7 Most Asked ReactJS Interview Questions & Answers

If you're a developer, you likely already know how widely React has taken over the development world. As one of the most popular JavaScript libraries, React has become the go-to solution for building front-end applications. Regardless of whether you're a seasoned developer or a beginner, gaining expertise in React can significantly boost your career prospects.

In this article, we have picked the 7 most asked ReactJS Interview Questions & Answers that will help you to understand the core concepts of React and to ace any interview.

1. How does the Virtual DOM work in React?

The Virtual DOM is a lightweight, in-memory representation of the real DOM, enabling React to optimize UI updates. It acts as an intermediary, reducing costly direct manipulations of the browser’s DOM, which is slow and resource-intensive.

How It Works:

1. Initial Render: When a component renders, React creates a Virtual DOM tree mirroring the real DOM’s structure.
2. State/Props Change: When a component’s state or props change, React generates a new Virtual DOM tree.
3. Diffing Algorithm: React compares the new Virtual DOM with the previous one (a process called reconciliation) to identify differences.
4. Minimal Updates: Only the changed parts are updated in the real DOM, minimizing performance overhead.
5. Batched Updates: React batches multiple state changes to optimize rendering efficiency.

Example:

import { useState } from 'react';

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

  return (
    <div>
      <p>Count: {count}</p>
      <button onClick={() => setCount(count + 1)}>Increment</button>
    </div>
  );
}

When count changes, React updates only the <p> element’s text in the real DOM, not the entire component.

Key Points:

• The Virtual DOM improves performance by reducing direct DOM manipulations.
• Reconciliation ensures efficient updates through diffing.
• For more details, see How React Works? and Virtual DOM.

2. What is JSX?

JSX (JavaScript XML) is a syntax extension for JavaScript used in React to write HTML-like code within JavaScript. It simplifies UI development by allowing developers to define component structures declaratively.

Features:

• HTML-like Syntax: JSX resembles HTML but is processed as JavaScript.

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

• JavaScript Expressions: Embed dynamic values using curly braces {}.

  const name = 'Ajay';
  const element = <h1>Hello, {name}!</h1>;
  

• Compiles to React.createElement: JSX is transpiled (by tools like Babel) into React.createElement calls, creating React elements.

  // JSX
  const element = <h1>Hello, world!</h1>;
  // Transpiled
  const element = React.createElement('h1', null, 'Hello, world!');
  

Key Points:

• JSX improves readability and maintainability over raw JavaScript.
• It requires a transpiler like Babel to convert to valid JavaScript.
• Always return a single root element in JSX.
• For more details, see React JSX.

3. What is ReactDOM, and what is the difference between ReactDOM and React?

ReactDOM is a separate package that bridges React’s Virtual DOM with the browser’s real DOM. It provides methods to render, update, and remove React components in the DOM.

Key Functions:

• Rendering Components: ReactDOM.createRoot and root.render mount components to a DOM node.
• Updating the DOM: ReactDOM applies Virtual DOM changes to the real DOM efficiently.
• Unmounting: ReactDOM.unmountComponentAtNode removes components from the DOM.

Difference Between ReactDOM and React:

Example (ReactDOM):

import ReactDOM from 'react-dom/client';
import App from './App';

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

Key Points:

• React handles component logic; ReactDOM handles DOM integration.
• React 18 introduced createRoot for concurrent rendering, replacing ReactDOM.render.
• For more details, see ReactDOM and React vs ReactDOM.

4. What is the difference between a Class Component and a Functional Component?

React supports two types of components: Class Components and Functional Components. Functional components with hooks are now the standard, but class components are still used in legacy code.

Comparison Table:

Functional Component: A functional component is a simpler function that returns JSX. Before React 16.8, functional components were stateless, but with hooks, they can now manage state and side effects.

import React, { useState } from "react";

const Welcome = () => {

    const [message, setMessage] = useState("Hello, World!");

    return (
        <div>
            <h1>{message}</h1>
            <button onClick={() => setMessage("Hello, React!")}>
                Change Message
            </button>
        </div>
    );
};

export default Welcome;

Class Component: A class component is created using the class keyword and extends React.Component. It requires a render() method to return JSX.

import React, { Component } from 'react';

class Welcome extends Component {
    constructor(props) {
        super(props);

        this.state = {
            message: 'Hello, World!'
        };
    }

    render() {
        return (
            <div>
                <h1>{this.state.message}</h1>
                <button onClick={() => this.setState({ message: 'Hello, React!' })}>
                    Change Message
                </button>
            </div>
        );
    }
}

export default Welcome;

The output for both Class component and functional component are same

In this code

1. Functional Component (Using useState Hook):

• useState is used to manage the message state.
• The button updates the message when clicked, changing it from "Hello, World!" to "Hello, React!".

