React JS Interview Questions and Answers

February 21, 2025

Top 40 Most Important React JS Interview Questions

Practice the Top 40 Most Asked React JS Interview Questions and Answers on this page.

React JS is one of the most popular JavaScript libraries for building user interfaces, especially for web applications. If you’re preparing for a React JS interview, it’s important to understand key concepts like components, state, props, hooks, and lifecycle methods.

In this article, we’ll go through some commonly asked React JS Interview Questions along with clear and simple answers. Whether you’re a beginner or an experienced developer, these questions will help you refresh your knowledge and boost your confidence before the interview.

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Top 40 React JS Interview Questions

Ques 1: What is ReactJS?

Answer.

  1. ReactJS is an open-source JavaScript library developed by Facebook for building user interfaces, particularly single-page applications where data changes over time.
  2. It allows developers to create large web applications that can update and render efficiently in response to data changes.

Ques 2. What are the key features of ReactJS?

Answer.

  • Virtual DOM: Enhances performance by minimizing direct DOM manipulations.
  • JSX: A syntax extension that combines JavaScript and HTML-like elements.
  • Component-Based Architecture: Encourages reusable and maintainable code by dividing the UI into independent components.
  • Unidirectional Data Flow: Simplifies data management and debugging.

Ques 3. Explain the concept of Virtual DOM in React.

Ans.

Answer:

  • The Virtual DOM is an in-memory representation of the real DOM elements generated by React components.
  • When a component’s state changes, a new Virtual DOM is created and compared to the previous one using a process called “reconciliation.”
  • Only the differences are then updated in the real DOM, leading to efficient rendering and improved performance.

Ques 4. What is JSX?

Answer:

JSX stands for JavaScript XML. It is a syntax extension for JavaScript that allows developers to write HTML-like code within JavaScript files.

JSX makes the code easier to understand and debug.

import React from 'react';

function WelcomeMessage() {
  return < h1>Welcome to Prepsters!!!</h1>
; } export default WelcomeMessage;

Ques 5. How do you create a React component?

Answer:

React components can be created as either function components or class components.

1. Function Component Example:

import React from 'react';
function Greeting() {
  return
< h1>Hello, Prepsters!</h1>

; } export default Greeting;

2. Class Component Example:

import React, { Component } from 'react';

class Greeting extends Component {
  render() {
    return < h1>Hello, Prepsters!</h1>
; } } export default Greeting;

Ques 6. What are props in React?

Answer:
  • Props (short for properties) are read-only attributes passed from a parent component to a child component.
  • They allow data to flow between components and enable component reusability.
  
import React from 'react';

function UserGreeting(props) {
  return < h1>Hello, {props.name}!;
}

// Usage
< UserGreeting name="Alice" />
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Ques 7. What is state in React?

Answer:

  • State is an object managed within a component that holds dynamic data influencing the component’s rendering and behavior.
  • Unlike props, state is mutable and can change over time, usually in response to user actions.
import React, { useState } from 'react';
function Counter() {
const [count, setCount] = useState(0);

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

export default Counter;

Ques 8.Differentiate between state and props.

Answer:

1. State:

  1. Managed within the component.
  2. Mutable and can change over time.
  3. Used for dynamic rendering.

2. Props:

  1. Passed from parent to child components.
  2. Immutable; cannot be modified by the receiving component.
  3. Used to pass data and event handlers to child components.

Ques 9. What are React Hooks?

Answer:

Introduced in React 16.8, Hooks are functions that let developers use state and other React features in function components without writing class components.

Common Hooks:

  • useState: Manages state in function components.
  • useEffect: Performs side effects in function components.
  • useContext: Accesses context values without wrapping components.

Ques 10. Explain the useState Hook with an example.

Answer:

The useState Hook allows you to add state to function components. It returns an array with two elements: the current state value and a function to update it.

import React, { useState } from 'react';

function ToggleButton() {
const [isOn, setIsOn] = useState(false);

const toggle = () => setIsOn(!isOn);

return (
<button onClick={toggle}>
{isOn ? 'ON' : 'OFF'}
</button>
);
}

export default ToggleButton;

Top 40 React JS Interview Questions with Answers

Ques 11. What is the useEffect Hook?

Answer:

The useEffect Hook lets you perform side effects in function components, such as data fetching, subscriptions, or manually changing the DOM. It runs after the first render and after every update by default.

import React, { useState, useEffect } from 'react';

function DataFetcher() {
const [data, setData] = useState([]);

useEffect(() => {
fetch('https://api.example.com/data')
.then(response => response.json())
.then(data => setData(data));
}, []); // Empty dependency array ensures this runs only once

return (
<ul>
{data.map(item => (
<li key={item.id}>{item.name}</li>
))}
</ul>
);
}

export default DataFetcher;

Ques 12. How does the Virtual DOM improve performance?

