React JS Interview Questions

Top React JS Interview Questions with Answers

Author: Vishal Thakur

Published: August 2021

React JS is a popular JavaScript library for building user interfaces. Below are some of the most frequently asked React JS interview questions with detailed answers to help you prepare for your next interview.

1. What is Virtual DOM?

Answer: The Virtual DOM is an abstraction of the HTML DOM. It allows React to render subtrees of nodes based on state changes with minimal DOM manipulation, keeping components up to date efficiently.

The DOM (Document Object Model) is an in-memory representation of HTML. React uses the Virtual DOM to optimize updates and improve performance by reducing direct DOM manipulations.

2. What are the Differences Between Functional and Class Components?

Answer: Before the introduction of Hooks, functional components were stateless and lacked features compared to class components. With Hooks, functional components can now manage state and lifecycle events, making them equivalent to class components.

Functional Component Example:

            function MyFunc(props) {
                return (
                    <div className="main-container">
                        <h2>Title of the MyFunc</h2>
                    </div>
                );
            }
        

Class Component Example:

            class MyFunc extends React.Component {
                constructor(props) {
                    super(props);
                }
                render() {
                    return (
                        <div className="main-container">
                            <h2>Title of the MyFunc</h2>
                        </div>
                    );
                }
            }
        

3. What are Hooks in React JS?

Answer: Hooks are a feature introduced in React 16.8 that allow functional components to use state and lifecycle methods. Basic Hooks include useState, useEffect, and useContext.

Example of useState Hook:

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

4. What is JSX?

Answer: JSX stands for JavaScript XML. It allows developers to write HTML-like syntax in JavaScript, making it easier to create React elements.

Example:

const element = <h1>Hello, World!</h1>;

5. What are Controlled and Uncontrolled Components?

Answer:

Controlled Components: The component's state is managed by React. Changes are handled via event callbacks like onChange.
Uncontrolled Components: The component's state is managed by the DOM. React does not control the input value directly.

6. What are the Lifecycle Methods in React JS?

Answer: React components go through three phases: Mounting, Updating, and Unmounting. Key lifecycle methods include:

Mounting: constructor, render, componentDidMount
Updating: shouldComponentUpdate, render, componentDidUpdate
Unmounting: componentWillUnmount

7. What is Strict Mode in React JS?

Answer: Strict Mode is a tool for highlighting potential problems in an application. It performs additional checks and warns about unsafe lifecycle methods, legacy APIs, and other issues.

Example:

            ReactDOM.render(
                <React.StrictMode>
                    <App />
                </React.StrictMode>,
                document.getElementById('root')
            );
        

Hope this will help! Kindly share your feedback so that I can improve my blogs.

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