
         });
         

1. Abort stale requests using AbortController, which createAsyncThunk supports via thunkAPI.signal — cancel the previous request when a new one starts.
2. Debounce/throttle the trigger (e.g., debounce keystrokes) so fewer overlapping requests are made in the first place.
3. Use RTK Query, which automatically de-duplicates in-flight requests, cancels/ignores stale ones, and keeps the cache consistent without manual bookkeeping.

redux-persist is the standard library for persisting and rehydrating a Redux store across page reloads/app restarts. It wraps your root reducer so selected slices are automatically saved to storage (e.g., localStorage on web, AsyncStorage on React Native) and restored on startup.

     import { combineReducers } from "redux";
     import { configureStore } from "@reduxjs/toolkit";
     import { persistStore, persistReducer } from "redux-persist";
     import storage from "redux-persist/lib/storage"; // localStorage

     const rootReducer = combineReducers({ user: userReducer, cart: cartReducer });

     const persistConfig = {
       key: "root",
       storage,
       whitelist: ["cart"], // only persist the `cart` slice
     };

     const persistedReducer = persistReducer(persistConfig, rootReducer);

     export const store = configureStore({
       reducer: persistedReducer,
       middleware: (getDefaultMiddleware) =>
         getDefaultMiddleware({ serializableCheck: false }),
     });

     export const persistor = persistStore(store);
     

     // index.js
     import { PersistGate } from "redux-persist/integration/react";

     <Provider store={store}>
       <PersistGate loading={null} persistor={persistor}>
         <App />
       </PersistGate>
     </Provider>;
     

Key points: use whitelist/blacklist to persist only the slices that need it (e.g., auth token, cart), disable serializableCheck for the persistence actions RTK flags by default, and wrap the app in <PersistGate> so rendering waits until the persisted state has been rehydrated.

A practical, step-by-step approach:

1. Confirm it's actually Redux: use the React DevTools Profiler to see which components re-render and how often, before assuming Redux is the culprit.
2. Inspect actions/state with Redux DevTools: check how frequently actions are dispatched and how large/deeply nested the state diffs are on each action.
3. Look for unstable selector results: a selector that returns a new object/array/function on every call (e.g., state => state.items.filter(...)) breaks useSelector's reference equality check and forces re-renders even when nothing meaningful changed. Wrap it in createSelector (Q335).
4. Check for over-broad useSelector calls: selecting an entire slice (state.user) instead of the specific field needed (state.user.name) causes re-renders on unrelated field changes (Q334).
5. Verify normalization: deeply nested/duplicated state (Q336) often causes broad reference changes on every update; normalizing narrows down what actually changes.
6. Check for missing memoization on connected children: wrap presentational components in React.memo so a parent re-render doesn't cascade if their own props are unchanged.
7. Use the why-did-you-render library (or React DevTools' "highlight updates") to visually confirm exactly which components re-render and why.
8. Measure, don't guess: use the Profiler's flame chart/ranked view to confirm the fix actually reduced render counts/time before moving on.

A common mistake is storing computed/derived values in Redux state itself, which then has to be kept in sync manually every time the source data changes. The rule of thumb: store the minimal raw data, and compute everything else with selectors.

| Store in Redux (source of truth) | Derive with selectors (computed) |
| --- | --- |
| Raw entities fetched from the server (items, users) | Filtered/sorted/paginated views of that list |
| Simple flags representing real state (isLoggedIn, selectedId) | Aggregates like totals, counts, averages |
| IDs of selected/active items | The full object looked up by that ID |
| Form field values | Validation results / "is form valid" flags |

     // BAD: `total` is derived data stored redundantly in state
     // Now every reducer that touches `items` must also recompute `total`
     { items: [...], total: 42 }

     // GOOD: store only `items`; compute `total` with a memoized selector
     const selectItems = (state) => state.cart.items;
     const selectTotal = createSelector([selectItems], (items) =>
       items.reduce((sum, i) => sum + i.price * i.qty, 0)
     );
     

Storing only raw data and deriving the rest avoids state getting out of sync, removes the need for extra reducer logic to keep computed fields updated, and keeps re-renders scoped to components that actually depend on the derived value (thanks to selector memoization).

No — not by default. Server data (see [client state vs server state](#what-is-the-difference-between-client-state-and-server-state)) has different needs than plain client state: caching, background refetching, deduplication of in-flight requests, invalidation, pagination, and loading/error tracking. Hand-rolling all of that inside Redux slices reinvents a data-fetching layer.

