Vue.js Interview Questions and Answers

You can use event handlers in vue similar to plain javascript. The method calls also support the special $event
variable.

<button v-on:click="show('Welcome to VueJS world', $event)">
  Submit
</button>

methods: {
  show: function (message, event) {
    // now we have access to the native event
    if (event) event.preventDefault()
    console.log(message);
  }
}

****

Normally, javascript provides event.preventDefault() or event.stopPropagation() inside event handlers. You can
use methods provided by vue, but these methods are meant for data logic instead of dealing with DOM events. Vue
provides below event modifiers for v-on and these modifiers are directive postfixes denoted by a dot.

1. .stop
2. .prevent
3. .capture
4. .self
5. .once
6. .passive

Let's take an example of stop modifier,

<!-- the click event's propagation will be stopped -->
<a v-on:click.stop="methodCall"></a>

You can also chain modifiers as below,

<!-- modifiers can be chained -->
<a v-on:click.stop.prevent="doThat"></a>

****

Vue supports key modifiers on v-on for handling keyboard events. Let's take an example of keyup event with enter
keycode.

<!-- only call `vm.show()` when the `keyCode` is 13 -->
<input v-on:keyup.13="show">

Remembering all the key codes is really difficult. It supports the full list of key codes aliases

1. .enter
2. .tab
3. .delete (captures both "Delete" and "Backspace" keys)
4. .esc
5. .space
6. .up
7. .down
8. .left
9. .right

Now the above keyup code snippet can be written with aliases as follows,

<input v-on:keyup.enter="submit" />
<!-- OR with shorthand notation -->
<input @keyup.enter="submit" />

Note: The use of keyCode events is deprecated and may not be supported in new browsers.

****

You can define custom key modifier aliases via the global config.keyCodes. There are few guidelines for the
properties

1. You can't use camelCase. Instead you can use kebab-case with double quotation marks
2. You can define multiple values in an array format

Vue.config.keyCodes = {
  f1: 112,
  "media-play-pause": 179,
  down: [40, 87]
}

****

Vue supports below modifiers to trigger mouse or keyboard event listeners when the corresponding key is pressed,

1. .ctrl
2. .alt
3. .shift
4. .meta

Lets take an example of control modifier with click event,

<!-- Ctrl + Click -->
<div @click.ctrl="doSomething">Do something</div>

****

Vue supports below mouse button modifiers

1. .left
2. .right
3. .middle

For example, the usage of .right modifier as below

 <button
   v-if="button === 'right'"
   v-on:mousedown.right="increment"
   v-on:mousedown.left="decrement"
 />

****

You can use the v-model directive to create two-way data bindings on form input, textarea, and select elements.

Lets take an example of it using input component,

<input v-model="message" placeholder="Enter input here">
<p>The message is: {{ message }}</p>

Remember, v-model will ignore the initial value, checked or selected attributes found on any form elements.
So it always use the Vue instance data as the source of truth.

****

There are three modifiers supported for v-model directive.

1. lazy: By default, v-model syncs the input with the data after each input event. You can add the lazy
modifier to instead sync after change events.

<!-- synced after "change" instead of "input" -->
<input v-model.lazy="msg" >

2. number: If you want user input to be automatically typecast as a number, you can add the number modifier to
your v-model. Even with type="number", the value of HTML input elements always returns a string. So, this typecast
modifier is required.

<input v-model.number="age" type="number">

3. trim: If you want whitespace from user input to be trimmed automatically, you can add the trim modifier to
your v-model.

<input v-model.trim="msg">

****

Components are reusable Vue instances with a name. They accept the same options as new Vue, such as data, computed,
watch, methods, and lifecycle hooks(except few root-specific options like el).

Lets take an example of counter component,

// Define a new component called button-counter
Vue.component('button-counter', {
  template: '<button v-on:click="count++">You clicked me {{ count }} times.</button>'
  data: function () {
    return {
      count: 0
    }
  },
})

Let's use this component inside a root Vue instance created with new Vue

<div id="app">
  <button-counter></button-counter>
</div>

var vm = new Vue({ el: '#app' });

****

Props are custom attributes you can register on a component. When a value is passed to a prop attribute, it becomes
a property on that component instance. You can pass those list of values as props option and use them as similar to
data variables in template.

Vue.component('todo-item', {
  props: ['title'],
  template: '<h2>{{ title }}</h2>'
})

Once the props are registered, you can pass them as custom attributes.

<todo-item title="Learn Vue conceptsnfirst"></todo-item>

****

In VueJS 2.x, every component must have a single root element when template has more than one element. In this
case, you need to wrap the elements with a parent element.

<template>
   <div class="todo-item">
       <h2>{{ title }}</h2>
       <div v-html="content"></div>
   </div>
</template>

Otherwise there will an error throwing, saying that "Component template should contain exactly one root
element...".

Whereas in 3.x, components now can have multiple root nodes. This way of adding multiple root nodes is called as
fragments.

<template>
     <h2>{{ title }}</h2>
     <div v-html="content"></div>
</template>

****

If you want child wants to communicate back up to the parent, then emit an event from child using $emit object to
parent,

Vue.component('todo-item', {
  props: ['todo'],
  template: `
    <div class="todo-item">
      <h3>{{ todo.title }}</h3>
      <button v-on:click="$emit('increment-count', 1)">
        Add
      </button>
      <div v-html="todo.description"></div>
    </div>
  `
})

Now you can use this todo-item in parent component to access the count value.

<ul v-for="todo in todos">
  <li>
    <todo-item
      v-bind:key="todo.id"
      v-bind:todo="todo"
      v-on:increment-count="total += 1"
    /></todo-item>
  </li>
</ul>
<span> Total todos count is {{total}}</span>

****

The custom events can also be used to create custom inputs that work with v-model. The <input> inside the
component must follow below rules,

1. Bind the value attribute to a value prop
2. On input, emit its own custom input event with the new value.

Let's take a custom-input component as an example,

Vue.component('custom-input', {
  props: ['value'],
  template: `
    <input
      v-bind:value="value"
      v-on:input="$emit('input', $event.target.value)"
    />
  `
})

Now you can use v-model with this component,

  <custom-input v-model="searchInput"></custom-input>
  

****

Vue implements a content distribution API using the <slot> element to serve as distribution outlets for content
created after the current Web Components spec draft.

Let's create an alert component with slots for content insertion,

Vue.component('alert', {
  template: `
    <div class="alert-box">
      <strong>Error!</strong>
      <slot></slot>
    </div>
  `
})

Now you can insert dynamic content as below,

<alert>
  There is an issue with in application.
</alert>

****

The components which are globally registered can be used in the template of any root Vue instance (new Vue) created
after registration.

In the global registration, the components created using Vue.component as below,

Vue.component('my-component-name', {
  // ... options ...
})

Let's take multiple components which are globally registered in the vue instance,

Vue.component('component-a', { /* ... */ })
Vue.component('component-b', { /* ... */ })
Vue.component('component-c', { /* ... */ })

new Vue({ el: '#app' })

The above components can be used in the vue instance,

<div id="app">
  <component-a></component-a>
  <component-b></component-b>
  <component-c></component-c>
</div>

Remember that the components can be used in subcomponents as well.

****

Due to global registration, even if you don't use the component it could still be included in your final build. So
it will create unnecessary javascript in the application. This can be avoided using local registration with the
below steps,

1. First you need to define your components as plain JavaScript objects

    var ComponentA = { /* ... */ }
    var ComponentB = { /* ... */ }
    var ComponentC = { /* ... */ }
    

Locally registered components will not be available in sub components. In this case, you need to add them in
components section

    var ComponentA = { /* ... */ }

    var ComponentB = {
      components: {
        'component-a': ComponentA
      },
      // ...
    }
    

1. You can use the components in the components section of the vue instance,

    new Vue({
      el: '#app',
      components: {
        'component-a': ComponentA,
        'component-b': ComponentB
      }
    })
    

****

In local registration, you need to create each component in components folder(optional but it is recommended)
and import them in another component file components section.

Let's say you want to register component A and B in component C, the configuration seems as below,

import ComponentA from './ComponentA'
import ComponentB from './ComponentB'

export default {
  components: {
    ComponentA,
    ComponentB
  },
  // ...
}

Now both ComponentA and ComponentB can be used inside ComponentC's template.

In global registration, you need to export all common or base components in a separate file. But some of the
popular bundlers like webpack make this process simpler by using require.context to globally register base
components in the below entry file(one-time).

import Vue from 'vue'
import upperFirst from 'lodash/upperFirst'
import camelCase from 'lodash/camelCase'

const requireComponent = require.context(
  // The relative path of the components folder
  './components',
  // Whether or not to look in subfolders
  false,
  // The regular expression used to match base component filenames
  /Base[A-Z]\w+\.(vue|js)$/
)

requireComponent.keys().forEach(fileName => {
  // Get component config
  const componentConfig = requireComponent(fileName)

  // Get PascalCase name of component
  const componentName = upperFirst(
    camelCase(
      // Strip the leading `./` and extension from the filename
      fileName.replace(/^\.\/(.*)\.\w+$/, '$1')
    )
  )

  // Register component globally
  Vue.component(
    componentName,
    // Look for the component options on `.default`, which will
    // exist if the component was exported with `export default`,
    // otherwise fall back to module's root.
    componentConfig.default || componentConfig
  )
})

****

You can declare props with type or without type. But it is recommended to have prop types because it provides the
documentation for the component and warns the developer for any incorrect data type being assigned.

props: {
  name: String,
  age: Number,
  isAuthenticated: Boolean,
  phoneNumbers: Array,
  address: Object
}

As mentioned in the above code snippet, you can list props as an object, where the properties' names and values
contain the prop names and types, respectively.

****

All props follows a one-way-down binding between the child property and the parent one. i.e, When the parent
property is updated then that latest prop value will be passed down to the child, but not the otherway(child to
parent) around. The child component should not mutate the prop otherwise it throws a warning in the console.
The possible mutation cases can be solved as below,

1. When you try to use parent prop as initial value for child property:

In this case you can define a local property in child component and assign parent value as initial value

   props: ['defaultUser'],
   data: function () {
     return {
       username: this.defaultUser
     }
   }
   

1. When you try to transform the parent prop:

You can define a computed property using the prop's value,

   props: ['environment'],
   computed: {
     localEnvironment: function () {
       return this.environment.trim().toUpperCase()
     }
   }
   

****

A non-prop attribute is an attribute that is passed to a component, but does not have a corresponding prop defined.

For example, If you are using a 3rd-party custom-input component that requires a data-tooltip attribute on the
input then you can add this attribute to component instance,

<custom-input data-tooltip="Enter your input" />

If you try to pass the props from parent component the child props with the same names will be overridden. But
props like class and style are exception to this, these values will be merged in the child component.

<!-- Child component -->
<input type="date" class="date-control">

<!-- Parent component -->
<custom-input class="custom-class" />

****

Vue provides validations such as types, required fields, default values along with customized validations. You can
provide an object with validation requirements to the value of props as below,

Let's take an example of user profile Vue component with possible validations,

Vue.component('user-profile', {
  props: {
    // Basic type check (`null` matches any type)
    age: Number,
    // Multiple possible types
    identityNumber: [String, Number],
    // Required string
    email: {
      type: String,
      required: true
    },
    // Number with a default value
    minBalance: {
      type: Number,
      default: 10000
    },
    // Object with a default value
    message: {
      type: Object,
      // Object or array defaults must be returned from
      // a factory function
      default: function () {
        return { message: 'Welcome to Vue' }
      }
    },
    // Custom validator function
    location: {
      validator: function (value) {
        // The value must match one of these strings
        return ['India', 'Singapore', 'Australia'].indexOf(value) !== -1
      }
    }
  }
})

****

The v-model directive on a component uses value as the prop and input as the event, but some input types
such as checkboxes and radio buttons may need to use the value attribute for a server side value. In this case,
it is preferred to customize model directive.

Let's take an example of checkbox component,

Vue.component('custom-checkbox', {
  model: {
    prop: 'checked',
    event: 'change'
  },
  props: {
    checked: Boolean
  },
  template: `
    <input
      type="checkbox"
      v-bind:checked="checked"
      v-on:change="$emit('change', $event.target.checked)"
    >
  `
})

Now you can use v-model on this customized component as below,

<custom-checkbox v-model="selectFramework"></custom-checkbox>

The selectFramework property will be passed to the checked prop and same property will be updated when custom
checkbox component emits a change event with a new value.

****

There are many ways Vue provides transition effects when items are inserted, updated, or removed from the DOM.

Below are the possible ways,

1. Automatically apply classes for CSS transitions and animations
2. Integrate 3rd-party CSS animation libraries. For example, Animate.css
3. Use JavaScript to directly manipulate the DOM during transition hooks
4. Integrate 3rd-party JavaScript animation libraries. For example, Velocity.js

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Vue Router is a official routing library for single-page applications designed for use with the Vue.js framework.

Below are their features,

1. Nested route/view mapping
2. Modular, component-based router configuration
3. Route params, query, wildcards
4. View transition effects powered by Vue.js' transition system
5. Fine-grained navigation control
6. Links with automatic active CSS classes
7. HTML5 history mode or hash mode, with auto-fallback in IE9
8. Restore scroll position when going back in history mode

****

It is easy to integrate vue router in the vue application.

Let us see the example with step by step instructions.

Step 1: Configure router link and router view in the template

   <script src="https://unpkg.com/vue/dist/vue.js"></script>
   <script src="https://unpkg.com/vue-router/dist/vue-router.js"></script>

   <div id="app">
     <h1>Welcome to Vue routing app!</h1>
     <p>
       <!-- use router-link component for navigation using `to` prop. It rendered as an `<a>` tag -->
       <router-link to="/home">Home</router-link>
       <router-link to="/services">Services</router-link>
     </p>
     <!-- route outlet in which component matched by the route will render here -->
     <router-view></router-view>
   </div>
   

Step 2: Import Vue and VueRouter packages and then apply router

   import Vue from 'vue';
   import VueRouter from 'vue-router';

   Vue.use(VueRouter)
   

Step 3: Define or import route components.

   const Home = { template: '<div>Home</div>' }
   const Services = { template: '<div>Services</div>' }
   

Step 4: Define your route where each one maps to a component

   const routes = [
     { path: '/home', component: Home },
     { path: '/services', component: Services }
   ]
   

Step 5: Create the router instance and pass the routes option

   const router = new VueRouter({
     routes // short for `routes: routes`
   })
   

Step 6: Create and mount the root instance.

   const app = new Vue({
     router
   }).$mount('#app')
   

Now you are able to navigate different pages(Home, Services) with in Vue application.

