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📘 Description:

Get ready for your next front-end interview with this comprehensive list of React interview questions and answers. Covers fundamental concepts like JSX, components, props, state, hooks, context, Redux, and error boundaries.

🔍 Introduction

React is one of the most widely used libraries for building interactive UIs, especially for single-page applications. Whether you're a beginner or an experienced developer brushing up before an interview, this guide covers the essential React questions and answers that will help you shine in any interview.

⚛️ React Core Concepts

1️⃣ What is React?

Answer:

React is an open-source JavaScript library developed by Facebook for building user interfaces. It promotes component-based architecture and is primarily used for building single-page applications with reusable UI elements.

2️⃣ What are the main features of React?

Answer:

• JSX: JavaScript syntax extension that looks like HTML.
• Components: Reusable and modular blocks of UI.
• Virtual DOM: Optimizes rendering by diffing changes.
• One-way Data Binding: Ensures predictable data flow.
• State & Props: Manage and pass data effectively between components.

3️⃣ What is JSX?

Answer:

JSX (JavaScript XML) allows you to write HTML-like code within JavaScript. It's compiled by Babel into standard JavaScript that creates React elements.

4️⃣ What is the Virtual DOM, and how does it work?

Answer:

React creates a virtual DOM to track UI changes. When a component’s state changes, React:

1. Updates the Virtual DOM.
2. Diffs the new and old Virtual DOMs.
3. Patches only the changed elements in the real DOM — making updates faster and more efficient.

🧩 Components, Props, and State

5️⃣ What are props in React?

Answer:

Props (short for “properties”) are used to pass data from parent to child components. They are read-only and help components stay pure and reusable.

6️⃣ What is state in React?

Answer:

State is a built-in object that stores property values that belong to a component. It can be modified with useState (in functional components) or this.setState (in class components) to trigger UI re-renders.

7️⃣ What is the difference between state and props?

FeaturePropsStateMutabilityImmutableMutableScopePassed from parentManaged within the componentPurposeConfigure a componentHandle dynamic data

8️⃣ What is a functional component?

Answer:

A functional component is a JavaScript function that returns React elements. With hooks, functional components can now manage state and side effects.

9️⃣ What is a class component?

Answer:

A class component extends React.Component, has its own state, and can use lifecycle methods like componentDidMount, componentDidUpdate, etc.

🔁 Hooks and Lifecycle

🔟 What are hooks in React?

Answer:

Hooks allow functional components to use state and lifecycle features. Common hooks include:

• useState
• useEffect
• useContext
• useReducer
• useRef

1️⃣1️⃣ What is the use of useEffect?

Answer:

useEffect is used to perform side effects such as data fetching, subscriptions, and DOM manipulations. It replaces lifecycle methods like componentDidMount.

js

useEffect(() => {
  console.log('Component mounted or updated');
}, [dependencies]);

1️⃣2️⃣ How do you handle forms in React?

Answer:

Use controlled components by binding form values to state and handling input changes:

jsx

function MyForm() {
  const [name, setName] = useState('');

  const handleSubmit = (e) => {
    e.preventDefault();
    alert(`Name: ${name}`);
  };

  return (
    <form onSubmit={handleSubmit}>
      <input value={name} onChange={(e) => setName(e.target.value)} />
      <button type="submit">Submit</button>
    </form>
  );
}

🌐 Advanced React Concepts

1️⃣3️⃣ What is the Context API?

Answer:

Context API provides a way to share state globally without prop drilling. You define a Context, wrap components with a Provider, and consume the context using useContext.

1️⃣4️⃣ What is a higher-order component (HOC)?

Answer:

A HOC is a function that takes a component and returns a new component with enhanced behavior.

js

const withLogging = (Component) => (props) => {
  console.log('Rendering', Component.name);
  return <Component {...props} />;
};

1️⃣5️⃣ What is Redux?

Answer:

Redux is a predictable state container that manages application state in a central store using:

• Actions (describe changes),
• Reducers (handle logic),
• Store (holds state).

1️⃣6️⃣ How do you connect React and Redux?

Answer:

• Use Provider to wrap your app.
• Use useSelector to access state.
• Use useDispatch to dispatch actions.

jsx

import { useSelector, useDispatch } from 'react-redux';

1️⃣7️⃣ What are pure components?

Answer:

A Pure Component performs a shallow comparison of props/state to avoid unnecessary re-renders. You can create them via React.PureComponent or memoized functional components using React.memo.

1️⃣8️⃣ What is the difference between useEffect and useLayoutEffect?

FeatureuseEffectuseLayoutEffectTimingAfter paintBefore paintUse caseAPI callsReading DOM layout

🧱 Rendering & Error Handling

1️⃣9️⃣ What are React Fragments?

Answer:

Fragments let you return multiple elements from a component without adding extra DOM nodes:

jsx

<>
  <h1>Title</h1>
  <p>Description</p>
</>

2️⃣0️⃣ How do you handle errors in React?

Answer:

Use Error Boundaries to catch rendering errors:

jsx

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

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

  componentDidCatch(error, info) {
    console.error(error, info);
  }

  render() {
    return this.state.hasError ? <h1>Error occurred</h1> : this.props.children;
  }
}

🧠 Conclusion

Mastering these questions will not only prepare you for React interviews but also help you understand the core mechanics of building modern web applications. Whether you're working with class-based or functional components, hooks or Redux — solid fundamentals will make you stand out.