Top 25 React Interview Questions and Answers

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In this guide, we will discuss the top 25 frequently asked React JS interview questions for both beginners and experienced developers.

Let's begin with simple questions.

1. What is React, and why is it used?

React is a JavaScript library created by Facebook for building fast, interactive user interfaces. Instead of treating an entire page as one DOM structure, React splits the UI into reusable components. Each component manages its own state and updates only when necessary.

React is used because it improves UI performance through the Virtual DOM, supports component-based architecture, provides unidirectional data flow for predictable behavior, and integrates well with modern tools like Redux, React Router, and Next.js.

Interviewers expect you to highlight React’s speed, simplicity, reusability, flexibility, and huge ecosystem.

2. What is JSX, and why is it used in React?

JSX is a syntax extension that lets you write HTML-like code inside JavaScript. Although browsers cannot interpret JSX directly, React transforms JSX into JavaScript functions that create UI elements.

JSX improves readability, gives developers visual clarity of component structure, reduces string concatenation errors, and allows embedding JavaScript expressions within markup.

Interviewers expect you to explain that JSX is not required but is preferred because it makes React development cleaner and more intuitive.

3. What is the Virtual DOM, and how does it improve performance?

React uses a Virtual DOM, a lightweight, in-memory representation of the real DOM.

Instead of updating the browser DOM directly, React updates the Virtual DOM first.

React then compares the difference (diffing) between the old and new Virtual DOM versions and updates only the changed parts in the real DOM.

This reduces expensive DOM operations, improves rendering speed, and enhances overall performance.

Interviewers expect you to emphasize that the Virtual DOM helps React render efficiently and maintain UI consistency.

4. What are React components?

React components are independent, reusable pieces of UI that return HTML-like code (JSX). Components make applications modular and easier to maintain.

There are two types of components: functional components and class components.

Functional components are simple JavaScript functions that return JSX.

Class components are ES6 classes extending React. Component and can use lifecycle methods.

Most modern React apps use functional components because hooks provide state and lifecycle features.

Interviewers want you to highlight that components increase reusability and maintainability.

5. What are props in React?

Props (short for “properties”) are read-only inputs passed from parent to child components. They allow data flow and UI customization.

Props are immutable, meaning the receiving component cannot modify them. This supports predictable UI behavior and one-way data flow.

Props can be strings, numbers, arrays, objects, functions, or even components.

Interviewers expect you to mention that props help create reusable and configurable components without mutating data.

6. What is the state in React? How is it different from props?

State represents data that changes over time inside a component. Updating the state re-renders the component.

Props are external and passed into a component.

State is internal and managed within the component.

Props are read-only, while state is mutable through setState or useState.

Interviewers expect a clear comparison:

Props is for external, immutable inputs.
State is for internal, changeable data.

7. What are React Hooks?

Hooks allow functional components to use features like state, lifecycle methods, and context without using classes.

Common hooks include useState for local state, useEffect for side effects, useContext for global data, useRef for referencing values, and useMemo for optimization.

Hooks make code simpler, cleaner, and easier to test.

Interviewers expect you to highlight that hooks replaced many class-component patterns and are the standard for modern React development.

8. What is useState, and how does it work?

useState is a React hook that allows functional components to store and update values.

It returns an array with two elements: the current state value and a function to update it.

Calling the update function triggers a re-render.

Interviewers expect you to explain that useState enables dynamic UI updates and is essential in creating interactive components.

9. What is useEffect, and why is it used?

useEffect handles side effects—operations that occur outside the UI rendering process.

Examples include API calls, timers, event listeners, or subscriptions.

Without useEffect, these operations would run on every render or clutter logic inside the component.

useEffect allows React to run side effects after rendering and clean them up when necessary.

Interviewers expect you to highlight the dependency array, which controls when the effect runs.

10. What is the difference between controlled and uncontrolled components?

A controlled component has its form input value managed by React state.

The component updates the state, and the UI reflects state changes.

Uncontrolled components rely on the DOM to manage values using refs. React does not fully control the input.

Interviewers expect you to mention that controlled components provide better validation and consistent UI behavior, while uncontrolled components are simpler for basic forms.

11. What is React Context? When should you use it?

React Context provides a way to share data across components without prop drilling.

It is used for global data like authentication, themes, languages, and user settings.

Context includes:

a provider that supplies values
and consumers who access those values.

Interviewers expect you to explain that Context is powerful but should be used carefully because too much global state can cause unnecessary re-renders.

12. What is React Router? Why is it used?

React Router is a library for building single-page application navigation.

Instead of reloading the page, it updates the UI when the URL changes.

It supports dynamic routing, nested routes, route parameters, redirects, and navigation hooks.

Interviewers expect you to highlight that React Router enables smooth client-side navigation and improves user experience.