****

Sometimes it may be required to map routes to the same component based on a pattern.

Let's take a user component with the mapped URLs like /user/john/post/123 and /user/jack/post/235 using dynamic
segments,

const User = {
  template: '<div>User {{ $route.params.name }}, PostId: {{ $route.params.postid }}</div>'
}

const router = new VueRouter({
  routes: [
    // dynamic segments start with a colon
    { path: '/user/:name/post/:postid', component: User }
  ]
})

****

When you navigate from one URL to other(mapped with a single component) using routes with params then the same
component instance will be reused. Even though it is more efficient than destroying the old instance and then
creating a new one, the lifecycle hooks of the component will not be called.

This problem can be solved using either of the below approaches,

1. Watch the $route object:

    const User = {
      template: '<div>User {{ $route.params.name }} </div>',
      watch: {
        '$route' (to, from) {
          // react to route changes...
        }
      }
    }
    

1. Use beforeRouteUpdate navigation guard: This is only available since 2.2 version.

    const User = {
      template: '<div>User {{ $route.params.name }} </div>',
      beforeRouteUpdate (to, from, next) {
        // react to route changes and then call next()
      }
    }
    

Note that the beforeRouteEnter guard does NOT have access to this. Instead you can pass a callback to next to
access the vm instance.

****

Sometimes the URL might be matched by multiple routes and the confusion of which route need to be mapped is
resolved by route matching priority. The priority is based on order of routes configuration. i.e, The route which
declared first has higher priority.

const router = new VueRouter({
       routes: [
         // dynamic segments start with a colon
         { path: '/user/:name', component: User } // This route gets higher priority
         { path: '/user/:name', component: Admin }
         { path: '/user/:name', component: Customer }
       ]
     })

****

Generally, the app is composed of nested components which are nested multiple levels deep. The segments of a URL
corresponds to a certain structure of these nested components. To render components into the nested outlet, you
need to use the children option in VueRouter constructor config.

Let's take a user app composed of profile and posts nested components with respective routes. You can also define a
default route configuration when there is no matching nested route.

const router = new VueRouter({
  routes: [
    { path: '/user/:id', component: User,
      children: [
        {
          // UserProfile will be rendered inside User's <router-view> when /user/:id/profile is matched
          path: 'profile',
          component: UserProfile
        },
        {
          // UserPosts will be rendered inside User's <router-view> when /user/:id/posts is matched
          path: 'posts',
          component: UserPosts
        },
          // UserHome will be rendered inside User's <router-view> when /user/:id is matched
        {  path: '',
           component: UserHome },
      ]
    }
  ]
})

****

Single File Components are an easy concept to understand. Earlier you might heard about all three parts(HTML,
JavaScript and CSS) of your application kept in different components. But Single File Components encapsulate the
structure, styling and behaviour into one file. In the beginning, it seems strange to have all three parts in one
file, but it actually makes a lot more sense.

Let's take an example of Singile File Components

<template>
  <div>
    <h1>Welcome {{ name }}!</h1>
  </div>
</template>

<script>
module.exports = {
  data: function() {
    return {
      name: 'John'
    }
  }
}
</script>

<style scoped>
h1 {
  color: #34c779;
  padding: 3px;
}
</style>

****

As for the latest modern UI development, separation of concerns is not equal to separation of file types. So it is
preferred to divide codebase layers into loosely-coupled components and compose them instead of dividing the
codebase into three huge layers that interweave with one another. This way makes Single File Components more
cohesive and maintainable by combining template, logic and styles together inside a component.
You can also still maintain javascript and CSS files separately with hot-reloading and pre-compilation features.

For example,

<template>
  <div>This section will be pre-compiled and hot reloaded</div>
</template>
<script src="./my-component.js"></script>
<style src="./my-component.css"></style>

****

The Single File Components solve the common problems occurred in a javascript driven application with a .vue
extension. The list of issues are,

1. Global definitions force unique names for every component
2. String templates lack syntax highlighting and require ugly slashes for multiline HTML
3. No CSS support means that while HTML and JavaScript are modularized into components, CSS is conspicuously left

out

1. No build step restricts us to HTML and ES5 JavaScript, rather than preprocessors like Pug (formerly Jade) and

Babel.

****

Filters can be used to apply common text formatting. These Filters should be appended to the end of the JavaScript
expression, denoted by the "pipe" symbol. You can use them in two specific cases:

1. mustache interpolations
2. v-bind expressions

For example, Let's define a local filter named capitalize in a component's options

filters: {
  capitalize: function (value) {
    if (!value) return ''
    value = value.toString()
    return value.charAt(0).toUpperCase() + value.slice(1)
  }
}

Now you can use the filter in either mustache interpolation or v-bind expression,

<!-- in mustaches -->
{{ username | capitalize }}

<!-- in v-bind -->
<div v-bind:id="username | capitalize"></div>

****

You can define filters in two ways,

1. Local filters:

You can define local filters in a component's options. In this case, filter is applicable to that specific
component.

 filters: {
   capitalize: function (value) {
     if (!value) return ''
     value = value.toString()
     return value.charAt(0).toUpperCase() + value.slice(1)
   }
 }
 

1. Global filters:

You can also define a filter globally before creating the Vue instance. In this case, filter is applicable to all
the components with in the vue instance,

 Vue.filter('capitalize', function (value) {
   if (!value) return ''
   value = value.toString()
   return value.charAt(0).toUpperCase() + value.slice(1)
 })

 new Vue({
   // ...
 })
 

****

You can chain filters one after the other to perform multiple manipulations on the expression. The generic
structure of filter chain would be as below,

{{ message | filterA | filterB | filterB ... }}

In the above chain stack, you can observe that message expression applied with three filters, each separated by a
pipe(|) symbol. The first filter(filterA) takes the expression as a single argument and the result of the
expression becomes an argument for second filter(filterB) and the chain continue for remaining filters.

For example, if you want to transform date expression with a full date format and uppercase then you can apply
dateFormat and uppercase filters as below,

{{ birthday | dateFormat | uppercase }}

****

Yes, you can pass arguments for a filter similar to a javascript function. The generic structure of filter
parameters would be as follows,

{{ message | filterA('arg1', arg2) }}

In this case, filterA takes message expression as first argument and the explicit parameters mentioned in the
filter as second and third arguments.

For example, you can find the exponential strength of a particular value

{{ 2 | exponentialStrength(10) }} <!-- prints 2 power 10 = 1024 -->

****

Plugins provides global-level functionality to Vue application. The plugins provide various services,

1. Add some global methods or properties. For example, vue-custom-element
2. Add one or more global assets (directives, filters and transitions). For example, vue-touch
3. Add some component options by global mixin. For example, vue-router
4. Add some Vue instance methods by attaching them to Vue.prototype.
5. A library that provides an API of its own, while at the same time injecting some combination of the above. For

example, vue-router

****

The Plugin is created by exposing an install method which takes Vue constructor as a first argument along with
options. The structure of VueJS plugin with possible functionality would be as follows,

 MyPlugin.install = function (Vue, options) {
   // 1. add global method or property
   Vue.myGlobalMethod = function () {
     // some logic ...
   }

   // 2. add a global asset
   Vue.directive('my-directive', {
     bind (el, binding, vnode, oldVnode) {
       // some logic ...
     }
     // ...
   })

   // 3. inject some component options
   Vue.mixin({
     created: function () {
       // some logic ...
     }
     // ...
   })

   // 4. add an instance method
   Vue.prototype.$myMethod = function (methodOptions) {
     // some logic ...
   }
 }
 

****

You can use plugin by passing your plugin to Vue's use global method. You need to apply this method before
start your app by calling new Vue().

// calls `MyPlugin.install(Vue, { someOption: true })`
Vue.use(MyPlugin)

new Vue({
  //... options
})

****

Mixin gives us a way to distribute reusable functionalities in Vue components. These reusable functions are merged
with existing functions. A mixin object can contain any component options. Let us take an example of mixin with
created lifecycle which can be shared across components,

const myMixin = {
  created(){
    console.log("Welcome to Mixins!")
  }
}
var app = new Vue({
  el: '#root',
  mixins: [myMixin]
})

Note: Multiple mixins can be specified in the mixin array of the component.

****

Sometimes there is a need to extend the functionality of Vue or apply an option to all Vue components available in
our application. In this case, mixins can be applied globally to affect all components in Vue. These mixins are
called as global mixins.

Let's take an example of global mixin,

Vue.mixin({
  created(){
    console.log("Write global mixins")
  }
})

new Vue({
  el: '#app'
})

In the above global mixin, the mixin options spread across all components with the console running during the
instance creation. These are useful during test, and debugging or third party libraries. At the same time, You need
to use these global mixins sparsely and carefully, because it affects every single Vue instance created, including
third party components.

****

Using Vue CLI, mixins can be specified anywhere in the project folder but preferably within /src/mixins for ease
of access. Once these mixins are created in a .js file and exposed with the export keyword, they can be
imported in any component with the import keyword and their file paths.

****

When a mixin and the component itself contain overlapping options, the options will be merged based on some
strategies.

1. The data objects undergo a recursive merge, with the component's data taking priority over mixins in cases of

overlapping or conflicts.

    var mixin = {
      data: function () {
        return {
          message: 'Hello, this is a Mixin'
        }
      }
    }
    new Vue({
      mixins: [mixin],
      data: function () {
        return {
          message: 'Hello, this is a Component'
        }
      },
      created: function () {
        console.log(this.$data); // => { message: "Hello, this is a Component'" }
      }
    })
    

1. The Hook functions which are overlapping merged into an array so that all of them will be called. Mixin hooks

will be called before the component's own hooks.

    const myMixin = {
      created(){
        console.log("Called from Mixin")
      }
    }

    new Vue({
      el: '#root',
      mixins: [myMixin],
      created(){
        console.log("Called from Component")
      }
    })

    // Called from Mixin
    // Called from Component
    

1. The options that expect object values(such as methods, components and directives) will be merged into the same

object. In this case, the component's options will take priority when there are conflicting keys in these
objects.

    var mixin = {
      methods: {
        firstName: function () {
          console.log('John')
        },
        contact: function () {
          console.log('+65 99898987')
        }
      }
    }

    var vm = new Vue({
      mixins: [mixin],
      methods: {
        lastName: function () {
          console.log('Murray')
        },
        contact: function () {
          console.log('+91 893839389')
        }
      }
    })

    vm.firstName() // "John"
    vm.lastName() // "Murray"
    vm.contact() // "+91 893839389"
    

****

Vue uses the default strategy which overwrites the existing value while custom options are merged. But if you want
a custom option merged using custom login then you need to attach a function to Vue.config.optionMergeStrategies

For the example, the structure of myOptions custom option would be as below,

Vue.config.optionMergeStrategies.myOption = function (toVal, fromVal) {
  // return mergedVal
}

Let's take below Vuex 1.0 merging strategy as an advanced example,

const merge = Vue.config.optionMergeStrategies.computed
Vue.config.optionMergeStrategies.vuex = function (toVal, fromVal) {
  if (!toVal) return fromVal
  if (!fromVal) return toVal
  return {
    getters: merge(toVal.getters, fromVal.getters),
    state: merge(toVal.state, fromVal.state),
    actions: merge(toVal.actions, fromVal.actions)
  }
}

****

Custom Directives are tiny commands that you can attach to DOM elements. They are prefixed with v- to let the
library know you're using a special bit of markup and to keep syntax consistent. They are typically useful if you
need low-level access to an HTML element to control a bit of behavior.

Let's create a custom focus directive to provide focus on specific form element during page load time,

// Register a global custom directive called `v-focus`
Vue.directive('focus', {
  // When the bound element is inserted into the DOM...
  inserted: function (el) {
    // Focus the element
    el.focus()
  }
})

Now you can use v-focus directive on any element as below,

<input v-focus>

****

You can also register directives locally(apart from globally) using directives option in component as below,

directives: {
  focus: {
    // directive definition
    inserted: function (el) {
      el.focus()
    }
  }
}

Now you can use v-focus directive on any element as below,

<input v-focus>

****

A directive object can provide several hook functions,

1. bind: This occurs once the directive is attached to the element.
2. inserted: This hook occurs once the element is inserted into the parent DOM.
3. update: This hook is called when the element updates, but children haven't been updated yet.
4. componentUpdated: This hook is called once the component and the children have been updated.
5. unbind: This hook is called only once when the directive is removed.

Note: There are several arguments that can be passed to the above hooks.

****

All the hooks have el, binding, and vnode as arguments. Along with that, update and componentUpdated
hooks expose oldVnode, to differentiate between the older value passed and the newer value. Below are the
arguments passed to the hooks,

1. el: The element the directive is bound to and it can be used to directly manipulate the DOM.
2. binding: An object containing the following properties.
3. name: The name of the directive, without the v- prefix.
4. value: The value passed to the directive. For example in v-my-directive="1 + 1", the value would be 2.
5. oldValue: The previous value, only available in update and componentUpdated. It is available whether or not

the value has changed.

1. expression: The expression of the binding as a string. For example in v-my-directive="1 + 1", the

expression would be "1 + 1".

1. arg: The argument passed to the directive, if any. For example in v-my-directive:foo, the arg would be "

foo".

1. modifiers: An object containing modifiers, if any. For example in v-my-directive.foo.bar, the modifiers

object would be { foo: true, bar: true }.

1. vnode: The virtual node produced by Vue's compiler.
2. oldVnode: The previous virtual node, only available in the update and componentUpdated hooks.

