Top React Interview Questions (2026)

React is one of the most widely used frontend libraries in 2026. Understanding its core concepts, hooks, and best practices is crucial for frontend interviews at top product-based companies.

In this guide, we cover essential React questions, with examples and explanations, so you can confidently prepare for interviews.

1. What is React?

React is a JavaScript library for building interactive, reusable UI components. It follows a declarative approach, making UI predictable and easier to debug.

2. What are Hooks?

Hooks are functions that let you "hook into" React features such as state and lifecycle methods from functional components.

import { useState, useEffect } from 'react';

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

  useEffect(() => {
    console.log('Count updated:', count);
  }, [count]);

  return <button onClick={() => setCount(count + 1)}>{count}</button>;
}

3. State vs Props

State is managed within a component and can change, while props are read-only and passed from parent to child components.

4. What is Virtual DOM?

Virtual DOM is a lightweight copy of the real DOM. React updates the virtual DOM first and then efficiently updates only the parts of the real DOM that changed.

5. What is JSX?

JSX is a syntax extension that allows you to write HTML-like code inside JavaScript. React uses it to define UI components.

6. What is useEffect and why is it used?

useEffect lets you perform side effects such as data fetching, subscriptions, or manually changing the DOM after a component renders.

7. What is Context API?

Context API provides a way to pass data through the component tree without passing props manually at every level.

8. What is React.memo?

React.memo is a higher-order component that memoizes functional components to avoid unnecessary re-renders.

9. What is the difference between class and functional components?

Class components can have state and lifecycle methods, while functional components use hooks to manage state and lifecycle.

10. What is a Pure Component?

A PureComponent implements shouldComponentUpdate with a shallow prop and state comparison to prevent unnecessary re-renders.

11. What is useCallback?

useCallback memoizes a function reference between renders, useful when passing callbacks to optimized child components.

12. What is useMemo?

useMemo memoizes a computed value between renders to avoid expensive calculations on every render.

13. What are keys in React?

Keys help React identify which items have changed, are added, or removed in a list of elements, optimizing re-renders.

14. What is lifting state up?

Lifting state up means moving the state to the closest common ancestor so that multiple child components can share it.

15. How to optimize React performance?

Use React.memo, useCallback, useMemo, lazy loading, code splitting, and avoid unnecessary re-renders.