13. What are Higher-Order Components (HOCs)?

A Higher-Order Component is a function that takes a component as input and returns a new enhanced component. HOCs allow code reuse, logic sharing, and transforming props without modifying the original component.

Examples include adding authentication checks, logging behavior, or subscription logic. HOCs wrap another component and provide extended features, similar to decorators.

Interviewers expect you to explain that HOCs help avoid duplication and improve the separation of concerns. However, modern React encourages using hooks instead of HOCs for most use cases.

14. What is reconciliation in React?

Reconciliation is React’s internal process of determining which parts of the UI need to be updated when state or props change. React compares the previous Virtual DOM with the new one.

If elements are similar, React updates only the changed parts. If elements differ, React replaces them.

This selective update process optimizes rendering and improves performance.

Interviewers expect you to highlight React’s diffing algorithm, which efficiently compares nodes by assuming that child elements of the same type will remain stable.

15. What is key in React lists and why is it important?

A key is a unique identifier assigned to list items. React uses keys to track each element when rendering lists.

Without keys, React cannot determine which items changed, were added, or were removed, resulting in incorrect UI updates.

Keys improve rendering efficiency and maintain component identity.

Interviewers expect you to mention that keys should be stable and unique. Using indexes as keys is discouraged when the list changes frequently.

16. What is React.memo and when should you use it?

React.memo is a higher-order component used to memoize functional components. It prevents unnecessary re-renders when props haven't changed.

React.memo checks the previous and current props. If they are the same, the component is skipped in the rendering process.

Use React.memo in performance-critical components, especially those receiving complex props or rendering large UI sections.

Interviewers expect you to mention that memoization works best when props are stable and when rendering costs are high.

17. What is useCallback, and how is it different from useMemo?

useCallback returns a memoized version of a callback function. It ensures the function reference remains stable unless dependencies change.

useMemo returns a memoized value, such as the result of a computation.

useCallback → memoizes functions
useMemo → memoizes values

Interviewers expect you to explain that both hooks improve performance by avoiding unnecessary re-renders or recalculations.

18. What is useRef and what are its use cases?

useRef stores a mutable value that persists across re-renders without causing a component re-render.

Common uses include:

tracking previous values,
referencing DOM elements,
storing timers,
caching values.
useRef is useful when you need to store information without affecting the UI.

Interviewers expect you to highlight that useRef is not for triggering UI updates.

19. What is prop drilling, and how can you avoid it?

Prop drilling occurs when parent components pass data to deeply nested children. This makes the component tree harder to maintain.

Prop drilling is avoided using:

React Context,
state management tools like Redux, Zustand, or Recoil,
or component composition patterns.

Interviewers expect you to show awareness of when drilling is acceptable and when global state is better.

20. What is Redux, and why do we use it?

Redux is a state-management library that stores global application state in a single central store.

It enforces predictable state updates using actions and reducers.

Redux solves issues like prop drilling, complex state sharing, and debugging inconsistent UI states.

Interviewers expect you to highlight Redux DevTools, middleware support, and predictable state flow.

21. What is the difference between Redux and Context API?

Context API is built into React and works well for small to medium global state needs such as theme, user data, or language.

Redux provides structured state management, predictable reducers, middleware, DevTools, and scaling capabilities.

Use Context for simple scenarios, and Redux when the application becomes large or when debugging and state predictability are crucial.

22. What is React Fiber?

React Fiber is the internal engine responsible for improving rendering performance. It allows React to break rendering into chunks, pause work, and resume later.

Fiber enables features such as Concurrent Mode, smoother animations, and improved responsiveness.

Interviewers expect you to mention that Fiber makes React more efficient when updating large or complex UIs.

23. What is lazy loading in React?

Lazy loading means loading components only when needed instead of loading everything upfront.

React provides React.lazy and Suspense to dynamically import components.

This improves application performance and reduces bundle size.

Interviewers expect you to highlight that lazy loading is crucial for large applications with multiple routes or heavy components.

24. What is hydration in React?

Hydration is the process of attaching React event handlers to server-rendered HTML.

It is used in frameworks like Next.js to enable server-side rendering.

React uses hydration to reuse the HTML generated on the server rather than recreating the DOM from scratch.

This improves initial load speed and SEO performance.

Interviewers expect you to mention that hydration is essential in SSR-based applications.

25. What is a Custom Hook, and when should you create one?

A Custom Hook is a JavaScript function that uses existing React Hooks to extract reusable logic.

If multiple components share logic—like fetching data, managing form inputs, or browser events—creating a Custom Hook avoids duplication and keeps components cleaner.

Custom Hooks support separation of concerns while keeping stateful logic reusable.

Interviewers expect you to mention that naming must start with “use” so React can track state correctly and avoid breaking the rules of hooks.

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