The arguments can be represented diagrammatically across the hooks as below,


****

A directive can take any valid javascript expression. So if you want to pass multiple values then you can pass in a
JavaScript object literal.

Let's pass object literal to an avatar directive as below

<div v-avatar="{ width: 500, height: 400, url: 'path/logo', text: 'Iron Man' }"></div>

Now let us configure avatar directive globally,

Vue.directive('avatar', function (el, binding) {
  console.log(binding.value.width) // 500
  console.log(binding.value.height)  // 400
  console.log(binding.value.url) // path/logo
  console.log(binding.value.text)  // "Iron Man"
})

****

In few cases, you may want the same behavior on bind and update hooks irrespective of other hooks. In this
situation you can use function shorthand,

Vue.directive('theme-switcher', function (el, binding) {
  el.style.backgroundColor = binding.value
})

****

The createElement accepts few arguments to use all the template features.

Let us see the basic structure of createElement with possible arguments,

// @returns {VNode}
createElement(
  // An HTML tag name, component options, or async function resolving to one of these.
  // Type is {String | Object | Function}
  // Required.
  'div',
 
  // A data object corresponding to the attributes you would use in a template.
  // Type is {Object}
  // Optional.
  {
    // Normal HTML attributes
    attrs: {
      id: 'someId'
    },
    // Component props
    props: {
      myProp: 'somePropValue'
    },
    // DOM properties
    domProps: {
      innerHTML: 'This is some text'
    },
    // Event handlers are nested under `on`
    on: {
      click: this.clickHandler
    },
    // Similar to `v-bind:style`, accepting either a string, object, or array of objects.
    style: {
      color: 'red',
      fontSize: '14px'
    },
    // Similar to `v-bind:class`, accepting either a string, object, or array of strings and objects.
    class: {
      className1: true,
      className2: false
    }
    // ....
  },
 
  // Children VNodes, built using `createElement()`, or using strings to get 'text VNodes'.
  // Type is {String | Array}
  // Optional.
  [
    'Learn about createElement arguments.',
    createElement('h1', 'Headline as a child virtual node'),
    createElement(MyComponent, {
      props: {
        someProp: 'This is a prop value'
      }
    })
  ]
)

See details of the date object in
official doc.

****

All virtual nodes(VNodes) in the component tree must be unique.i.e, You can't write duplicated nodes in a
straightforward way. If you want to duplicate the same element/component many times then you should use factory
function.

The below render function is invalid where you are trying to duplicate h1 element 3 times,

render: function (createElement) {
  var myHeadingVNode = createElement('h1', 'This is a Virtual Node')
  return createElement('div', [
    myHeadingVNode, myHeadingVNode, myHeadingVNode
  ])
}

You can make duplicates with factory function,

render: function (createElement) {
  return createElement('div',
    Array.apply(null, { length: 3 }).map(function () {
      return createElement('h1', 'This is a Virtual Node')
    })
  )
}

****

The functional components are just simple functions to create simple components just by passing a context. Every
functional component follows two rules,

1. Stateless: It doesn't keep any state by itself
2. Instanceless: It has no instance, thus no this

You need to define functional: true to make it functional. Let's take an example of functional components,

Vue.component('my-component', {
  functional: true,
  // Props are optional
  props: {
    // ...
  },
  // To compensate for the lack of an instance,
  // we are now provided a 2nd context argument.
  render: function (createElement, context) {
    // ...
  }
})

Note: The functional components are quite popular in React community too.

****

Even though ReactJS and VueJS are two different frameworks there are few similarities(apart from the common goal of
utilized in interface design) between them.

1. Both frameworks are based on the Virtual DOM model
2. They provide features such Component-based structure and reactivity
3. They are intended for working with the root library, while all the additional tasks are transferred to other

libraries(routing, state management etc).

****

Even though VueJS and ReactJS share few common features there are many difference between them.

Let's list down them in a table format.

| Feature | VueJS | ReactJS |
| --------------------- | ------------------------------------ | -------------------------------------------------- |
| Type | JavaScript MVVM Framework | JavaScript Library |
| Platform | Primarily focused on web development | Both Web and Native |
| Learning Curve | Easy to learn the framework | A steep learning curve and requires deep knowledge |
| Simplicity | Vue is simpler than React | React is more complex than Vue |
| Bootstrap Application | Vue-cli | CRA (Create React App) |

****

Vue has the following advantages over React

1. Vue is smaller and faster
2. The convenient templates ease the process of developing
3. It has simpler javascript syntax without learning JSX

****

React has the following advantages over Vue

1. ReactJS gives more flexibility in large apps developing
2. Easy to test
3. Well-suited for mobile apps creation
4. The eco system is quite big and well matured.

****

The the syntax of Vue and Angular is common at some points because Angular is the basis for VueJS development in
the beginning.

But there are many differences between VueJS and Angular as listed,

| Feature | VueJS | Angular |
| --------------- | ------------------------------------------- | ------------------------------------------------------------------------ |
| Complexity | Easy to learn, simple API and design | The framework is bit huge and need some learning curve on typescript etc |
| Binding of Data | One-way binding | Two-way binding |
| Learning Curve | Easy to learn the framework | A steep learning curve and requires deep knowledge |
| Founders | Created by Former Google Employee | Powered by Google |
| Initial Release | February 2014 | September 2016 |
| Model | Based on Virtual DOM(Document Object Model) | Based on MVC(Model-View-Controller) |

****

The dynamic component will allow you to dynamically switch between multiple components without updating the route
of the application and even retaining the state of the component when switching back to the initial component. It
works by using <component> tag with v-bind:is attribute which accept dynamic component.

Let's create a dynamic component vue instance to switch between different pages of a website,

new Vue({
  el: '#app',
  data: {
    currentPage: 'home'
  },
  components: {
    home: {
      template: "<p>Home</p>"
    },
    about: {
      template: "<p>About</p>"
    },
    contact: {
      template: "<p>Contact</p>"
    }
  }
})

Now you can use the dynamic component which holds the current page,

<div id="app">
   <component v-bind:is="currentPage">
       <!-- component changes when currentPage changes! -->
       <!-- output: Home -->
   </component>
</div>

The component will be unmounted when it is switched away but it is possible to force the inactive component alive
using <keep-alive> component

Note: The value of :is attribute can be either name of the registered component or the actual imported object
itself.

****

Keep-alive tag is an abstract component used to preserve component state or avoid re-rendering. When you wrapped <
keep-alive> tag around a dynamic component, it caches the inactive component instances without destroying them.

Let's see the example usage of it,

<!-- Inactive components will be cached! -->
<keep-alive>
  <component v-bind:is="currentTabComponent"></component>
</keep-alive>

When there are multiple conditional children, it requires that only one child is rendered at a time.

<!-- multiple conditional children -->
<keep-alive>
  <comp-a v-if="a > 1"></comp-a>
  <comp-b v-else></comp-b>
</keep-alive>

Note: Remember that keep-alive tag doesn't render a DOM element itself, and doesn't show up in the component
parent chain.

****

If you keep an inline-template on a child component then it will use its inner content as a template instead of
treating as reusable independent content.

<my-component inline-template>
   <div>
       <h1>Inline templates</h1>
       <p>Treated as component component owne content</p>
   </div>
</my-component>

Note: Even though this inline-templates gives more flexibility for template authoring, it is recommended to
define template using template property or <template> tag inside .vue component.

****

Apart from regular templates and inline templates, you can also define templates using a script element with the
type text/x-template and then referencing the template by an id.

Let's create a x-template for simple use case as below,

<script type="text/x-template" id="script-template">
  <p>Welcome to X-Template feature</p>
</script>

Now you can define the template using reference id,

Vue.component('x-template-example', {
  template: '#script-template'
})

****

The Components that can recursively invoke themselves in their own template are known as recursive components.

Vue.component('recursive-component', {
  template: `<!--Invoking myself!-->
             <recursive-component></recursive-component>`
});

Recursive components are useful for displaying comments on a blog, nested menus, or basically anything where the
parent and child are the same, eventhough with different content.

Note: Remember that recursive component can lead infinite loops with max stack size exceeded error, so make
sure recursive invocation is conditional(for example, v-if directive).

****

In complex applications, vue components will actually be each other's descendent and ancestor in the render tree.

Let's say componentA and componentB included in their respective templates which makes circular dependency,

//ComponentA
<div>
  <component-b >
</div>

//ComponentB
<div>
  <component-a >
</div>

This can be solved by either registering(or wait until) the child component in beforeCreate hook or using
webpack's asynchronous import while registering the component,

Solution1:

beforeCreate: function () {
 this.$options.components.componentB = require('./component-b.vue').default
}

Solution2:

components: {
 componentB: () => import('./component-b.vue')
}

****

Some environments(Google Chrome Apps) prohibits the usage of new Function() for evaluating expressions and the
full builds of vue applications depends on this feature to compile templates. Due to this reason, the full builds
of VueJS application are not CSP complaint.

In this case you can use runtime-only builds with Webpack + vue-loader or Browserify + vueify technology stack
through which templates will be precompiled into render functions. This way you can make sure VueJS applications
are 100% CSP complaint.

****

There are two types of builds provided by VueJS,

1. Full: These are the builds that contain both the compiler and the runtime.

2. Runtime Only: These builds doesn't include compiler but the code is responsible for creating Vue instances,
rendering and patching virtual DOM. These are about 6KB lighter min+gzip.

****

Below are the list of different builds of VueJS based on type of build,

| Type | UMD | CommonJS | ES Module (for bundlers) | ES Module (for browsers) |
| ------------------------- | ------------------ | --------------------- | ------------------------ | ------------------------ |
| Full | vue.js | vue.common.js | vue.esm.js | vue.esm.browser.js |
| Runtime only | vue.runtime.js | vue.runtime.common.js | vue.runtime.esm.js | NA |
| Full (production) | vue.min.js | NA | NA | vue.esm.browser.min.js |
| Runtime-only (production) | vue.runtime.min.js | NA | NA | NA |

****

You can configure vueJS in webpack using alias as below,

   module.exports = {
     // ...
     resolve: {
       alias: {
         'vue$': 'vue/dist/vue.esm.js' // 'vue/dist/vue.common.js' for webpack 1
       }
     }
   }
   

****

DevTools is a browser extension allowing you to inspect and debug your Vue applications in a more user-friendly
interface. You can find the below extensions for different browsers or environments,

1. Chrome Extension
2. Firefox Addon
3. Standalone Electron app (works with any environment)

The DevTools plugins can be used as shown in the below snapshot,

<img src="https://github.com/sudheerj/vuejs-interview-questions/blob/master/images/DevTools.png" width="700" height="500">

Note:

1. If the page uses a production/minified build of Vue.js, devtools inspection is disabled by default so the Vue

pane won't show up.

1. To make it work for pages opened via file:// protocol, you need to check "Allow access to file URLs" for this

extension in Chrome's extension management panel.

****

It supports all ECMAScript5 complaint browsers as mentioned in this url. VueJS
doesn't support IE8 browser and below, because it uses ECMAScript 5 features that are un-shimmable(require support
from the underlying JS engine) in IE8.

****

VueJS is available in jsdelivr, unpkg and cdnjs etc CDNs. Normally you can use them for prototyping or learning
purposes.

For example, you can use them using jsdelivr with latest versions as below,

<script src="https://cdn.jsdelivr.net/npm/vue@2.6.7/dist/vue.js"></script>

You can use it for native ES modules as below,

<script type="module">
  import Vue from 'https://cdn.jsdelivr.net/npm/vue@2.6.7/dist/vue.esm.browser.js'
</script>

Note: You can remove version number to get latest version.

****

It is extremely rare situation of having to manually force an update despite the fact that no reactive data has
changed. i.e, To force the Vue instance to re-render manually. You can do it force update using **vm.$forceUpdate()
** API method.

Note: It does not affect all child components but only the instance itself and child components with inserted
slot content.

****

If you want to render a lot of static content then you need to make sure it only evaluated once and then cached
thereafter. In this case, you can use v-once directive by wrapping at the root level.

The example usage of v-once directive would be as below,

Vue.component('legal-terms', {
  template: `
    <div v-once>
      <h1>Legal Terms</h1>
      ... a lot of static content goes here...
    </div>
  `
})

Note: It is recommended not to overuse unless there is slow rendering due to lot of static content.

****

The root instance(new Vue()) can be accessed with the $root property.

Let's see the usage of root instance with an example.

First let's create a root instance with properties and methods as below,

// The root Vue instance
new Vue({
  data: {
    age: 26
  },
  computed: {
    fullName: function () { /* ... */ }
  },
  methods: {
    interest: function () { /* ... */ }
  }
})

Now you can access root instance data and it's methods with in subcomponents as below,

// Get root data
this.$root.age

// Set root data
this.$root.age = 29

// Access root computed properties
this.$root.fullName

// Call root methods
this.$root.interest()

It is recommend using Vuex to manage state instead of using root instance as a global store.

****

Below are the top 10 organizations using VueJS for their applications or products,

1. Facebook - Used on marketing side of its Newsfeed
2. Netflix - Used in two internal apps for building movie streaming interfaces
3. Adobe - Used for Portfolio, a custom website builder designed to help users showcase their creative work
4. Xiaomi - Used for products where it sells from consumer electronics to software
5. Alibaba - Provide their apps an excellent experience to its customers
6. WizzAir - A budget airline WizzAir used for their customers user interface
7. EuroNews
8. Laracasts
9. GitLab
10. BMW

****

When the default render function encounters an error then you can use rennderError as an alternative render output.
The error will be passed to renderError as the second argument.

The example usage of renderError is as below,

new Vue({
  render (h) {
    throw new Error('An error')
  },
  renderError (h, err) {
    return h('div', { style: { color: 'red' }}, err.stack)
  }
}).$mount('#app')

****

The $parent object refers to the immediate outer scope. The parent will be accessible as
this.$parent for the child, and the child will be pushed into the parent's $children array. It establishes a
parent-child relationship between the two instances(parent and child). You can access parent data and properties
similar to $root.

