React JS Interview Questions - 2025

What is React?

React is a JavaScript library developed by Facebook for building user interfaces, primarily for single-page applications. .

What is JSX?

JSX, or JavaScript XML, enables us to write HTML-like syntax within JavaScript, allowing it to function seamlessly as JavaScript code.

How does the Virtual DOM work?

The Virtual DOM is a lightweight copy of the actual DOM. When an object's state changes, React generates a new Virtual DOM, compares it with the previous version, and updates only the modified elements in the real DOM.

Explain the component lifecycle in React.

React components include various lifecycle methods, such as componentDidMount, shouldComponentUpdate, and componentWillUnmount, enabling code execution at specific stages of a component's lifecycle.

What are controlled components?

Controlled components are form elements, such as inputs and textareas, whose values are managed by the component's state. Any changes to their values are handled through functions.

Why do class methods need to be bound to a class instance, and how can you avoid the need for binding?

In JavaScript, the value of this varies based on the execution context. In React class component methods, developers typically expect this to reference the current component instance. To ensure this behavior, these methods need to be bound to the instance, usually within the constructor. For example:

class SubmitButton extends React.Component {

  constructor(props) {

    super(props);

    this.state = {

      isFormSubmitted: false

    };

    this.handleSubmit = this.handleSubmit.bind(this);

  }

  handleSubmit() {

    this.setState({

      isFormSubmitted: true

    });

  }

  render() {

    return (

      <button onClick={this.handleSubmit}>Submit</button>

    )

  }

}

What are the most common approaches for styling a React application?

CSS Classes

In React, class names can be assigned to components just as they are to DOM elements in HTML.

When transitioning from traditional web development to React, developers often rely on CSS classes for styling since they are already familiar with this method.

Inline CSS

Applying inline CSS to React elements ensures styles are fully scoped to a specific element using a familiar and straightforward approach. However, inline styles have limitations, such as the inability to style :hover pseudo-classes.

Pre-processors (Sass, Stylus, Less)

Pre-processors are commonly used in React projects because they follow the traditional CSS approach, making them easy for developers to adopt. They are particularly useful when integrating React into an existing application with established styling practices.

CSS-in-JS Modules (Styled Components, Emotion, Styled-jsx)

CSS-in-JS solutions are a popular choice for styling React applications due to their seamless integration with React components. They enable dynamic styling based on React props at runtime and typically scope styles to individual components by default.

What is prop drilling and how can you avoid it?

Managing Deeply Nested Data in React

When developing a React application, there are scenarios where a deeply nested component needs access to data from a higher-level component.

Consider the following component structure:

<EditUsersPage /> maintains selectedUserAddress in its state and renders a <User /> component.

<User /> renders a <UserDetails /> component.

<UserDetails /> renders a <UserAddress /> component.

<UserAddress /> requires access to selectedUserAddress from <EditUsersPage />.

Prop Drilling

A straightforward way to pass selectedUserAddress down the hierarchy is by forwarding it as a prop through each intermediate component. This approach, known as prop drilling, ensures that the necessary data reaches <UserAddress />.

However, prop drilling can lead to unnecessary complexity. Components such as <User /> and <UserDetails />, which do not directly need selectedUserAddress, are forced to handle it, making the code harder to manage and maintain.

Using React Context

To avoid prop drilling, React context provides a more efficient solution. By defining a Provider component at a higher level, nested components can directly access the required data using either a Consumer component or the useContext hook.

Context and State Management

While context can be used independently to share global state, it can also work alongside state management libraries like Redux, which offer additional features for managing complex application state efficiently.