10 Most Important React.js Interview Questions [2022 Updated]

React.js is a very dominant JavaScript library for User Interface. This library was initially used by Meta(Facebook) and later on it became an open source library which is used by many developers and Big Enterprises companies. This library is supported by a Big community where every developer contributes and adds a new feature(version). If you want to check the latest version then you can follow this link https://reactjs.org/versions

Today, this library has become much popular and also widely used due to various advantages provided by react.js.

In this blog, I will be covering the top 10 most important React.Js interview questions.

What is React.js?

It is an open-source free front-end Javascript library for building user interfaces for web and mobile applications. It helps you to create collective UI’s. React can also be used in the SPA(single page application), mobile or server rendering frameworks such as Next.js

What Are Some Features of React.js?

• JSX: It is a syntax extension of Javascript. React uses React to describe what the user interface should look like. JSX is closer to JavaScript instead to HTML. ReactDOM uses camelCase property naming. e.g. class becomes ClassName.
• Components: It is the building block of React elements. using components lets you split the UI into independent, reusable pieces. components are like JS functions.
• Virtual DOM: is a “virtual” representation of UI that is kept in memory and synchronized with the “real DOM” by a library using ReactDOM.
• One-way data-binding: React uses one-way data binding. In one-way data-binding one of the following conditions can be followed:
  • Component to View: any change in component data would get reflected in the view.
  • View to Component: any change in view would get reflected in the component’s data.
• High performance: React updates only those components that have changed rather than updating the whole component. This leads to react.js working faster. Also, react uses virtual DOM which makes the app runs faster.

What Is the Difference Between Real DOM and React’s Virtual DOM?

Real DOM

• DOM stands for “Document Object Model” and is also an application programming interface for manipulating HTML documents.
• It represents an HTML document as a tree of nodes. DOM also provides you features to add, remove, update and delete the parts of the document.
• In the DOM, all the HTML documents are Objects and the programming interface is the properties and methods of each object. (properties – is the value that changes the content of HTML elements, method – is the action that we performed like add or delete)

React Virtual DOM

• React use the virtual DOM and it is a lightweight copy of the actual DOM.(i.e it is a virtual representation of the DOM). So, every object that is present in the original DOM, is also present in React Virtual DOM.
• The Virtual DOM is an abstraction of the HTML DOM. It is lightweight and detached from the browser-specific implementation details. Since the DOM itself was already an abstraction, the virtual DOM is, in fact, an abstraction of an abstraction.

What Is the Difference Between React Component State and Props?

State	Props
In-state, we cannot pass data from one component to another component.	In props, data can be passed from one component to another component.
In-state, we can modify the data.	Props cannot be modified.
In-state, both read and write operations can be done.	In props, are read-only.
States are mutable.	Props are immutable.

Explain the Reconciliation Process in React

Reconciliation is the process through which react updates the Real DOM(browser DOM). It uses the diffing algorithm to update the react component faster.

These two important concepts are behind the working of the reconciliation process: –

• Virtual DOM: React renders JSX components to the Browser DOM and keeps the copy of the real DOM. This copy is the virtual DOM.
• Diffing Algorithm: React uses the diffing algorithm to update the react nodes.

What Are Hooks in React?

React version 16.8 introduces the feature “Hooks”. A hook is a function that lets you “hook into” React features. Using these features you can able to use state and other react features without writing class components. Hooks are the JavaScript function. Functional Components were stateless components. but, know hooks came into existence to help with the state and props.

While using hooks you need to follow a few rules:

• Don’t call Hooks inside loops, conditions or nested functions.
• Hooks can be used only in functional components.

What is the Life Cycle of a React.js Class Component?

Each component in react goes through the life cycle of React which keeps monitoring and updating the component. The lifecycle of the component is divided into four phases.

1. Mounting: – Mounting is the first phase in the life cycle. Mounting means putting the elements into the DOM. It has four built-in methods that are used during mounting.

constructor:-

• The constructor for react component is used before it is mounted.
• In class component, while implementing the constructor for a React. component subclass, always you should use super(props) before any other statement.
• Otherwise, this.props will be undefined in the constructor.

getDerivedStateFromProps:- getDerivedStateFromProps() the method is called right before rendering the element(s) in the DOM.

render():- The render() a method is the only required method in a class component. render() should always be pure, which means it does not modify the component state, it returns the same result every time it gets called.

componentDidMount():- componentDidMount() gets invoked immediately after the component gets inserted (mounted).

2. Updating:- In this phase when the component gets rendered whenever there is a change in the component’s state or properties.

React has five built-in methods that get called, in this order, when a component is updated:

getDerivedStateFromProps():- getDerivedStateFromProps() are invoked right before invoking the render method, both on the initial mount and on subsequent updates.

shouldComponentUpdate():- shouldComponentUpdate() let’s react to know if a component’s output is not affected by the current change in state or props.

render(): – render() the method is of course called when a component gets updated, it has to re-render the HTML to the DOM, with the new changes.

getSnapshotBeforeUpdate(): –getSnapshotBeforeUpdate() is invoked right before the recently rendered output is done.

componentDidUpdate(): –componentDidUpdate() is invoked after the updating occurs in the DOM.

3. Updating: -in this phase, when a component is removed from the DOM or unmounted then React calls it.

componentWillUnmount():- componetWillUnmount() is called immediately before the component is unmounted or removed.

What Is PureComponent in React?

Based on the concept of purity in function programming paradigms, a function is said to be pure if it meets the following two conditions:

• Its return values are only determined by its input values.
• Its return values are always the same for the same input values.

React Component is considered pure, it renders the same output for the same state and props.

What Are a Few Differences Between a Class and Functional Component?

Functional Components	Class Components
Functional components are basic JavaScript functions. These are typically arrow functions but can also be created with the regular function keyword.	Class components make use of the ES6 class and extend the Component class in React.
Earlier referred to as stateless components as they simply accept data and display them in some form; that is they are mainly responsible for rendering UI. Now they can also be stateful thanks to hooks.	Sometimes called stateful components as they tend to implement logic and state.
React lifecycle methods (for example, componentDidMount()) cannot be used in functional components. It hooks like useEffect have to be used.	React lifecycle methods can be used inside class components (for example, componentDidMount()).
There is no render method used in functional components. Whatever is returned from the function is the component code.	We pass props down to class components and access them with this.props.
Functional components should be favoured moving forward.

What is Higher-order-Component (HoC) in React?

• It is a function that takes a component and returns a new component.
• It is an advanced technique in React for reusing the component logic.
• They are the pattern that emerges from React’s compositional nature. The component transforms props into UI, and a higher-order component converts a component into another component.

Conclusion