****

The state management has state, view and actions as major components. The pattern followed by these components in a
application is known as State Management Pattern. Below are the components in a detail,

1. The state, which is the source of truth that drives our app
2. The view, which is just a declarative mapping of the state
3. The actions, which are the possible ways the state could change in reaction to user inputs from the view.

Let us take a counter example which follows state management pattern with the above 3 components,

    new Vue({
      // state
      data () {
        return {
          count: 0
        }
      },
      // view
      template: `
        <div>{{ count }}</div>
      `,
      // actions
      methods: {
        increment () {
          this.count++
        }
      }
    })
   

****

Vue.js has a one-way data flow model, through the props property. The same concept can be represented in vuex has
below,

<img src="https://github.com/sudheerj/vuejs-interview-questions/blob/master/images/flow.png" width="400" height="500">

****

Vue loader is a loader for webpack that allows you to author Vue components in a format called Single-File
Components (SFCs).

For example, it authors HelloWorld component in a SFC,

<template>
  <div class="greeting">{{ message }}</div>
</template>

<script>
export default {
  data () {
    return {
      message: 'Hello world for vueloader!'
    }
  }
}
</script>

<style>
.greeting {
  color: blue;
}
</style>

****

Vue Loader's configuration is a bit different from other loaders by adding Vue Loader's plugin to your webpack
config. The vue loader plugin is required for cloning any other rules(js and css rules) defined and applying them
to the corresponding language blocks(<script> and <style>) in .vue files.

For example, the simple demonistration of webpack configuration for vue loader would be as below,

// webpack.config.js
const VueLoaderPlugin = require('vue-loader/lib/plugin')

module.exports = {
  mode: 'development',
  module: {
    rules: [
      {
        test: /\.vue$/,
        loader: 'vue-loader'
      },
      // this will apply to both plain `.js` files and `<script>` blocks in `.vue` files
      {
        test: /\.js$/,
        loader: 'babel-loader'
      },
      // this will apply to both plain `.css` files and `<style>` blocks in `.vue` files
      {
        test: /\.css$/,
        use: [
          'vue-style-loader',
          'css-loader'
        ]
      }
    ]
  },
  plugins: [
    // make sure to include the plugin for cloning and mapping them to respective language blocks
    new VueLoaderPlugin()
  ]
}

****

Below are the list of Asset URL transform rules

1. Absolute path: If the URL is an absolute path (for example, /images/loader.png)then it will be preserved

as-is.

1. Relative path: If the URL starts with . (for example, ./images/loader.png) then it will be interpreted as

a relative module request and resolved based on the folder structure on your file system.

1. URLs starts with ~ symbol: If the URL starts with ~ symbol(for example, ./some-node-package/loader.png)

then it is interpreted as a module request. This way it can reference assets inside node modules too.

1. URLs starts with @ symbol: If the URL starts with @ symbol then it is interpreted as a module request.

This is useful if your webpack config has an alias for @, which by default points to /src path.

****

Vue-loader will automatically infer the proper loaders to use based on the lang attribute of a language block
and the rules defined in webpack config. You can use pre-processors such as SASS,LESS, Stylus and PostCSS using
vuejs loader.

****

Scoped CSS is a mechanism in VueJS Single File Components(SFC) that prevents styles from leaking out of the current
component and affecting other unintended components on your page. i.e, When a <style> tag has the scoped attribute,
its CSS will apply to elements of the current component only. It uses PostCSS to transform scoped css to plain CSS.

Let's take an example usage of scoped css,

<style scoped>
.greeting {
  color: green;
}
</style>

<template>
  <div class="greeting">Let's start Scoped CSS</div>
</template>

The above code will be converted to plain CSS,

  <style scoped>
 .greeting[data-v-f3f3eg9] {
   color: green;
 }
 </style>

 <template>
   <div class="greeting" data-v-f3f3eg9>Let's start Scoped CSS</div>
 </template>

****

Yes, you can include both scoped and non-scoped styles in the same component. If you don't mention scoped attribute
then it will become global style.

<style>
/* global styles */
</style>

<style scoped>
/* local styles */
</style>

****

In scoped css, if you need to modify the styles of a child component using deep selectors(i,e from parent scoped
css) then you need to use >>> combinator.

For example, the scoped deep selector on parent scoped css would be as below,

<style scoped>
.class1 >>> .class2 { /* ... */ }
</style>

It will be converted as,

.class1[data-v-f3f3eg9] .class2 { /* ... */ }

Note: If you preprocessors such as SASS then it may not be able to processs >>> properly. In such cases use the
/deep/ or ::v-deep combinator instead >>> combinator.

****

The parent component's styles will not leak into child components. But a child component's root node will be
affected by both the parent's scoped CSS and the child's scoped CSS. i.e, your child component's root element has a
class that also exists in the parent component, the parent component's styles will leak to the child. Anyway this
is by design so that the parent can style the child root element for layout purposes.

For example, the background color property of parent component leaked into child component as below,

//parent.vue

<template>
  <div class="wrapper">
    <p>parent</p>
    <ChildMessageComponent/>
  </div>
</template>

<script>
import ChildMessageComponent from "./components/child";

export default {
  name: "App",
  components: {
    ChildMessageComponent
  }
};
</script>
<style scoped>
.wrapper {
  background: blue;
}
</style>

//child.vue

<template>
  <div class="wrapper">
    <p>child</p>
  </div>
</template>

<script>
export default {
  name: "Hello, Scoped CSS",
};
</script>
<style scoped>
.wrapper {
  background: red;
}
</style>

Now the background color of child wrapper is going to be blue instead red.

****

The scoped css style doesn't impact v-html directive's dynamically generated content. In this case, you can use
deep selectors to solve this styling issue.

****

Yes, Vue.js provides first-class, built-in support for CSS Modules through vue-loader (in Webpack) and @vitejs/plugin-vue (in Vite) as an alternative or complement to standard scoped CSS (<style scoped>).

### How to Use CSS Modules in Vue:
Add the module attribute to your <style> block:

<template>
  <div :class="$style.cardContainer">
    <h2 :class="$style.title">Vue CSS Modules</h2>
  </div>
</template>

<style module>
.cardContainer {
  padding: 1.5rem;
  border-radius: 8px;
  background-color: #ffffff;
}
.title {
  color: #2c3e50;
  font-weight: bold;
}
</style>

### Named Modules:
You can also assign custom module names if you have multiple style blocks:

<style module="theme">
.primaryBtn { background: blue; }
</style>
<!-- In template: :class="theme.primaryBtn" -->

### Why Use CSS Modules over Scoped CSS?
CSS Modules generate globally unique hashed class names (.cardContainer_a8f9z), ensuring zero style leakage across components, explicit JavaScript class binding, and full compatibility with automated design tokens.

No. The lightweight runtime-only build of Vue.js (vue.runtime.esm-bundler.js) does not include the in-browser template compiler.

### Understanding Vue Builds:

1. Runtime-Only Build (~30% smaller):

• Does not contain the compiler.
• Requires all templates to be pre-compiled into JavaScript render functions at build time (by Vite, Webpack, or vue-loader) inside .vue Single File Components (SFCs).
• Recommended for 99% of modern production web applications because of smaller bundle size and faster initial load.

1. Full Build (Runtime + Compiler):

• Includes the in-browser compiler.
• Required if you mount templates directly from DOM string templates or inline strings at runtime:

     // Requires full build with compiler:
     const app = Vue.createApp({
       template: '<div>{{ message }}</div>'
     });
     

If you use string templates without pre-compilation on a runtime build, Vue will log a console warning explaining that template compilation is not supported in the runtime-only build.

Below are the steps to use css modules in VueJS,

1. Enable CSS modules: CSS Modules must be enabled by passing modules: true option to css-loader

   // webpack.config.js
   {
     module: {
       rules: [
         // ... other rules omitted
         {
           test: /\.css$/,
           use: [
             'vue-style-loader',
             {
               loader: 'css-loader',
               options: {
                 // enable CSS Modules
                 modules: true,
                 // customize generated class names
                 localIdentName: '[local]_[hash:base64:8]'
               }
             }
           ]
         }
       ]
     }
   }
   

1. Add module attribute: Add the module attribute to your <style>

   <style module>
   .customStyle {
     background: blue;
   }
   </style>
   

1. Inject CSS modules: You can inject CSS modules object with computed property $style

   <template>
     <div :class="$style.blue">
       Background color should be in blue
     </p>
   </template>
   

It can work with object/array syntax of :class binding.

****

Yes, you can use preprocessors with CSS Modules.

For example, sass-loader can configured in webpack file for sass preprocessor.

// webpack.config.js -> module.rules
{
  test: /\.scss$/,
  use: [
    'vue-style-loader',
    {
      loader: 'css-loader',
      options: { modules: true }
    },
    'sass-loader'
  ]
}

****

You can customize the name of the injected computed property by giving the module attribute a value. This will be
helpful to avoid overwriting injected styled if you have more than one <style> tags in a single *.vue component.

For example, you can use module attribute as below,

<style module="a">
  /* identifiers injected as a */
</style>

<style module="b">
  /* identifiers injected as b */
</style>

****

Hot reloading is not about reloading the page when you edit any .vue file. Instead, when you edit a *.vue file, all
instances of that component will be swapped in without reloading the page. It improves the development experience
when you are tweaking the templates or styling of your components.

****

Hot Reload is always enabled except below situations:

1. webpack target is node (SSR)
2. webpack minifies the code
3. process.env.NODE_ENV === 'production'

****

You can use hotReload: false option to disable the Hot Reload explicitly.

It can be configured as below,

module: {
  rules: [
    {
      test: /\.vue$/,
      loader: 'vue-loader',
      options: {
        hotReload: false // disables Hot Reload
      }
    }
  ]
}

****

The vue loader plugin internally uses hot reloading. If you are scaffolding project with vue-cli, hot reloading
comes out of the box but if you are manually setting up the project then hot reloading is enabled by default with
webpack-dev-server --hot command.

****

Below are the state preservation rules in hot reloading,

1. When editing the <template> of a component, instances of the edited component will re-render in place,

preserving all current private state.

1. When editing the <script> part of a component, instances of the edited component will be destroyed and

re-created in place.

1. When editing the <style> hot reload operates on its own via vue-style-loader without affecting application

state.

****

You can create functional components by adding functional attribute to template block,

<template functional>
  <div>{{ props.msg }}</div>
</template>

****

If you need to access properties defined globally on Vue.prototype then you can access them on parent,

<template functional>
  <div>{{ parent.$someProperty }}</div>
</template>

****

You can perform testing in two ways,

1. Using vue-cli: It offers pre-configured unit testing and e2e testing setups
2. Manual setup: You can manually setting up unit tests for *.vue files using either mocha-webpack or jest

****

The stylelint linter supports linting style parts of Vue single file components. You can run linter on particular
vue file as below,

stylelint MyComponent.vue

Other option is configuring stylelint-webpack-plugin in webpack. It can be configured as a dev dependency.

// webpack.config.js
const StyleLintPlugin = require('stylelint-webpack-plugin');
module.exports = {
  // ... other options
  plugins: [
    new StyleLintPlugin({
      files: ['**/*.{vue,htm,html,css,sss,less,scss,sass}'],
    })
  ]
}

****

The official eslint-plugin-vue supports linting both the template and script parts of Vue single file components.
You can configure plugin in your ESLint config,

// .eslintrc.js
module.exports = {
  extends: [
    "plugin:vue/essential"
  ]
}

You can run linter on particular component as below,

eslint --ext js,vue MyComponent.vue

****

You can use eslint-loader for *.vue files in order to automatically linted on save during development. It can be
installed as npm module,

npm install -D eslint eslint-loader

After that you need to add it as pre-loader,

// webpack.config.js
module.exports = {
  // ... other options
  module: {
    rules: [
      {
        enforce: 'pre',
        test: /\.(js|vue)$/,
        loader: 'eslint-loader',
        exclude: /node_modules/
      }
    ]
  }
}

****

CSS Extraction is used to extract all the processed CSS in all Vue components into a single CSS file. For
webpack4, you need to install below npm command,

npm install -D mini-css-extract-plugin

You can configure this plugin in webpack as below,

// webpack.config.js
var MiniCssExtractPlugin = require('mini-css-extract-plugin')

module.exports = {
  // other options...
  module: {
    rules: [
      // ... other rules omitted
      {
        test: /\.css$/,
        use: [
          process.env.NODE_ENV !== 'production'
            ? 'vue-style-loader'
            : MiniCssExtractPlugin.loader,
          'css-loader'
        ]
      }
    ]
  },
  plugins: [
    // ... Vue Loader plugin omitted
    new MiniCssExtractPlugin({
      filename: 'style.css'
    })
  ]
}

****

You can define custom language blocks inside *.vue files based on the lang attribute of the block, the block's
tag name, and the rules in your webpack config. You can also use resourceQuery to match a rule against a custom
block with no lang.

For example, to match against <message> custom blocks.

{
  module: {
    rules: [
      {
        resourceQuery: /blockType=message/,
        loader: 'loader-to-use'
      }
    ]
  }
}

****

Below are the list of major stylelint features

1. It has more than 160 built-in rules to catch errors, apply limits and enforce stylistic conventions
2. Understands latest CSS syntax including custom properties and level 4 selectors
3. It extracts embedded styles from HTML, markdown and CSS-in-JS object & template literals
4. Parses CSS-like syntaxes like SCSS, Sass, Less and SugarSS
5. Supports Plugins for reusing community plugins and creating own plugins

****

Vuex enforces below rules to structure any application.

1. Application-level state is centralized in the store.
2. The only way to mutate the state is by committing mutations, which are synchronous transactions.
3. Asynchronous logic should be encapsulated in, and can be composed with actions.

The project structure for any non-trivial application would be as below,
<img src="https://github.com/sudheerj/vuejs-interview-questions/blob/master/images/vuex-app-structure.png" width="700" height="500">

****

Yes, vuex supports hot-reloading for mutations, modules, actions and getters during development. You need to use
either webpack's hot module replacement API or browserify's hot module replacement plugin.

****

The store.hotUpdate() API method is used for mutations and modules.

