Q831. How do you check an object is a promise or not?
If you don't know if a value is a promise or not, wrapping the value as `Promise.resolve(value)` which returns a promise ```javascript function isPromise(object) { if (Pr...
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Q832. What is the main purpose of constructor?
The constructor is mainly used for two purposes, 1. To initialize local state by assigning object to this.state 2. For binding event handler methods to the instance For e...
Q833. How to detect if a function is called as constructor?
You can use `new.target` pseudo-property to detect whether a function was called as a constructor(using the new operator) or as a regular function call. 1. If a construct...
Q834. Is it mandatory to define constructor for React component?
No, it is not mandatory. i.e, If you don’t initialize state and you don’t bind methods, you don’t need to implement a constructor for your React component.
Q835. What are the differences between arguments object and rest parameter?
There are three main differences between arguments object and rest parameters 1. The arguments object is an array-like but not an array. Whereas the rest parameters are a...
Q836. Why should not call setState in componentWillUnmount?
You should not call `setState()` in `componentWillUnmount()` because once a component instance is unmounted, it will never be mounted again.
Q837. What are the differences between spread operator and rest parameter?
Rest parameter collects all remaining elements into an array. Whereas Spread operator allows iterables( arrays / objects / strings ) to be expanded into single arguments/...
Q838. What is the purpose of getDerivedStateFromError?
This lifecycle method is invoked after an error has been thrown by a descendant component. It receives the error that was thrown as a parameter and should return a value ...
Q839. What are the different kinds of generators?
There are five kinds of generators, 1. **Generator function declaration:** ```javascript function* myGenFunc() { yield 1; yield 2; yield 3; } const genObj = myGenFunc(); ...
Q840. What is the methods order when component re-rendered?
An update can be caused by changes to props or state. The below methods are called in the following order when a component is being re-rendered. 1. static getDerivedState...