Answer:

  1. Virtual DOM enhances performance by reducing direct manipulations of the real DOM, which are typically slow operations.
  2. When a component’s state changes, React creates a new Virtual DOM and compares it to the previous one (a process called “reconciliation”).
  3. It then updates only the parts of the real DOM that have changed, minimizing the number of manipulations and leading to faster updates.

Ques 13. What is the importance of keys in React lists?

Answer:

Keys are used in React to help identify unique elements in a list. They improve performance by allowing React to track changes efficiently when rendering dynamic lists.

Without keys, React may incorrectly update or reorder list items.

import React from 'react';

const users = [
{ id: 1, name: 'Alice' },
{ id: 2, name: 'Bob' },
];

function UserList() {
return (
<ul>
{users.map(user => (
<li key={user.id}>{user.name}</li> // Unique key ensures efficient re-rendering
))}
</ul>
);
}

export default UserList;

Ques 14. What is the difference between controlled and uncontrolled components?

Answer:

 

Controlled ComponentUncontrolled Component
State is managed by React using useState or this.stateState is managed by the DOM itself
Uses value prop and onChange eventUses ref to access input value
More predictable behaviorLess predictable, useful in specific cases

Ques 15. What is React Context API?

Answer:

React Context API is a built-in state management feature that allows data to be shared globally across components without manually passing props at every level (prop drilling).

It is useful when many components need access to the same global state, such as theme settings, authentication status, and user preferences.

Ques 16. What are synthetic events in React?

Answer:

Synthetic Event in React is a wrapper around the native browser event. It normalizes events across different browsers, ensuring consistent behavior.

React’s synthetic events wrap native events like click, keydown, focus, etc., making them cross-browser compatible.

  • handleClick function logs details about the event.
  • event is not the native browser event, but a React SyntheticEvent.
  • React automatically pools events for performance optimization.
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Ques 17: Why is there a need to use keys in Lists?

Answer:

  • Keys in React lists help identify which items have changed, been added, or removed.

  • They optimize rendering by allowing React to track elements efficiently, avoiding unnecessary re-renders and UI inconsistencies.

  • Without keys, React may re-render elements incorrectly, causing UI bugs.

  • The best practice is to use unique IDs as keys rather than array indices, as index keys can cause issues when items are added, removed, or reordered.

Ques 18: How do you create forms in React?

Answer:

Forms in React are created using controlled components, where form elements like inputs, textareas, and selects are controlled by React state. The useState hook is commonly used to handle form data.

  • value attribute is set to state (formData.name and formData.email).
  • onChange event updates state using setFormData.
  • handleSubmit function prevents default form submission and logs form data.
  • This ensures a controlled form where React manages input values.
import React, { useState } from "react";

function ContactForm() {
const [formData, setFormData] = useState({ name: "", email: "" });

function handleChange(event) {
setFormData({ ...formData, [event.target.name]: event.target.value });
}

function handleSubmit(event) {
event.preventDefault();
console.log("Form Submitted:", formData);
}

return (
<form onSubmit={handleSubmit}>
<label>
Name:
<input type="text" name="name" value={formData.name} onChange={handleChange} />
</label>
<label>
Email:
<input type="email" name="email" value={formData.email} onChange={handleChange} />
</label>
<button type="submit">Submit</button>
</form>
);
}

export default ContactForm;

Ques 19: What is useEffect, and how does it differ from lifecycle methods in class components?

Answer:

The useEffect hook in React handles side effects in functional components, replacing class lifecycle methods. It runs after render and can mimic:

  • componentDidMount → useEffect(() => {...}, []) (runs once)
  • componentDidUpdate → useEffect(() => {...}, [dependencies]) (runs on updates)
  • componentWillUnmount → Cleanup inside useEffect(() => {...}, [])

It simplifies lifecycle management and runs only when dependencies change.

Ques 20: What is Memoization in React?

Answer:

Memoization in React is an optimization technique that caches computed values to prevent unnecessary re-computations or re-renders.

Memoization Techniques in React

  1. React.memo(Component) → Memoizes functional components to avoid re-rendering if props don’t change.
  2. useMemo(callback, [dependencies]) → Memoizes computed values.
  3. useCallback(callback, [dependencies]) → Memoizes function references to prevent unnecessary re-creations.

Top 40 React JS Interview Questions With Answer

Ques 21: What is a dispatcher?

Answer:

In React, a dispatcher is an internal mechanism that handles state updates and schedules re-renders for components. It is primarily used in React’s Hooks system, ensuring that useState, useReducer, and other hooks work correctly.

How the Dispatcher Works

  • When a hook like useState is called, the dispatcher registers the state and returns a function to update it.
  • When the update function is called, the dispatcher triggers a re-render with the new state.