For example, you need to configure vuex store as below,

// store.js
import Vue from 'vue'
import Vuex from 'vuex'
import mutations from './mutations'
import myModule from './modules/myModule'

Vue.use(Vuex)

const state = { message: "Welcome to hot reloading" }

const store = new Vuex.Store({
  state,
  mutations,
  modules: {
    moduleA: myModule
  }
})

if (module.hot) {
  // accept actions and mutations as hot modules
  module.hot.accept(['./mutations', './modules/newMyModule'], () => {
    // Get the updated modules
    const newMutations = require('./mutations').default
    const newMyModule = require('./modules/myModule').default
    //swap in the new modules and mutations
    store.hotUpdate({
      mutations: newMutations,
      modules: {
        moduleA: newMyModule
      }
    })
  })
}

****

Since mutations are just functions that completely rely on their arguments it will be easier to test. You need to
keep mutations inside your store.js file and should also export the mutations as a named export apart from default
export.

Let's take an example of increment mutations,

// mutations.js
export const mutations = {
  increment: state => state.counter++
}

And test them using mocha and chai as below,

// mutations.spec.js
import { expect } from 'chai'
import { mutations } from './store'

// destructure assign `mutations`
const { increment } = mutations

describe('mutations', () => {
  it('INCREMENT', () => {
    // mock state
    const state = { counter: 10 }
    // apply mutation
    increment(state)
    // assert result
    expect(state.counter).to.equal(11)
  })
})

****

It is easier to test getters similar to mutations. It is recommended to test these getters if they have complicated
computation.

Let's take a simple todo filter as a getter

// getters.js
export const getters = {
  filterTodos (state, status) {
    return state.todos.filter(todo => {
      return todo.status === status
    })
  }
}

And the test case for above getter as follows,

// getters.spec.js
import { expect } from 'chai'
import { getters } from './getters'

describe('getters', () => {
  it('filteredTodos', () => {
    // mock state
    const state = {
      todos: [
        { id: 1, title: 'design', status: 'Completed' },
        { id: 2, title: 'testing', status: 'InProgress' },
        { id: 3, title: 'development', status: 'Completed' }
      ]
    }
    // mock getter
    const filterStatus = 'Completed'

    // get the result from the getter
    const result = getters.filterTodos(state, filterStatus)

    // assert the result
    expect(result).to.deep.equal([
      { id: 1, title: 'design', status: 'Completed' },
      { id: 2, title: 'development', status: 'Completed' }
    ])
  })
})

****

By proper mocking, you can bundle tests with webpack and run them on node without having depenceny on Browser API.
It involves 2 steps,

1. Create webpack config: Create webpack config with proper .babelrc

   // webpack.config.js
   module.exports = {
     entry: './test.js',
     output: {
       path: __dirname,
       filename: 'test-bundle.js'
     },
     module: {
       loaders: [
         {
           test: /\.js$/,
           loader: 'babel-loader',
           exclude: /node_modules/
         }
       ]
     }
   }
   

1. Run testcases: First you need to bundle and then run them using mocha as below,

    webpack
    mocha test-bundle.js
    

****

Below are the steps to run tests in real browser,

1. Install mocha-loader.
2. Configure webpack config entry point to 'mocha-loader!babel-loader!./test.js'.
3. Start webpack-dev-server using the config.
4. Go to localhost:8080/webpack-dev-server/test-bundle to see the test result

****

In strict mode, whenever Vuex state is mutated outside of mutation handlers, an error will be thrown. It make sure
that all state mutations can be explicitly tracked by debugging tools. You can just enable this by passing
strict: true while creating the vuex store.

const store = new Vuex.Store({
  // ...
  strict: true
})

****

No, it is not recommended to use strict mode in production environment. Strict mode runs a synchronous deep watcher
on the state tree for detecting inappropriate mutations and it can be quite expensive when you perform large amount
of mutations. i.e, It can impact performance if you enable in production mode.
Hence, it should be handled through build tools,

const store = new Vuex.Store({
  // ...
  strict: process.env.NODE_ENV !== 'production'
})

****

The vuex plugin is an option hat exposes hooks for each mutation. It is a normal function that receives the store
as the only argument. You can create your own plugin or use built-in plugins.
The plugin skeleton would be as below,

const myPlugin = store => {
  // called when the store is initialized
  store.subscribe((mutation, state) => {
    // called after every mutation.
    // The mutation comes in the format of `{ type, payload }`.
  })
}

After that plugin can be configured for plugins options as below,

const store = new Vuex.Store({
  // ...
  plugins: [myPlugin]
})

****

Similar to components you can't mutate state directly but they can trigger changes by by committing mutations. This
way a plugin can be used to sync a data source to the store.

For example, createWebSocketPlugin plugin is used to sync a websocket data source to the store.

export default function createWebSocketPlugin (socket) {
  return store => {
    socket.on('data', data => {
      store.commit('receiveData', data)
    })
    store.subscribe(mutation => {
      if (mutation.type === 'UPDATE_DATA') {
        socket.emit('update', mutation.payload)
      }
    })
  }
}

And then configure plugin in vuex store as below

const plugin = createWebSocketPlugin(socket)

const store = new Vuex.Store({
  state,
  mutations,
  plugins: [plugin]
})

****

A Vuex "store" is basically a container that holds your application state. The store creation is pretty
straightforward.

Below are the list of instructions to use vuex in an increment application,

1. Configure vuex in vuejs ecosystem

   import Vuex from "vuex";
   Vue.use(Vuex)
   

1. Provide an initial state object and some mutations

   // Make sure to call Vue.use(Vuex) first if using a module system

   const store = new Vuex.Store({
     state: {
       count: 0
     },
     mutations: {
       increment (state) {
         state.count++
       }
     }
   })
   

1. Trigger state change with commit and access state variables,

   store.commit('increment')

   console.log(store.state.count) // -> 1
   

****

Below are the two major differences between vuex store and plain global object,

1. Vuex stores are reactive: If the store's state changes then vue components will reactively and efficiently

get updated

1. Cannot directly mutate the store's state: The store's state is changed by explicitly committing mutations to

ensure that every state change leaves a track-able record for tooling purpose

****

We want to explicitly track application state in order to implement tools that can log every mutation, take state
snapshots, or even perform time travel debugging. So we need to commit a mutation instead of changing store's state
directly.

****

Vuex's single state tree is single object contains all your application level state and serves as the "single
source of truth". It does not conflict with modularity when you split state and mutations into sub modules.

****

You can install vuex using npm or yarn as below,

npm install vuex --save
(or)
yarn add vuex

In a module system, you must explicitly install Vuex via Vue.use()

import Vue from 'vue'
import Vuex from 'vuex'

Vue.use(Vuex)

(OR)

You can also install it using CDN links such as unpkg.cpm which provides NPM-based CDN links. Just include vuex
after Vue and it will install itself automatically.

<script src="https://unpkg.com/vue.js"></script>
<script src="https://unpkg.com/vuex.js"></script>

Note: You can use a specific version/tag via URLs like https://unpkg.com/vuex@2.0.0. If you don't mention any
version then it will point to latest version.

****

Yes, vuex requires Promise. If your supporting browsers do not implement Promise (e.g. IE), you can use a polyfill
library, such as es6-promise using npm or yarn.

npm install es6-promise --save # NPM
yarn add es6-promise # Yarn

After that import into anywhere in your application,

import 'es6-promise/auto'

****

Since Vuex stores are reactive, you can retrieve" state from store by simply returning store's state from within a
computed property. i.e, Whenever store state changes, it will cause the computed property to re-evaluate, and
trigger associated DOM updates.

Let's take a hello word component which display store's state in the template,

// let's create a hello world component
const Greeting = {
  template: `<div>{{ greet }}</div>`,
  computed: {
    greet () {
      return store.state.msg
    }
  }
}

****

Vuex provides a mechanism to "inject" the store into all child components from the root component with the store
option. It will be enabled by vue.use(vuex).

For example, let's inject into our app component as below,

const app = new Vue({
  el: '#app',
  // provide the store using the "store" option.
  // this will inject the store instance to all child components.
  store,
  components: { Greeting },
  template: `
    <div class="app">
      <greeting></greeting>
    </div>
  `
})

Now the store will be injected into all child components of the root and will be available on them as **this.$store
**

 // let's create a hello world component
     const Greeting = {
       template: `<div>{{ greet }}</div>`,
       computed: {
         greet () {
           return this.$store.state.msg
         }
       }
     }

****

In Vuex application, creating a computed property every time whenever we want to access the store's state property
or getter is going to be repetitive and verbose, especially if a component needs more than one state property. In
this case, we can make use of the mapState helper of vuex which generates computed getter functions for us.

Let's take an increment example to demonstrate mapState helper,

// in full builds helpers are exposed as Vuex.mapState
import { mapState } from 'vuex'

export default {
  // ...
  computed: mapState({
    // arrow functions can make the code very succinct!
    username: state => state.username,

    // passing the string value 'username' is same as `state => state.username`
    usernameAlias: 'username',

    // to access local state with `this`, a normal function must be used
     greeting (state) {
      return this.localTitle + state.username
    }
  })
}

We can also pass a string array to mapState when the name of a mapped computed property is the same as a state sub
tree name

computed: mapState([
  // map this.username to store.state.username
  'username'
])

****

You can use object spread operator syntax in order to combine mapState helper(which returns an object) with other
local computed properties. This way it simplify merging techniques using utilities.

computed: {
  localComputed () { /* ... */ },
  // mix this into the outer object with the object spread operator
  ...mapState({
    // ...
  })
}

****

No, if a piece of state strictly belongs to a single component, it could be just fine leaving it as local state.
i.e, Even though vuex used in the application, it doesn't mean that you need to keep all the local state in vuex
store. Other than that the code becomes more verbose and indirect although it makes your state mutations more
explicit and debuggable.

****

Vuex getters acts as computed properties for stores to compute derived state based on store state. Similar to
computed properties, a getter's result is cached based on its dependencies, and will only re-evaluate when some of
its dependencies have changed.

Let's take a todo example which as completedTodos getter to find all completed todos,

const store = new Vuex.Store({
  state: {
    todos: [
      { id: 1, text: 'Vue course', completed: true },
      { id: 2, text: 'Vuex course', completed: false },
      { id: 2, text: 'Vue Router course', completed: true }
    ]
  },
  getters: {
    completedTodos: state => {
      return state.todos.filter(todo => todo.completed)
    }
  }
})

Note: Getters receive state as first argument.

****

You can access values of store's getter object(store.getters) as properties. This is known as property style
access.

For example, you can access todo's status as a property,

store.getters.todosStatus

The getters can be passed as 2nd argument for other getters. For example, you can derive completed todo's count
based on their status as below,

getters: {
  completedTodosCount: (state, getters) => {
    return getters.todosStatus === 'completed'
  }
}

Note: The getters accessed as properties are cached as part of Vue's reactivity system.

****

You can access store's state in a method style by passing arguments.

For example, you can pass user id to find user profile information as below,

getters: {
  getUserProfileById: (state) => (id) => {
    return state.users.find(user => user.id === id)
  }
}

After that you can access it as a method call,

store.getters.getUserProfileById(111); {id: '111', name: 'John', age: 33}

****

The mapGetters is a helper that simply maps store getters to local computed properties.

For example, the usage of getters for todo app would be as below,

import { mapGetters } from 'vuex'

export default {
  computed: {
    // mix the getters into computed with object spread operator
    ...mapGetters([
      'completedTodos',
      'todosCount',
      // ...
    ])
  }
}

****

Vuex mutations are similar to any events with a string type and a handler. The handler function is where we
perform actual state modifications, and it will receive the state as the first argument.

For example, the counter example with increment mutation would be as below,

const store = new Vuex.Store({
  state: {
    count: 0
  },
  mutations: {
    increment (state) {
      // mutate state
      state.count++
    }
  }
})

You can't directly invoke mutation instead you need to call store.commit with its type. The above mutation would
be triggered as follows

store.commit('increment')

****

You can also pass payload for the mutation as an additional argument to store.commit.

For example, the counter mutation with payload object would be as below,

mutations: {
  increment (state, payload) {
    state.count += payload.increment
  }
}

And then you can trigger increment commit

store.commit('increment', {
  increment: 20
})

Note: You can also pass primitives as payload.

****

You can also commit a mutation is by directly using an object that has a type property.

store.commit({
  type: 'increment',
  value: 20
})

Now the entire object will be passed as the payload to mutation handlers(i.e, without any changes to handler
signature).

mutations: {
  increment (state, payload) {
    state.count += payload.value
  }
}

****

Since a Vuex store's state is made reactive by Vue, the same reactivity caveats of vue will apply to vuex
mutations. These are the rules should be followed for vuex mutations,

1. It is recommended to initialize store's initial state with all desired fields upfront
2. Add new properties to state Object either by set method or object spread syntax

   Vue.set(stateObject, 'newProperty', 'John')
   

(OR)

   state.stateObject = { ...state.stateObject, newProperty: 'John' }
   

****

You need to remember that mutation handler functions must be synchronous. This is why because any state mutation
performed in the callback is essentially un-trackable. It is going to be problematic when the devtool will need to
capture a "before" and "after" snapshots of the state during the mutations.

mutations: {
  someMutation (state) {
    api.callAsyncMethod(() => {
      state.count++
    })
  }
}

****

You can commit mutations in components with either this.$store.commit('mutation name') or mapMutations helper
to map component methods to store.commit calls.

For example, the usage of mapMutations helper on counter example would be as below,

import { mapMutations } from 'vuex'

export default {
  methods: {
    ...mapMutations([
      'increment', // map `this.increment()` to `this.$store.commit('increment')`

      // `mapMutations` also supports payloads:
      'incrementBy' // map `this.incrementBy(amount)` to `this.$store.commit('incrementBy', amount)`
    ]),
    ...mapMutations({
      add: 'increment' // map `this.add()` to `this.$store.commit('increment')`
    })
  }
}

****

No, it is not mandatory. But you might observed that State management implementations such Flux and Redux use
constants for mutation types. This convention is just a preference and useful to take advantage of tooling like
linters, and putting all constants in a single file allows your collaborators to get an at-a-glance view of what
mutations are possible in the entire application.