2. Class Component

• The component uses this.state to manage the message state.
• The button updates the state using this.setState, changing the message in the same way as the functional component.

5. What is the Difference Between State and Props?

Here is the difference between state and props.

For more details follow this article => What are the differences between props and state ?

6. What is the Higher-Order Component?

A Higher-Order Component (HOC) is a pattern in React used to enhance or modify a component by wrapping it with another component.

• HOC is a function that takes a component and returns a new component with added functionality or behavior.
• It’s commonly used for cross-cutting concerns like authentication, fetching data, or adding styling.
• Example: Adding extra functionality (like logging) to an existing component without modifying the original component.

import React from 'react';

function withLogging(Component) {
    return function (props) {
        console.log('Rendering component with props:', props);
        return <Component {...props} />; 
    };
}

export default withLogging;

import React from 'react';

const MyComponent = (props) => {
    return <div>{props.message}</div>;
};

export default MyComponent;

import React from "react";
import MyComponent from "./MyComponent";
import withLogging from "./withLogging";

const ComponentLogging = withLogging(MyComponent);

const App = () => {
    return (
        <div>
            <ComponentLogging message="Hello, World!" />
        </div>
    );
};

export default App;

Output

In this code

• withLogging.js: A HOC that logs props and returns a new component.
• MyComponent.js: Displays a message passed as a prop.
• App.js: Wraps MyComponent with withLogging to log props and display the message.

For more details follow this article => React.js Higher-Order Components

7. What is Redux?

Redux is a great way to store the entire application's state in a single store. When your application is small, you wouldn't be facing issues in handling the state. But when it starts growing you will find that state in various components is becoming unmanageable. Here Redux solves your problem. 

Redux mainly works on three components

• Action: Actions are payloads of information that send data from the application to the store. Actions are the only source of information for the store. We send them to the store using the store.dispatch().
• Reducer: Reducer specifies how the applications' state changes in response to actions sent to the store. Actions describe what happened, but it doesn't describe how the application's state changes. Basically, a reducer determines how the state will change to action.
• Store: Store objects bring the action and reducer together. You can access the state via getState(); It allows the state to be updated via dispatch (action);

Store contains JavaScript objects. You can change the state by firing actions from your application. After that, you can write reducers for these actions and modify the state. The whole transition is kept inside the reducer, and it should not have any side effects. 

import React from 'react';
import { Provider, useDispatch, useSelector } from 'react-redux';
import store from './store'; // Import the Redux store

const Counter = () => {
    const dispatch = useDispatch(); // Dispatch actions to the store
    const count = useSelector((state) => state.count); // Get count from Redux state

    return (
        <div>
            <h1>Count: {count}</h1>
            <button onClick={() => dispatch({ type: 'INCREMENT' })}>Increment</button>
            <button onClick={() => dispatch({ type: 'DECREMENT' })}>Decrement</button>
        </div>
    );
};

const App = () => (
    <Provider store={store}>
        <Counter />
    </Provider>
);

export default App;

import { createStore } from 'redux';

// Reducer function to manage the state
const initialState = { count: 0 };

const counterReducer = (state = initialState, action) => {
    switch (action.type) {
        case 'INCREMENT':
            return { count: state.count + 1 };
        case 'DECREMENT':
            return { count: state.count - 1 };
        default:
            return state;
    }
};

// Create the Redux store
const store = createStore(counterReducer);

export default store;

Output

In this code

• Store: A Redux store is created with a counterReducer to manage a count state.
• Provider: Provider from react-redux makes the Redux store available to components.
• Counter Component: useSelector fetches the count from the Redux store and useDispatch dispatches INCREMENT and DECREMENT actions to update the state.

For more details follow this article => What is Redux

Conclusion

React has become a leading JavaScript library for building modern front-end applications, offering a component-based architecture and efficient rendering through the Virtual DOM. Understanding core concepts such as State vs. Props, Class vs. Functional Components, and Higher-Order Components (HOCs) is essential for mastering React. Additionally, tools like Redux provide powerful state management for larger applications