10 React JS Interview Questions and Answers for 2026

Why React JS Interview Questions Matter in 2026

React JS remains one of the most in-demand front-end libraries worldwide. Companies continue to adopt React for building scalable user interfaces, especially in single-page applications, e-commerce platforms, dashboards, and mobile apps through React Native.

Interviewers test both theoretical knowledge and practical problem-solving skills. Questions often cover core concepts like the virtual DOM, component lifecycle (now largely replaced by hooks), state management patterns, and performance optimization techniques.

Mastering these topics demonstrates deep understanding of React's architecture. It also shows readiness to handle real-world challenges such as large-scale state management and efficient rendering.

Understanding React interview questions thoroughly improves confidence during technical rounds. Many companies now include live coding sessions focused on hooks and functional components.

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1. What is React and what are its main benefits?

React is an open-source JavaScript library developed by Facebook for building user interfaces, particularly single-page applications. It allows developers to create reusable UI components that manage their own state.

Key benefits include:

Component-based architecture – Encourages modular, reusable code.
Virtual DOM – Improves rendering performance by minimizing direct DOM manipulation.
Declarative approach – Developers describe what the UI should look like, and React handles updates.
One-way data flow – Makes debugging easier and predictable.
Strong ecosystem – Integrates well with tools like Redux, React Router, and Next.js.

React powers millions of applications worldwide, including Facebook, Instagram, Netflix, and Airbnb.

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2. What is the Virtual DOM and how does React use it?

The Virtual DOM is a lightweight, in-memory representation of the actual DOM. React maintains this copy to track changes efficiently.

When state or props change:

React creates a new Virtual DOM tree.
It compares the new tree with the previous one (diffing/reconciliation).
Only the differences (minimal set of updates) are applied to the real DOM.

This approach reduces expensive DOM operations. Traditional frameworks often re-render the entire page, while React targets only changed parts.

Reconciliation uses a heuristic algorithm called Fiber (introduced in React 16) to prioritize updates and support features like concurrent mode.

3. How does React handle updates and rendering?

React uses a reconciliation process to determine what needs updating. When state or props change, React schedules a re-render.

Key steps:

setState() / useState triggers a re-render.
React builds a new Virtual DOM.
Diffing compares old and new trees.
Only necessary DOM mutations occur.

React batches multiple state updates for performance. In React 18, automatic batching extends to promises and timeouts.

Fiber architecture allows pausing, aborting, or prioritizing renders, improving responsiveness in large applications.

4. Explain the concept of Components in React

Components are the building blocks of React applications. They are reusable pieces of UI that can be functions or classes (though functions dominate since hooks).

Two main types:

Functional Components – Simple JavaScript functions that return JSX.
Class Components – ES6 classes with lifecycle methods (less common now).

Components accept props as input and can maintain internal state. They compose together to form complex UIs.

Best practice: Keep components small and focused on single responsibilities.

5. What is JSX and why is it used in React?

JSX (JavaScript XML) is a syntax extension that looks like HTML but gets transformed into JavaScript function calls.

Example:

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

Babel transpiles this to:

JavaScript

React.createElement('h1', null, 'Hello, world!');

JSX makes UI code more readable and maintainable. It supports embedding expressions with curly braces:

jsx

<h1>Hello, {user.name}!</h1>

JSX is optional but widely adopted for its clarity.

6. What are React Hooks and how do they work?

React Hooks are functions that let functional components use state and lifecycle features. Introduced in React 16.8, they eliminate the need for class components in most cases.

Common hooks:

useState – Adds local state.
useEffect – Handles side effects.
useContext – Accesses context values.
useReducer – Manages complex state logic.
useRef – Persists values across renders.

Hooks must follow two rules:

Call them only at the top level (not inside loops, conditions, or nested functions).
Call them only from React function components or custom hooks.

This ensures consistent state preservation between renders.

7. Explain the useState Hook in detail

useState returns a pair: current state value and a function to update it.

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

Initial value can be a function for expensive computations (lazy initialization).

Updates are asynchronous and batched for performance. Multiple setState calls in one event handler merge into one re-render.

Functional updates help when new state depends on previous state:

setCount(prevCount => prevCount + 1);

8. How does the useEffect Hook work?

useEffect runs side effects after render. It replaces componentDidMount, componentDidUpdate, and componentWillUnmount.

useEffect(() => {
  // Runs after every render
}, []); // Empty array = runs once on mount

Cleanup function prevents memory leaks:

useEffect(() => {
  const timer = setInterval(() => {}, 1000);
  return () => clearInterval(timer); // Cleanup
}, []);

Dependency array controls execution:

No array: Runs after every render.
Empty array: Runs once on mount.
With values: Runs when dependencies change.

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

Controlled components let React manage form input state:

<input value={value} onChange={handleChange} />

Uncontrolled components use DOM default behavior:

<input ref={inputRef} />

Controlled components provide:

Instant validation
Easier formatting
Consistent state management

Uncontrolled components suit simple forms or integration with non-React libraries.

10. Explain React Context and when to use it

Context provides a way to pass data through the component tree without props drilling.

const ThemeContext = createContext('light');
function App() {
  return (
<ThemeContext.Provider value="dark">
<Toolbar />
</ThemeContext.Provider>
);
}