Ques 22: What is the use of render() in React?

Answer:

In React, the render() method is used in class components to return JSX that defines the component’s UI. It is required in class components and must be a pure function (returning the same output for the same state and props).

Ques 23: Explain the lifecycle methods of components.

Answer:

React class components have 3 lifecycle phases:

1. Mounting (Component Creation)

  • constructor() → Initializes state.
  • render() → Returns JSX.
  • componentDidMount() → Runs after first render (API calls, subscriptions).

2. Updating (Re-rendering)

  • shouldComponentUpdate() → Controls re-rendering.
  • render() → Updates UI.
  • componentDidUpdate() → Runs after state/props change (side effects).

3. Unmounting (Component Removal)

  • componentWillUnmount() → Cleanup tasks (remove listeners, clear timers).

Ques 24: Explain React fragments.

Answer:

  • React Fragments allow grouping multiple elements without adding extra DOM nodes. They help keep the DOM structure clean and improve performance by avoiding unnecessary wrapper elements like <div>.

    Fragments are useful when returning multiple elements from a component, especially in lists where extra elements might interfere with styling or layout. They come in two syntaxes:

    1. Short Syntax: <>...</> (cannot take attributes).
    2. Full Syntax: <React.Fragment>...</React.Fragment> (allows key attribute for lists).

    Using fragments improves efficiency and prevents unwanted nesting in the DOM.

Ques 25: Describe the lifting state up in React.

Answer:

Lifting State Up in React is a pattern used to share state between sibling components by moving the state to their nearest common parent. This ensures that multiple components can access and update shared data.

Why Lift State Up?

  • Avoids redundant state in multiple components.
  • Ensures data consistency across the app.
  • Enables better communication between child components.

How It Works:

    1. Move the state from child components to a common parent.
    2. Pass the state down as props to child components.
    3. Update the state using callback functions from children.

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Ques 26: Tell us about Strict mode.

Answer:

Strict Mode in React is a development-only feature that helps identify potential problems in an application. It does not render any visible UI but activates additional checks and warnings for components.

Key Benefits of Strict Mode

  • Detects unsafe lifecycle methods in class components.
  • Warns about deprecated API usage.
  • Identifies side effect issues in useEffect.
  • Helps find accidental re-renders by double-invoking certain functions in development mode.
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Ques 27: What is Redux?

Answer:

Redux is a state management library for JavaScript applications, commonly used with React. It helps manage global state in a predictable way using a unidirectional data flow.

Concepts in Redux:

  1. Store → A single JavaScript object that holds the global state.
  2. Actions → Plain objects describing what should change in the state.
  3. Reducers → Pure functions that take the current state and an action, then return a new state.
  4. Dispatch → A function that sends actions to the reducer to update the state.
  5. Selectors → Functions to extract specific data from the store.

Ques 28. Difference Between Context API and Redux.

Answer:
Context API Redux
Passes state down the component tree without prop drilling. Manages complex global state with predictable updates.
Lightweight, suitable for small-scale state sharing. Centralized and structured, ideal for large applications.
Can cause unnecessary re-renders if not optimized. Optimized with reducers and middleware for efficient updates.
Directly provides state via useContext. Uses actions, reducers, and dispatch for structured state updates.
Minimal setup (useContext + useState). Requires setup with store, reducers, and actions.
No built-in middleware support. Supports middleware like Redux Thunk and Redux Saga.
Best for simple or localized state management. Best for large applications with complex state logic.

Ques 29: How do you style React components?

Answer:

In React, you can style components using:

  1. Inline Styles → Directly applied using the style attribute ({{ color: "blue" }}). Quick, but hard to maintain.
  2. CSS Stylesheets → External .css files (import "./styles.css"). Global styling, but can cause conflicts.
  3. CSS Modules → Scoped styles using .module.css (import styles from "./styles.module.css"). Prevents conflicts.
  4. Styled Components → CSS-in-JS using styled-components. Supports dynamic styling.
  5. Tailwind CSS → Utility-first CSS framework (className="text-blue-500"). Fast and scalable.

Ques 30: Explain the use of CSS modules in React.

Answer:

CSS Modules in React provide scoped styling by ensuring that CSS class names are locally scoped to the component, preventing conflicts with global styles.

  • Each component gets unique class names to avoid collisions.
  • CSS files use the .module.css extension.
  • Imported as a JavaScript object in the component.

Example

styles.module.css

.heading {
color: blue;
font-size: 24px;
}

Component File (Example.js)

import styles from "./styles.module.css";

function Example() {
return <h1 className={styles.heading}>Hello, React!</h1>;
}

export default Example;

Benefits of CSS Modules

  • Scoped Styles: Prevents global class name conflicts.
  • Better Maintainability:  Each component has its own styles.
  • Works with CSS Preprocessors like SASS.

