Top 10 ReactJS Interview Questions for 2025 Frontend + Backend

ReactJS continues to shape the landscape of modern web development by offering a robust library for building dynamic user interfaces. It powers countless applications, from single-page apps to complex front-end-heavy systems, and is increasingly pivotal in full-stack development. If you’re preparing for a ReactJS interview in 2025, understanding the latest trends, tools, and challenges is essential to showcase your expertise.

This blog explores the Top 10 ReactJS Interview Questions for both frontend and backend developers, providing insights into core concepts, practical applications, and real-world examples.

What Are React Hooks and How Do They Replace Class Components?
useEffect vs useLayoutEffect
Controlled vs Uncontrolled Components
Virtual DOM vs Real DOM
Prop Drilling and Context API
Memoization: React.memo, useMemo, and useCallback
React Router v6 Navigation
Error Boundaries in React
React Performance Optimization Tips
State Management Options (Redux, Zustand, Jotai)
FAQs

1. What Are React Hooks and How Do They Replace Class Components?

React Hooks, introduced in React 16.8, allow developers to use state and lifecycle methods within functional components instead of relying on class components.

Why Were Hooks Introduced?

Simplification: Hooks eliminate the boilerplate code associated with classes, such as constructors and this binding.
Reusability: They enhance the ability to reuse logic by extracting stateful behavior into custom hooks.
Improved Readability: Functional components with hooks are typically easier to read and debug.

Common Hooks:

useState: Handles local state in a functional component.

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

useEffect: Manages side effects like fetching data or subscribing to events.

Example: Replacing a class component using useState and useEffect:

   // Functional Component with Hooks
   function Counter() {
       const [count, setCount] = useState(0);

       useEffect(() => {
           document.title = `Count is ${count}`;
       }, [count]);

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

---  

## 2. useEffect vs useLayoutEffect  

Both `useEffect` and `useLayoutEffect` are used for handling side effects, but their execution timing differs.  

### Key Differences:
| **useEffect**                   | **useLayoutEffect**             |
|----------------------------------|----------------------------------|
| Runs **after render** in an asynchronous manner. | Runs **synchronously after render** but before the browser paints the UI. |
| Suitable for non-blocking tasks like data fetching. | Ideal for measuring layout changes or DOM mutations. |

### Example Use Case:
- Use `useLayoutEffect` to measure DOM nodes before rendering transitions or animations.  
- Use `useEffect` for API calls or setting document titles.  

---  

## 3. Controlled vs Uncontrolled Components  

### Controlled Components  
A component whose form data is fully managed by React state.  
- Example:
   ```javascript
   function ControlledInput() {
       const [value, setValue] = useState('');
       return <input value={value} onChange={(e) => setValue(e.target.value)} />;
   }

Uncontrolled Components

Form inputs manage their own state internally, accessed via ref.

Example:

   function UncontrolledInput() {
       const inputRef = useRef();
       return <input ref={inputRef} />;
   }

Best Practice: Use controlled components when possible for predictability and validation.

4. Virtual DOM vs Real DOM

The Virtual DOM is a lightweight representation of the Real DOM that React uses to optimize UI rendering.

Differences:

Virtual DOM	Real DOM
Faster updates by comparing “diffs.”	Slower updates due to direct manipulation.
Systematically calculates changes needed.	Directly manipulates all properties at once.

How It Works:

React creates a Virtual DOM.
Changes are computed using the “diffing” algorithm.
Only the affected parts of the Real DOM are updated.

5. Prop Drilling and Context API

Prop Drilling

Passing props through multiple layers of components even when only the last layer requires the data.

Challenge: Leads to unnecessarily verbose code.
Example:

   <Parent>
       <Child message="Hello" />
   </Parent>

Context API

The Context API provides a way to share state globally without prop drilling.

Example:

   const MyContext = React.createContext();

   function App() {
       return (
           <MyContext.Provider value="Hello">
               <Child />
           </MyContext.Provider>
       );
   }

Pro Tip: Use Context sparingly to avoid overcomplicated state management.

6. Memoization: React.memo, useMemo, and useCallback

React.memo

Wraps a component to avoid re-rendering unless props change.

   const MemoizedComponent = React.memo(MyComponent);

useMemo

Computes and caches values derived from dependencies.

   const computedValue = useMemo(() => expensiveFunction(input), [input]);

useCallback

Memoizes functions to prevent unnecessary re-creation.

   const memoizedCallback = useCallback(() => doSomething(a, b), [a, b]);

7. React Router v6 Navigation

React Router v6 introduced simplified routing and nested route support.

Key Features:

New <Routes> component replaces <Switch>.
Dynamic routing now uses path parameters like /:id.

Example:

   <Routes>
       <Route path="/" element={<Home />} />
       <Route path="/about" element={<About />} />
       <Route path="/user/:id" element={<User />} />
   </Routes>

8. Error Boundaries in React

An error boundary is a component that “catches” JavaScript errors in child components.

Example:

   class ErrorBoundary extends React.Component {
       state = { hasError: false };

       static getDerivedStateFromError() {
           return { hasError: true };
       }

       render() {
           return this.state.hasError ? <h1>Something went wrong.</h1> : this.props.children;
       }
   }

Note: Error boundaries only catch errors in the component tree, not in handlers.

9. React Performance Optimization Tips

Use React.memo: Prevent unnecessary re-renders.
Split Code Dynamically: Use React.lazy and Suspense.
Avoid Anonymous Functions: Use useCallback for stable references.
Batch State Updates: Use React’s built-in event batching for faster updates.

10. State Management Options (Redux, Zustand, Jotai)

Redux

Offers predictable state management with reducers and middleware.

Best for large applications with complex state requirements.

Zustand

A lightweight, simpler alternative to Redux focusing on stores.

Jotai

A minimalistic library for atomic state management.

FAQs

How does React improve performance with its Virtual DOM?

The Virtual DOM optimizes updates by re-rendering only the components that actually changed instead of the entire DOM tree.

Can React be used for backend development?

While React is primarily frontend-focused, pairing it with Node.js can enable efficient full-stack applications.

Are React hooks backward compatible?

Yes, hooks are fully backward compatible and can be gradually adopted in legacy applications.

Summary

With its unparalleled versatility and efficiency, React remains the top choice for frontend and full-stack developers. From mastering hooks to implementing the latest features in the Context API and React Router, understanding these concepts will help you build scalable and performant applications. Start preparing for your next interview or project with the ReactJS interview essentials discussed here, and solidify your expertise in modern web development!

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