Guidance:

• Use RTK Query (or React Query/SWR) for server data — these libraries already integrate with the Redux store (RTK Query lives inside your Redux store) and handle caching/invalidation/refetching for you.
• Use plain Redux slices for genuine client state: UI state, auth/session flags, user preferences, multi-step form state, feature toggles — anything that isn't just a mirror of server data.
• Avoid duplicating server data into a separate hand-written slice "just in case" — it creates two sources of truth that can drift out of sync (e.g., a users slice updated manually alongside an RTK Query cache for the same endpoint).
• It's fine to combine both: e.g., store the *currently selected* item's ID in a normal slice, while the actual item data comes from an RTK Query/React Query cache keyed by that ID.

In short, Redux (or any store) is best reserved for state your UI truly owns; let a dedicated data-fetching layer own anything that mirrors the server.

All three are drop-in <Router> implementations from react-router-dom (see [What are the <Router> components of React Router v6?](#what-are-the-router-components-of-react-router-v6)) that wrap a different history implementation, so your app's routes work the same way — only how the URL is stored/read differs.

| Aspect | <BrowserRouter> | <HashRouter> | <MemoryRouter> |
| --- | --- | --- | --- |
| URL format | Clean URLs using the HTML5 History API (/about) | Uses a URL hash fragment (/#/about) | No URL at all — history is kept purely in memory |
| Where history lives | Browser's window.history | Browser's window.location.hash | An in-memory array/stack, not tied to the browser |
| Server requirement | Server must be configured to return index.html for every route (fallback routing), otherwise a hard refresh on /about 404s | None — the part after # is never sent to the server, so any static server works with zero config | Not applicable — there's no browser/server involved |
| Typical use case | Standard web apps deployed behind a server/CDN you control (or configure) for SPA fallback | Static hosting with no server-side routing support (e.g., plain GitHub Pages, legacy browsers) | Unit/integration tests, Storybook, React Native, or any non-DOM environment |
| Back/forward buttons | Fully supported via native browser history | Supported, but the hash shows up in the URL/bookmark | Not applicable — no visible URL to bookmark/share |
| Bookmarking/sharing links | Works naturally | Works, but URLs look less clean (example.com/#/about) | N/A — not meant to be visible to a user |

     import { BrowserRouter } from "react-router-dom";
     // Standard choice for most deployed web apps
     <BrowserRouter>
       <App />
     </BrowserRouter>;
     

     import { HashRouter } from "react-router-dom";
     // Good when you can't configure server-side rewrites
     <HashRouter>
       <App />
     </HashRouter>;
     

     import { MemoryRouter } from "react-router-dom";
     // Great for tests — you can even seed the starting route
     render(
       <MemoryRouter initialEntries={["/dashboard"]}>
         <App />
       </MemoryRouter>
     );
     

In short: use BrowserRouter for real, deployed apps with clean URLs (as long as your server supports SPA fallback), HashRouter when you can't control server routing (plain static hosting), and MemoryRouter for tests or non-browser environments where there's no real URL bar to sync with.

A dynamic route is a route path that contains a placeholder segment (a *route parameter*) instead of a fixed, hard-coded value, so a single route definition can match many different URLs. Route parameters are the named placeholder segments (prefixed with : in React Router) whose actual values are extracted from the matched URL at runtime.

     import { BrowserRouter, Routes, Route, useParams } from "react-router-dom";

     function UserProfile() {
       // useParams() returns an object of all dynamic segments matched for this route
       const { userId } = useParams();
       return <div>Profile for user: {userId}</div>;
     }

     function App() {
       return (
         <BrowserRouter>
           <Routes>
             {/* :userId is a route parameter — matches /users/1, /users/abc, etc. */}
             <Route path="/users/:userId" element={<UserProfile />} />
           </Routes>
         </BrowserRouter>
       );
     }
     

A route can also define multiple parameters and an optional catch-all (splat) parameter:

     // Multiple params: /posts/2024/react-hooks-guide
     <Route path="/posts/:year/:slug" element={<Post />} />

     // Optional segment (v6.4+): matches /shop and /shop/electronics
     <Route path="/shop/:category?" element={<Shop />} />

     // Splat/wildcard param: matches any depth, e.g. /docs/a/b/c
     <Route path="/docs/*" element={<Docs />} />
     

     function Post() {
       const { year, slug } = useParams();
       // year -> "2024", slug -> "react-hooks-guide"
       return <h1>{slug} ({year})</h1>;
     }