React JS Interview Questions

Ques 31: Explain conditional rendering in React.

Answer:

Conditional rendering in React means displaying different UI elements based on certain conditions, just like regular JavaScript conditions. It allows you to show or hide components, manage authentication, control permissions, or display API data dynamically.

Ways to Implement Conditional Rendering:

  1. If-Else Statements – Useful for small to medium applications.
  2. Ternary Operator (? :) – A shorter alternative to if-else.
  3. Element Variables – Helps keep the code cleaner and more readable.
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Ques 32: Do Hooks cover all the functionalities provided by the classes?

Answer:

Hooks aim to replace class components, but they do not yet cover all functionalities. Some lifecycle methods still lack direct Hook equivalents, such as:

  • getSnapshotBeforeUpdate() – Captures DOM information before an update.
  • getDerivedStateFromError() – Handles state updates when an error occurs.
  • componentDidCatch() – Catches errors in child components.

Additionally, some third-party libraries may not yet fully support Hooks, but compatibility is improving over time.

Ques 33: What are Higher Order Components(HOC)?

Answer:

  • Higher-Order Component (HOC) in React is a function that takes a component as an argument and returns a new component with additional functionality.
  • It is a pattern used for reusing component logic without modifying the original component.
  • Used for code reusability (e.g., authentication, logging, styling).
  • Do not modify the original component but wrap it.
  • Commonly used in libraries like Redux’s connect().

Ques 34: Explain the difference between server side rendering and client side rendering in React.

Answer:
Server-Side Rendering (SSR) Client-Side Rendering (CSR)
Processed on the server, and a fully rendered page is sent to the browser. Processed in the browser, JavaScript loads and renders content dynamically.
Faster initial load as HTML is ready. Slower initial load as JavaScript fetches and renders content.
Better SEO as search engines can easily crawl content. Weaker SEO unless using techniques like prerendering or hydration.
Good for content-heavy pages, as processing is done before reaching the client. Good for interactive apps but may suffer from slow first load.
Faster first-page load, but interactions may require additional requests. Richer user experience with fast navigation after initial load.
Used for news websites, blogs, and e-commerce platforms (e.g., Next.js). Used for single-page applications (SPAs) like dashboards or social media apps.

Ques 35: Explain the concept of a Middleware in Redux.

Answer:

  1. Middleware in Redux is a function that sits between the dispatching of an action and the reducer, allowing you to modify or intercept actions before they reach the store.
  2. It is used to handle side effects such as API calls, logging, authentication, or asynchronous operations.

Ques 36. What is the difference between a React component and a React element?

Answer:
React Component React Element
A function or class that returns a React element. An object that describes the UI structure.
Defines how the UI should behave and render. Represents a single UI node in the Virtual DOM.
Can manage state and lifecycle methods (Class components) or use Hooks (Function components). Immutable and cannot hold state or lifecycle methods.
Can contain multiple elements and logic. Represents only a single instance of a UI component.
Example: function Button() { return <button>Click</button>; } Example: const button = <button>Click</button>;
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Ques 37. What is React Fiber, and how does it improve React’s performance?

Answer:

React Fiber is the reconciliation engine introduced in React 16. It improves rendering performance by breaking updates into smaller units and using a priority-based scheduling system. This allows React to handle animations, user interactions, and updates more efficiently.

Key Benefits:

  • Supports asynchronous rendering
  • Improves UI responsiveness
  • Helps with features like Suspense & Concurrent Mode

Ques 38. What is the purpose of the key prop in React lists?

Answer:

const items = ["Apple", "Banana", "Orange"];
return (
<ul>
{items.map((item, index) => (
<li key={index}>{item}</li>
))}
</ul>
);

Ques 39. What are React Portals, and when should you use them?

Answer:

  1. Assume Referential Integrity is an option in Tableau that ensures the integrity of relationships between tables when blending data from multiple sources.
  2. When this option is enabled, Tableau assumes that the relationships between the fields in the different data sources are consistent and that matching records across those sources are accurate.

Ques 40: What are React Portals, and when should you use them?

Answer:

React Portals allow rendering a component outside its parent DOM hierarchy, while still being part of React’s virtual DOM. They are useful for modals, tooltips, and pop-ups where the UI should not be restricted by parent overflow styles.

Conclusion:

In React JS development, different file types and structures help manage components, state, and configurations efficiently. From JSX files for UI components to JSON for dependencies (package.json), each file plays a crucial role in maintaining a scalable and modular application.

Similarly, in Full Stack Development, managing both frontend (React, HTML, CSS, JavaScript) and backend (Node.js, Express, databases like MongoDB or PostgreSQL) requires understanding various files and frameworks. Proper structuring ensures better performance, maintainability, and seamless collaboration in a project.

🚀 Good luck with your interview!

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