For example, the mutations can be declared as below,

// mutation-types.js
export const SOME_MUTATION = 'SOME_MUTATION'

And you can configure them in store as follows,

// store.js
import Vuex from 'vuex'
import { SOME_MUTATION } from './mutation-types'

const store = new Vuex.Store({
  state: { ... },
  mutations: {
    // ES2015 computed property name feature to use a constant as the function name
    [SOME_MUTATION] (state) {
      // mutate state
    }
  }
})

****

In Vue application architecture, asynchronous operations (such as API data fetching, asynchronous imports, or timer delays) are handled differently depending on whether you use the modern Pinia store, Vuex, or the Composition API:

### 1. In Modern Pinia (Recommended):
Actions can be directly declared as async functions with no distinction between synchronous mutations and asynchronous actions:

export const useUserStore = defineStore('user', {
  state: () => ({ user: null, loading: false }),
  actions: {
    async fetchUser(id: number) {
      this.loading = true;
      try {
        this.user = await api.getUser(id);
      } finally {
        this.loading = false;
      }
    }
  }
});

### 2. In Vuex:
Vuex strictly requires mutations to be synchronous to ensure DevTools state snapshots remain deterministic. Asynchronous logic must reside inside Actions, which commit mutations once resolved:

actions: {
  async fetchUserData({ commit }, userId) {
    commit('SET_LOADING', true);
    const data = await api.getUser(userId);
    commit('SET_USER', data);
    commit('SET_LOADING', false);
  }
}

### 3. In Components (Composition API):
Perform async calls inside lifecycle hooks like onMounted() or using <Suspense> wrappers.

Actions are similar to mutations, but there are two main differences,

1. Mutations perform mutations on the state, actions commit mutations.
2. Actions can contain arbitrary asynchronous operations unlike mutations.

****

Vuex provides actions property similar mutations property in order to define action handlers. These action handlers
receive context object as an argument which has same properties and methods of store instance.

Let's see counter example to demonstrate increment action which commits respective mutation,

const store = new Vuex.Store({
  state: {
    count: 0
  },
  mutations: {
    increment (state) {
      state.count++
    }
  },
  actions: {
    increment (context) {
      context.commit('increment')
    }
  }
})

****

In Vue state management, actions are dispatched to initiate asynchronous logic or multi-step operations:

### 1. In Vuex:
Actions are triggered using store.dispatch():

// In Options API
this.$store.dispatch('fetchUserProfile', { userId: 123 });

// In Composition API
import { useStore } from 'vuex';
const store = useStore();
store.dispatch('fetchUserProfile', { userId: 123 });

### 2. In Modern Pinia (Standard Practice):
Pinia eliminates the string-based dispatch syntax entirely. Actions are simply invoked as standard typed methods directly on the store instance:

import { useUserStore } from '@/stores/user';

const userStore = useUserStore();
// Direct type-safe method invocation:
await userStore.fetchUserProfile(123);

### Best Practice:
Always handle errors when dispatching actions by wrapping with try/catch or handling returned promises to prevent unhandled rejection errors in the UI.

Yes, actions support both payload and object style format similar to mutations.

// dispatch with a payload
store.dispatch('incrementAsync', {
  amount: 10
})

// dispatch with an object
store.dispatch({
  type: 'incrementAsync',
  amount: 10
})

****

Styled components is a CSS-in-JS library used mainly for ReactJS applications. If you want to use it for VueJS
applications, there is a VueJS styled components library port available, however, it is not a common practice.

****

You can dispatch actions in components with this.$store.dispatch('action name'), or use the mapActions
helper which maps component methods to store.dispatch calls.

For example, you can dispatch increment actions in counter component as below,

import { mapActions } from 'vuex'

export default {
  // ...
  methods: {
    ...mapActions([
      'increment', // map `this.increment()` to `this.$store.dispatch('increment')`

      // `mapActions` also supports payloads:
      'incrementBy' // map `this.incrementBy(amount)` to `this.$store.dispatch('incrementBy', amount)`
    ]),
    ...mapActions({
      add: 'increment' // map `this.add()` to `this.$store.dispatch('increment')`
    })
  }
}

****

You can write multiple actions together to handle more complex async flows either by chaining promises or
async/await. i.e, store.dispatch can handle Promise returned by the triggered action handler and it also returns
Promise.

Let's take two actions to see how they are combined and handled async flows,

actions: {
  actionOne ({ commit }) {
    return new Promise((resolve, reject) => {
      setTimeout(() => {
        commit('first mutation')
        resolve()
      }, 1000)
    })
  },
  actionTwo ({ dispatch, commit }) {
    return dispatch('actionA').then(() => {
      commit('second mutation')
    })
  }
}

As per the above example, When you try to dispatch actionTwo it dispatches actionOne first and then commits
respective mutation. You can still simplify with async/await as below,

actions: {
  async actionOne ({ commit }) {
    commit('first mutation', await getDataAsPromise())
  },
  async actionTwo ({ dispatch, commit }) {
    await dispatch('actionOne') // wait for `actionA` to finish
    commit('second mutation', await getSomeDataAsPromise())
  }
}

****

If you keep all state of our application in a single big state, the store can get really bloated. To solve this
problem, Vuex allows us to divide our store into modules. Here, each module can contain its own state, mutations,
actions, getters, and even nested modules.

Let's take an example with multiple modules, configuring them in vuex and accessing different modules,

const moduleOne = {
  state: { ... },
  mutations: { ... },
  actions: { ... },
  getters: { ... }
}

const moduleTwo = {
  state: { ... },
  mutations: { ... },
  actions: { ... },
  getters: { ... }
}

const store = new Vuex.Store({
  modules: {
    one: moduleOne,
    two: moduleTwo
  }
})

store.state.one // -> `moduleOne's state
store.state.two // -> `moduleTwo's state

****

When you use modules the local state will be available to mutations, getters and actions in different ways.

1. Both mutations and getters will receive module local state as first argument.

   const moduleOne = {
     state: { count: 0 },
     mutations: {
       increment (state) {
         state.count++; // Here state refers local module state
       }
     },

     getters: {
       average (state) {
         return state.count / 2
       }
     }
   }
   

1. In actions, local state will be available as first argument.

   const moduleOne = {
     actions: {
       incrementConditional ({ state, commit, rootState }) {
         if (state.count < rootState.count) {
           commit('increment')
         }
       }
     }
   }
   

****

By default in Vuex, all actions, mutations, and getters inside modules are registered under the global namespace—meaning multiple modules can react to the exact same action or mutation name.

### Enabling Namespacing:
To make a module self-contained and isolated, specify namespaced: true:

const cartModule = {
  namespaced: true,
  state: () => ({ items: [] }),
  getters: {
    totalCount(state) { return state.items.length; }
  },
  mutations: {
    ADD_ITEM(state, item) { state.items.push(item); }
  },
  actions: {
    checkout({ commit }) { /* ... */ }
  }
};

### Accessing Namespaced Modules:
When namespacing is enabled, getters, mutations, and actions are accessed with the module path prefix:

// Dispatching an action
store.dispatch('cart/checkout');

// Committing a mutation
store.commit('cart/ADD_ITEM', newItem);

// Reading a getter
store.getters['cart/totalCount'];

### Benefits of Namespacing:

• Eliminates naming collisions in enterprise applications.
• Makes state relationships explicit and modular.
• Supports reusable, pluggable module instances.

By default, actions, mutations and getters inside modules are still registered under the global namespace. Because
of that multiple modules react to the same mutation/action type.

****

Sometime you may need to create multiple instances of a module.

For example, it is needed in the below cases,

1. If multiple stores that use the same module
2. Register the same module multiple times in the same store.

In those cases, you need to assign to a variable and export it for reusability,

const MyReusableModule = {
  // state
  // mutations, actions, getters...
}

****

Vuex enforces below high-level principles,

1. The Application-level state need to be centralized in the store
2. The state should be mutated by committing mutations only(i.e, for synchronous transactions)
3. The actions should be used for asynchronous transactions.

****

In strict mode, you can't mutate state directly using v-model attribute. If you use v-model it throws an error
because mutation is not performed inside an explicit Vuex mutation handler.

For example, the below input throws an error due to v-model usage

<input v-model="stateObject.message">

In this case, you need to bind the <input>'s value. It can be resolved using value attribute as below,

<input :value="username" @input="updateProfile">

computed: {
  ...mapState({
    username: state => state.user.username
  })
},
methods: {
  updateProfile (e) {
    this.$store.commit('updateProfile', e.target.value)
  }
},
mutations: {
  updateProfile (state, username) {
    state.user.username = username
  }
}

****

You can still use model directive using two-way computed property with a setter.

 <input v-model="username">
 computed: {
  username: {
    get () {
      return this.$store.state.user.username
    },
    set (value) {
      this.$store.commit('updateProfile', value)
    }
  }
 }
 mutations: {
       updateProfile (state, username) {
         state.user.username = username
       }
 }

****

VueCLI provides below major features,

1. Interactive project scaffolding via @vue/cli
2. A rich collection of official plugins integrating the best tools in the frontend ecosystem.
3. A full graphical user interface to create and manage Vue.js projects.
4. Zero config rapid prototyping via combination of @vue/cli and @vue/cli-service-global
5. A runtime dependency (@vue/cli-service) built on top of webpack and extensible via plugins

In Vue CLI, Instant prototyping is known as rapid prototyping with just a single *.vue file with the vue serve(
similar to vue create) and vue build commands. But you need to install below global addon for this.

npm install -g @vue/cli-service-global
# or
yarn global add @vue/cli-service-global

You can also provide entry component for vue serve and target file for vue build using below commands

vue serve MyComponent.vue
vue build MyComponent.vue

****

You can create project using vue create command

vue create my-app

You can either choose the default preset or select "Manually select features" to pick the features you need.

The default preset prompt would be as below,

<img src="https://github.com/sudheerj/vuejs-interview-questions/blob/master/images/cli-default-presets.png" width="400" height="500">

and the manual select features would be as below,

<img src="https://github.com/sudheerj/vuejs-interview-questions/blob/master/images/cli-manual-features.png" width="400" height="500">

****

You can also create and manage projects using a graphical interface with the vue ui command. Once you apply this
command, it opens a browser window with a GUI that guides you through the project creation process.

<img src="https://github.com/sudheerj/vuejs-interview-questions/blob/master/images/cli-gui.png" width="400" height="500">

****

Vue CLI uses a plugin-based architecture where each plugin can modify the internal webpack configuration and inject
commands to vue-cli-service. i.e, Each feature is implemented as a plugin. This architecture makes Vue CLI
flexible and extensible.

****

You can install a plugin into an already created project with the vue add command.

vue add @vue/eslint
(OR)
vue add @vue/cli-plugin-eslint

You can also add options for plugin

vue add @vue/eslint --config airbnb --lintOn save

If a plugin is already installed, you can skip the installation and only invoke its generator with the vue invoke
command.

****

If you need access to the plugin API in your project without creating a full plugin, you can use the
vuePlugins.service option in your package.json file

{
  "vuePlugins": {
    "service": ["my-service.js"]
  }
}

****

You can also add files that will behave like UI plugins with the vuePlugins.ui option

{
  "vuePlugins": {
    "ui": ["my-ui.js"]
  }
}

****

A Vue CLI preset is a JSON object that contains pre-defined options and plugins for creating a new project without
interactive prompts to select them. During project creation(using vue create), the presets will be saved in a
~/.vuerc which can modified at any time.

For example, the generated JSON object(or preset) would be as below

{
  "useConfigFiles": true,
  "router": true,
  "vuex": true,
  "cssPreprocessor": "sass",
  "plugins": {
    "@vue/cli-plugin-babel": {},
    "@vue/cli-plugin-eslint": {
      "config": "airbnb",
      "lintOn": ["save", "commit"]
    }
  }
}

****

You can explicitly specify versions of the plugins being used.

{
  "plugins": {
    "@vue/cli-plugin-eslint": {
      "version": "^3.0.0",
      // ... other options for this plugin
    }
  }
}

For official plugins, the CLI will automatically use the latest version available in the registry

****

Each plugin can inject its own prompts during the project creation process irrespective of preset declarations
using prompts: true setting

For example, user can pick their own ESLint config using the below configuration

{
  "plugins": {
    "@vue/cli-plugin-eslint": {
      // let the users pick their own ESLint config
      "prompts": true
    }
  }
}

****

You can share a preset with other developers by publishing it in a git repo. The repo can be published in either
github, GitLab or BitBucket.
The repo will contain below files,

1. preset.json: The main file containing the preset data and it is required.
2. generator.js: A generator that can inject or modify files in the project.
3. prompts.js: A prompts file that can collect options for the generator.

You can apply --preset option to use remote presets while creating the project

   # use preset from GitHub repo
   vue create --preset username/repo my-project
   

****

Yes, Vue CLI will load local presets if the value for the --preset option is a relative or absolute file path, or
ends with .json. i.e, You can work with local presets directly. These local presets avoids repeatedly pushing the
preset to a remote repo to test.

// Directory contains preset.json file
vue create --preset ./my-preset my-project
(OR)
vue create --preset my-preset.json my-project

****

The browserslist option is available in package.json file in order to specify a range of browsers the project is
supported. This value is going to be used by babel and autoprefixer to transpile javascript features and applying
vendor prefixes.

For example, you can declare it as follows,

"browserslist": [
    "last 1 version",
    "> 1%",
    "IE 10"
  ]

****

The community plugins and components might need different strategies for different versions. In this case, you can
use Vue.version which provides installed version of Vue as a string.

For example, you can implement different logic based on different versions

let version = Number(Vue.version.split('.')[0])

if (version === 2) {
  // Vue v2.x.x
} else if (version === 1) {
  // Vue v1.x.x
} else {
  // Unsupported versions of Vue
}

****

In version 3.0 you can create a reactive object with the reactive() API.

const reactiveState = reactive({
count: 0
 })
 

In version 2.6, you can create reactive objects with Vue.observable() global API.

const reactiveState = Vue.observable({
  count: 0
})

These observable objects can be used directly in computed properties and render functions.

const Demo = {
  render(h) {
    return h('button', {
      on: { click: () => { reactiveState.count++ }}
    }, `count is: ${state.count}`)
  }
}

****

In Vue 2.6 version, the new slot syntax is provided using v-slot directive which aligns syntax with Vue 3.0. This
is going to be replacement for old slot syntax.

The comparison for old and new slot syntax:

<!-- old -->
<user>
  <template slot="header" slot-scope="{ msg }">
    text slot: {{ msg }}
  </template>
</user>

<!-- new -->
<user>
  <template v-slot:header="{ msg }">
    text slot: {{ msg }}
  </template>
</user>

****

VueJS provides compile method which is used to compile a template string into a render function. This method is
only available in the full build.

For example, you can compile template message:

var result = Vue.compile('<div><span>{{ msg }}</span></div>')

new Vue({
  data: {
    msg: 'Welcome to Vue world'
  },
  render: result.render,
  staticRenderFns: result.staticRenderFns
})

****

The nextTick method is just a comfortable way to execute a function after the data has been set, and the DOM has
been updated. As an example, the usage is going to be similar to setTimeout:

// modify data
vm.msg = 'Welcome to Vue'
// DOM not updated yet
Vue.nextTick(function () {
  // DOM updated
})

// usage as a promise (2.1.0+)
Vue.nextTick()
  .then(function () {
    // DOM updated
  })

****

From 2.6 version onwards, Vue's built-in error handling mechanism can capture errors inside v-on handlers. Also,if
any of your lifecycle hooks or event handlers performs asynchronous operations, you can now return a Promise from
the function so that any uncaught errors from that Promise chain are also sent to your error handlers.

Let's take an example of mounted lifecycle hook,

export default {
  async mounted() {
    // if an async error is thrown here, it now will get
    // caught by errorCaptured and Vue.config.errorHandler
    this.todos = await api.getTodos()
  }
}

****

In Vue 2.6 release onwards, Directive arguments can now accept dynamic JavaScript expressions. i.e, the specific
argument that we want to use is only known at runtime.

Let's assign dynamic key and event directives for a div element,

<div v-bind:[key]="value"></div>
<div v-on:[event]="handler"></div>

****

Apart from the benefits of dynamic directives arguments, it brings two drawbacks or considerations on the usage

1. Constraints on expressions: When you perform complex JavaScript expressions, make sure that html attribute

names cannot contain spaces and quotes.
The below expression doesn't work as expected

  <div :[key + 'unique']="value"></div>
  

Instead you may need to use string template syntax

  <div :[`${key}unique`]="value"></div>
  

1. Custom Directives: The custom directive implementations need to have potential argument changes in addition

to value changes.

****

Dynamic argument values are expected to be strings but it allows null as a special value that explicitly
indicates that the binding should be removed. Other types will be treated as mistakes and will trigger a warning.
So null value can be applied for v-bind and v-on.

****

No, you cannot pass null or undefined to dynamically remove a slot binding using v-slot (#).

### Why?
In Vue directive syntax:

• For general directives like v-bind:[attribute]="value" or v-on:[event]="handler", passing null explicitly unbinds the attribute or event listener.
• However, v-slot is a structural compilation directive, not a standard runtime attribute binding. Slots represent scoped template render functions created during compilation. Vue does not support conditionally unbinding or setting slot names to null.

### How to Conditionally Render Slots:
Instead of trying to pass null to the slot name, use conditional rendering (v-if) inside or around the slot:

<template #header v-if="hasHeader">
  <h1>Custom Header Content</h1>
</template>

Or check if the parent passed content to a slot using $slots.header in the child component.

Vue I18n is an internationalization plugin of Vue.js. It easily integrates some localization features to your
Vue.js Application.

The simple usage with in html would be as below,

<script src="https://unpkg.com/vue/dist/vue.js"></script>
<script src="https://unpkg.com/vue-i18n/dist/vue-i18n.js"></script>

<div id="app">
  <p>{{ $t("user.message") }}</p>
</div>

and after that configure them in javascript

// Ready translated locale messages
const messages = {
  en: {
    user: {
      message: 'Good morning'
    }
  },
  de: {
    user: {
      message: 'Guten Morgen'
    }
  }
}

// Create VueI18n instance with options
const i18n = new VueI18n({
  locale: 'de', // set locale
  messages, // set locale messages
})


// Create a Vue instance with `i18n` option
new Vue({ i18n }).$mount('#app')

The output is going to be like this,
<div id="#app">
<p>Guten Morgen</p>
</div>

****

Basically there are 4 types of formatting available in i18n plugin,

1. Named formatting: First You need to define the message keys in curly braces({})

   const messages = {
     en: {
       message: {
         greeting: '{msg} Morning'
       }
     }
   }
   

After that pass argument value along with key in the template

   <p>{{ $t('message.greeting', { msg: 'Good' }) }}</p>
   

It outputs the result as below,

   <p>Good Morning</p>
   

1. List formatting: First you need to define zero index based keys in the messages,

   const messages = {
     en: {
       message: {
         greeting: '{0} Morning'
       }
     }
   }
   

After that pass argument value with in an array

   <p>{{ $t('message.greeting', ['Good']) }}</p>
   

Finally it outputs the result as below,

   <p>Good morning</p>
   

Note: It also accepts array-like object

   <p>{{ $t('message.greeting', {'0': 'Good'}) }}</p>
   

1. HTML formatting: This formatting is required when want to render your translation as an HTML message and not

a static string.

   const messages = {
     en: {
       message: {
         greeting: 'Good <br> Morning'
       }
     }
   }
   

After that use it in the html directive template as below

   <p v-html="$t('message.greeting')"></p>
   

Finally it outputs the result as below

   <p>Good
   <!--<br> exists but is rendered as html and not a string-->
   Morning</p>
   

1. Ruby on rails format: First you need to define with percentile and curly braces as below,

   const messages = {
     en: {
       message: {
         greeting: '%{msg} Morning'
       }
     }
   }
   

After that pass argument with key similar to named formatting

   <p>{{ $t('message.greeting', { msg: 'Good' }) }}</p>
   

Finally it renders the output as below,

   <p>Good Morning</p>
   

****

You can use custom formatting for some of the formatting cases such as ICU formatting syntax (message "pattern"
strings with variable-element placeholders enclosed in {curly braces}). It implement Formatter Interface.

// Custom Formatter implementation
class CustomFormatter {
     constructor (options) {
       // ...
     }
interpolate (message, values) {
       // return the interpolated array
       return ['resolved message string']
     }
}

// register with `formatter` option
const i18n = new VueI18n({
  locale: 'en-US',
  formatter: new CustomFormatter(/* here the constructor options */),
  messages: {
    'en-US': {
      // ...
    },
    // ...
  }
})

// Run!
new Vue({ i18n }).$mount('#app')

****

You can translate with pluralization by defining the locale that have a pipe | separator, and define plurals in
pipe separator. Remember that template should use $tc() instead of $t().

First you need to define the messages,

const messages = {
  en: {
    user: 'user | users',
    friend: 'no friend | one friend | {count} friends'
  }
}

And the template can configure the messages with values

<p>{{ $tc('user', 1) }}</p>
<p>{{ $tc('user', 10) }}</p>

<p>{{ $tc('friend', 0) }}</p>
<p>{{ $tc('friend', 1) }}</p>
<p>{{ $tc('friend', 10, { count: 10 }) }}</p>

Finally it outputs the result as below

<p>user</p>
<p>users</p>

<p>no friend</p>
<p>one friend</p>
<p>10 friends</p>

****

You can localize the datetime with definition formats(e.g. short, long, etc).

Lets follow below steps to localize date and time,

1. For example, you can add definition formats for English and Jappan locale as below

    const dateTimeFormats = {
      'en-US': {
        short: {
          year: 'numeric', month: 'short', day: 'numeric'
        },
        long: {
          year: 'numeric', month: 'short', day: 'numeric',
          weekday: 'short', hour: 'numeric', minute: 'numeric'
        }
      },
      'ja-JP': {
        short: {
          year: 'numeric', month: 'short', day: 'numeric'
        },
        long: {
          year: 'numeric', month: 'short', day: 'numeric',
          weekday: 'short', hour: 'numeric', minute: 'numeric', hour12: true
        }
      }
    }
    

1. After that You need to specify the dateTimeFormats option of VueI18n constructor

    const i18n = new VueI18n({
      dateTimeFormats
    })

    new Vue({
      i18n
    }).$mount('#app')
    

1. And then add them to the template

    <div id="app">
      <p>{{ $d(new Date(), 'short') }}</p>
      <p>{{ $d(new Date(), 'long', 'ja-JP') }}</p>
    </div>
    

1. Finally it outputs the result

    <div id="app">
      <p>May 20, 2019</p>
      <p>2019年5月20日</p>
    </div>
    

****

You can localize the number with definition formats(e.g. currency, etc)

Lets follow below steps to localize numbers,

1. You need to add definition formats. For example, lets add it for English and Japanese locales

     const numberFormats = {
      'en-US': {
        currency: {
          style: 'currency', currency: 'USD'
        }
      },
      'ja-JP': {
        currency: {
          style: 'currency', currency: 'JPY', currencyDisplay: 'symbol'
        }
      }
    }
  

1. After that specify the numberFormats option of VueI18n constructor

    const i18n = new VueI18n({
      numberFormats
    })

    new Vue({
      i18n
    }).$mount('#app')
    

1. Now let's configure them in template

    <div id="app">
      <p>{{ $n(10, 'currency') }}</p>
      <p>{{ $n(50, 'currency', 'ja-JP') }}</p>
    </div>
    

1. Finally it outputs the result

    <div id="app">
      <p>$10.00</p>
      <p>￥50</p>
    </div>
    

****

All child components of a root instance are localized using the locale property of the VueI18n class. You can
change the value of the locale property of the VueI18n instance as below.

const i18n = new VueI18n({
  locale: 'de', // set locale
  ...
})

// create root Vue instance
new Vue({
  i18n,
  ...
}).$mount('#app')

// change other locale
i18n.locale = 'en'

You can also use component's VueI18n instance referenced as the $i18n property which will be used to change the
locale.

<template>
  <div class="locale-changer">
    <select v-model="$i18n.locale">
      <option v-for="(lang, i) in langs" :key="`Lang${i}`" :value="lang">{{ lang }}</option>
    </select>
  </div>
</template>

<script>
export default {
  name: 'locale-changer',
  data () {
    return { langs: ['de', 'en'] }
  }
}
</script>

****

The loading of all translation files at once is unnecessary and it may impact the performance too. It will be easy
for lazy loading or asynchronously loading the translation files when you use webpack. i.e, You can dynamically
load or import language translations using webpack as below,

//i18n-setup.js
import Vue from 'vue'
import VueI18n from 'vue-i18n'
import messages from '@/lang/en'
import axios from 'axios'

Vue.use(VueI18n)

export const i18n = new VueI18n({
  locale: 'en', // set locale
  fallbackLocale: 'en',
  messages // set locale messages
})

const loadedLanguages = ['en'] // our default language that is preloaded

function setI18nLanguage (lang) {
  i18n.locale = lang
  axios.defaults.headers.common['Accept-Language'] = lang
  document.querySelector('html').setAttribute('lang', lang)
  return lang
}

export function loadLanguageAsync (lang) {
  if (i18n.locale !== lang) {
    if (!loadedLanguages.includes(lang)) {
      return import(/* webpackChunkName: "lang-[request]" */ `@/lang/${lang}`).then(msgs => {
        i18n.setLocaleMessage(lang, msgs.default)
        loadedLanguages.push(lang)
        return setI18nLanguage(lang)
      })
    }
    return Promise.resolve(setI18nLanguage(lang))
  }
  return Promise.resolve(lang)
}

After that loadLanguageAsync function can be used inside a vue-router beforeEach hook.

router.beforeEach((to, from, next) => {
  const lang = to.params.lang
  loadLanguageAsync(lang).then(() => next())
})

****

The main difference between a computed property and a method is that computed properties are cached and
invoke/change only when their dependencies change. Whereas a method will evaluate every time it's called.

****

You can use deep watcher by setting deep: true in the options object. This option enables us to detect nested
value changes inside Objects.

vm.$watch('someObject', callback, {
  deep: true
})
vm.someObject.nestedValue = 123
// callback is fired

Note: This is not required to listen for Array mutations.

****

You can use immediate: true option in order to trigger watchers when the vue instance (or component) is being
created. i.e This option will trigger the callback immediately with the current value of the expression.

watch: {
  test: {
    immediate: true,
    handler(newVal, oldVal) {
      console.log(newVal, oldVal)
    },
  },
},

****

When comments option enabled, it will preserve and render HTML comments found in templates. By default, it's
value is false. Let's see the action in an example,

<template>
  <div class="greeting">
    <!--greeting-->
    <h1>{{ msg }}</h1>
  </div>
</template>

<script>
export default {
  comments: true,
  data () {
    return {
      msg: 'Good morning'
    }
  }
}
</script>

Note: This option is only available in the full build, with in-browser compilation. i.e, It won't work with
Single File Components(SFC).

****

You can use vm.$isServer method to know whether the current Vue instance is running on the server or client.

The usage would be as below,

const Vue = require('vue');
Vue.prototype.$isServer
(OR)
this.$isServer // With in component

****

You can setup a watcher on the $route in your component. It observes for route changes and when changed ,sets the
message property.

watch:{
    $route (to, from){
        this.message = 'Welcome';
    }
}

****

You can use vue-router-sync library to sync current $route object in vuex store's state.

The usage is quite straight forward with two steps

1. Installation:

   npm install vuex-router-sync
   

1. Sync router and store:

   import { sync } from 'vuex-router-sync'
   import store from './vuex/store' // vuex store instance
   import router from './router' // vue-router instance

   const unsync = sync(store, router) // Returns an unsync callback function
   unsync() // Unsyncs store from router
   

****

The navigation guards of vue-router are used to protect navigations either by redirecting it or canceling it.

Below are the 3 different ways to hook into router navigations

1. Global:
2. Per-route:
3. In-component:

****

Yes, you can access it directly as you would data props.

For example, the comboTwo computed property uses comboOne computed property as below,

data() {
    return {
        propOne: 'prop1',
        propTwo: 'prop2'
    }
},

computed: {
     comboOne() {
         return this.propOne + ',' + this.propTwo;
     },

     comboTwo() {
        return this.comboOne.split(',').join('-');
    }
}

****

The variables need to be exposed on your data in order to use them in template section. i.e, You can't use them
directly on template.

<span>
   CREATE: {{CREATE_PROP}}
   UPDATE: {{UPDATE_PROP}}
   DELETE: {{DELETE_PROP}}
</span>
<script>
import {CREATE_DATA, UPDATE_DATA, DELETE_DATA} from 'constants';
new Vue({
    ...
    data:{
        CREATE_PROP: CREATE_DATA,
        UPDATE_PROP: UPDATE_DATA,
        DELETE_PROP: DELETE_DATA
    }
    ...
})
</script>

****

No, it is not recommended. Computed properties should be synchronous. But if you still use asynchronous actions
inside them, they may not work as expected and can lead to an unexpected behaviour.

For example, the below usage of async/await is not recommended,

 async someComputedProperty () {
      return await someFunction()
    },

Note: If you still prefer to use async computed properties for some reason then you can consider using
additional plugin such as vue-async-computed.

****

In Vue.js components, if you define the same property name across conflicting component options (such as having a property named profile simultaneously in props, data, computed, and methods), Vue's component instance initialization will raise conflicts:

### Resolution Hierarchy & Conflicts:

1. Props Collision with Data:

If a property is declared in props and again in data(), Vue issues a runtime console warning:
*[Vue warn]: The data property "profile" is already declared as a prop. Use prop default value instead.*
The data declaration will overwrite or collide with the prop reference.

1. Computed Collision with Data/Props:

A computed property sharing a name with props or data triggers a warning because all properties are proxied onto the same root component instance (this).

1. Methods Collision:

Methods defined with duplicate names overwrite earlier declarations, resulting in lost functions and difficult-to-trace bugs.

### Prevention in Composition API:
With the modern Composition API (<script setup>), JavaScript lexical scoping prevents variable shadowing within the same script block, turning what used to be subtle runtime warnings into immediate compile-time errors in your IDE.

The component data must be a function instead directly providing the object. This is because each instance needs to
maintain an independent copy of the returned data object. Otherwise one component instance data changes will impact
the data of all other instances.

For example, the below code snippets gives an idea on correct approach,

    data: { // Bad
      message: 'Hello'
    }
    data: function () { //Good
      return {
        message: 'Hello'
      }
    }

****

Component names should always be multi-word, except for root level or built-in vue components(such as
<transition> or <component> etc). This recommendation is to prevent conflicts with existing and future HTML
elements, since all HTML elements are a single word.

Vue.component('user', { //bad approach
  // ...
})
Vue.component('user-profile', { //good approach
       // ...
     })

****

The best way to force Vue to re-render a component is to set a :key on the component. i.e, Whenever the component
to be re-rendered, just change the value of the key then Vue will re-render the component.
****

VueJS automatically detects the changes to ref's value and updates the DOM with a dependency-tracking based
reactivity system.

1. When a component is rendered for the first time, it tracks every ref that was used during the render.
2. Whenever a ref is mutated, it will trigger a re-render of the component.

Vue 2.0 does not have native support for the Composition API, but you can use it via the official plugin:
@vue/composition-api.

Let's see the usage in step-by-step instructions,

1. Install the Plugin

    npm install @vue/composition-api
    # or
    yarn add @vue/composition-api
    

1. Register the plugin in your main.js file,

     import Vue from 'vue';
     import VueCompositionAPI from '@vue/composition-api';

     Vue.use(VueCompositionAPI);

     new Vue({
       render: h => h(App),
     }).$mount('#app');
   

1. Using Composition API in Components

You can now use ref, reactive, computed, watch, onMounted, etc., in your Vue 2 components.

#### Example: Counter Component

     <template>
       <div>
         <p>Count: {{ count }}</p>
         <button @click="increment">Increment</button>
       </div>
     </template>

     <script>
     import { ref } from '@vue/composition-api';

     export default {
       setup() {
         const count = ref(0);

         const increment = () => {
           count.value++;
         };

         return {
           count,
           increment,
         };
       },
     };
     </script>
     

Note:

• The @vue/composition-api plugin is compatible with Vue 2.6+.
• It does not include all Vue 3 features (e.g., <script setup>, Suspense, Fragments).
• Great for gradual migration or using modern Vue patterns in Vue 2.

****

The Composition API in Vue 3 is a new way to organize and reuse logic in Vue components. It was introduced to
address some limitations of the Options API (used in Vue 2), especially when dealing with large or complex
components where logic can become scattered and hard to manage.

The Composition API is a set of APIs (e.g., setup(), ref, reactive, computed, watch, etc.) that allow
developers to compose component logic more flexibly and cleanly, especially when logic is reused across
components.

##### Key Concepts

1. The setup() function

The heart of the Composition API. It's called before the component is created, and it's the place where you
declare reactive state, computed properties, methods, and lifecycle hooks.

        <template>
          <button @click="increment">Count is: {{ count }}</button>
        </template>
        <script setup>
        import { ref } from 'vue'
    
        const count = ref(0)
        function increment() {
          count.value++
        }
        </script>
     

• ref() creates a reactive reference to a primitive value.
• count.value is how you access/update the value inside a ref.

1. Reactive State: ref() and reactive()

##### ref()

• Used for primitive values.
• Wraps a value in a reactive object.

         const message = ref('Hello');
         message.value = 'Hi';
         

##### reactive()

• Used for objects.
• Deeply reactive, making every property reactive.

         const user = reactive({  name: 'Alice',  age: 25 });
         user.age++;
         

1. Computed Properties

Creates values that automatically update when dependencies change.

   import { ref, computed } from 'vue';
    
   setup() {
     const firstName = ref('John');
     const lastName = ref('Doe');
     const fullName = computed(() => `${firstName.value} ${lastName.value}`);

     return { fullName };
   }
   

1. Watchers

Both watch() and watchEffect() are used to run side effects when data changes:

       watch(count, (newVal, oldVal) => {
       console.log('Count changed:', newVal);
       });
   
       watchEffect(() => {
       console.log('Count is now', count.value);
       });
     

1. Lifecycle Hooks

Composition API uses new lifecycle hooks imported from Vue.

       import { onMounted, onUnmounted } from 'vue';

       onMounted(() => { 
           console.log('Component is mounted'); 
       });

       onUnmounted(() => {
           console.log('Component is unmounted');
       });
      

1. Reusable Logic with Composables

You can encapsulate logic in composable functions and reuse across components:

Example: Reuse logic of counter using useCounter.js

       import { ref } from 'vue';
       export function useCounter(initialValue = 0) { 
         const count = ref(initialValue);
         const increment = () => count.value++;
         const decrement = () => count.value--;
         const reset = () => count.value = initialValue;
         return { count, increment, decrement, reset }; 
       }
     

Using it in a component

     <script setup>
      import { useCounter } from '@/composables/useCounter';
   
       const { count, increment, decrement, reset } = useCounter(5);
       </script>
   
       <template>
         <div>
           <p>Count: {{ count }}</p>
           <button @click="increment">+</button>
           <button @click="decrement">-</button>
           <button @click="reset">Reset</button>
         </div>
       </template>
     

****

The composition API provides several benefits over the traditional Options API as listed below.

1. Better logic reuse and code organization:
2. Better typescript support:
3. Easier testing
4. Smaller production bundles

****

Composition functions (commonly called Vue Composables) are functions that leverage Vue's Composition API to encapsulate, manage, and reuse stateful logic across components.

Composables are Vue's architectural equivalent to React Custom Hooks. By convention, their names start with use....

### Practical Composable Example:

// composables/useWindowSize.ts
import { ref, onMounted, onUnmounted } from 'vue';

export function useWindowSize() {
  const width = ref(window.innerWidth);
  const height = ref(window.innerHeight);

  function update() {
    width.value = window.innerWidth;
    height.value = window.innerHeight;
  }

  onMounted(() => window.addEventListener('resize', update));
  onUnmounted(() => window.removeEventListener('resize', update));

  return { width, height };
}

### Using It in Any Component:

<script setup>
import { useWindowSize } from '@/composables/useWindowSize';

const { width, height } = useWindowSize();
</script>

<template>
  <p>Viewport: {{ width }} x {{ height }}</p>
</template>

### Advantages over Mixins:

• Explicit Data Sources: You see exactly where every reactive variable originated.
• No Namespace Collisions: Destructured properties can be renamed easily (const { width: screenWidth } = useWindowSize()).
• Full TypeScript Support: Provides end-to-end type inference.

The <Teleport> component allows you to render a part of your component's template into a different location in
the DOM — outside the current component's hierarchy.

#### Example:

1. Wrap teleport component around the modal template inside app.vue file

<template>
 <div>
   <h1>Page Content</h1>
    
   <!-- This modal will render in the #modals element -->
   <teleport to="#modals">
     <div class="modal">
       <p>This is a modal rendered with teleport</p>
     </div>
   </teleport>
 </div>
</template>

1. Render modal component inside main index.html file

<body>
 <div id="app"></div>
 <div id="modals"></div> <!-- This is where the modal will appear -->
</body>

In Vue 2, this functionality was possible via the community plugin portal-vue

#### Use cases

1. Modals
2. Toasts
3. Tooltips
4. Context menus
5. Dropdowns
6. Anything that needs to break out of DOM nesting

****

The v-html directive is used to update the inner html of a DOM element with latest data. It is similar to
innerHTML property in DOM.

The example usage of this directive as shown below,

<div id="app">
   <div>{{ htmlContent }}</div>
   <div v-html="htmlContent"></div>
</div>

Note: This directive should be used with trused content only but not on user provided content. Otherwise it can
leads to XSS vulnerabilities.

****

| Feature | Vue 2 | Vue 3 |
| ------------------ | ------------------------- | ------------------------------------ |
| API Style | Options API | Composition API + Options API |
| Reactivity System | Object.defineProperty() | Proxy-based (faster & more reliable) |
| Performance | Moderate | Improved (~2x faster in some cases) |
| Tree-shaking | Limited | Fully tree-shakable |
| TypeScript Support | Partial | Built-in and first-class support |
| Fragment Support | Not supported | Supported |
| Teleport/Portals | Plugin-based | Native support |
| Suspense | Not available | Native support for async components |

****

The watch() function observes a reactive source((like ref, reactive, getter functions or computed values)) and
runs a callback function whenever that source changes.

Syntax:

 watch(source, callback, options ?) 

• source: a ref, reactive property, getter function, or array of them.
• callback(newValue, oldValue): function to run when the value changes.
• options (optional): object to control behavior (e.g., immediate, deep).

#### Example 1: Watching a ref

import {ref, watch} from 'vue';
    
const count = ref(0);
    
watch(count, (newVal, oldVal) => {
   console.log(`Count changed from ${oldVal} to ${newVal}`);
});
count.value = 5; // triggers the watch callback

#### Example 2: Watching a function (getter)

const firstName = ref('Sudheer');
const lastName = ref('Jonna');
    
watch(
   () => `${firstName.value} ${lastName.value}`,
   (newFullName, oldFullName) => {
       console.log(`Full name changed from ${oldFullName} to ${newFullName}`);
   }
);

#### Example 3: Watching a reactive object deeply

    import {reactive, watch} from 'vue';
   
    const user = reactive({name: 'Sudheer', age: 38});
   
    watch(
       () => user,
       (newUser, oldUser) => {
           console.log('User object changed:', newUser);
       },
       {deep: true}
    );

The main uses of watch() are:

1. Watching route changes
2. Triggering API calls
3. Responding to complex data changes
4. Manually reacting to specific state updates

****

In Composition API, lifecycle hooks are functions you call inside the setup() function to hook into different
stages of a component's lifecycle. These hooks replace the traditional options API lifecycle methods (like created(),
mounted(), etc.) with function-based hooks that are imported from Vue.

#### Main Lifecycle Hooks

| Lifecycle Stage | Composition API Hook | Description |
| ------------------------- | -------------------- | ------------------------------------------------------- |
| Before component creation | onBeforeMount() | Called right before the component is mounted |
| Component mounted | onMounted() | Called after the component has been mounted |
| Before update | onBeforeUpdate() | Called before the component updates the DOM |
| After update | onUpdated() | Called after the component updates the DOM |
| Before unmount | onBeforeUnmount() | Called right before the component is unmounted |
| Component unmounted | onUnmounted() | Called after the component is unmounted |
| Error captured | onErrorCaptured() | Called when an error from a child component is captured |
| Activated (keep-alive) | onActivated() | Called when a kept-alive component is activated |
| Deactivated (keep-alive) | onDeactivated() | Called when a kept-alive component is deactivated |

The above hooks can be imported from vue and used inside setup() function. For example, the usage of hooks will be
as follows,

   import {onMounted, onBeforeUnmount} from 'vue'

    export default {
    setup() {
        onMounted(() => {
            console.log('Component is mounted!')
        })

        onBeforeUnmount(() => {
            console.log('Component is about to unmount')
        })
    }
    }
   

****

Both watch and watchEffect are Composition API functions used for reactivity, but they serve different purposes and
behave differently.

watch: It is used to watch specific reactive sources (like ref or computed) and respond to changes. This
function is lazy. i.e, The callback is not called immediately but only on changes. It is useful when you need to
access both old and new values, and performs side-effects conditionally.

         watch(count, (newVal, oldVal) => {
            console.log('Count changed:', newVal);
         });
      

watchEffect: This function is automatic and eager. It runs a function immediately and tracks its dependencies
automatically. It cannot access old values directly.

      watchEffect(() => {
        console.log(`Count is ${count.value}`);
      });
      

****
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