ReactJS is among the most favored front-end libraries used for the development of dynamic and interactive web applications. It is necessary for you to gain a strong command over the basic concepts, higher-level ideas, and best practices if you’re getting ready to face a ReactJS interview Questions. Following are some of the best ReactJS interview questions and their answers provided to help you prepare efficiently.
1. What is ReactJS?
ReactJS is an open-source JavaScript library developed by Facebook for building user interfaces, especially for single-page applications where UI updates dynamically.
2. What are the key features of React?
- JSX (JavaScript XML): A JavaScript syntax extension used to write HTML-like code within JavaScript files.
- Virtual DOM: Makes performance better by only updating parts of the real DOM that need it.
- Component-Based Architecture: The UI is broken down into reusable components.
- Unidirectional Data Flow: Data moves in one direction, making control and debugging better.
- Hooks: Functional components can now leverage state and lifecycle hooks.
3. What is JSX?
JSX is a syntax extension that looks similar to HTML but is used with JavaScript. It makes writing UI components easier and more readable.
const element = <h1>Hello, World!</h1>;
4. What is Virtual DOM, and how does it function?
The Virtual DOM is a representation of the actual DOM elements in memory. React renders to the Virtual DOM and then updates the actual DOM efficiently only where changes have taken place, enhancing performance.
5. What is the difference between functional and class components?
| Feature | Functional Components | Class Components |
| State | Uses Hooks (useState) | Uses this.state |
| Lifecycle Methods | Uses Hooks (useEffect) | Uses component lifecycle methods |
| Syntax | Simple and concise | More boilerplate |
| Performance | Better for stateless components | Slightly heavier due to this binding |
6. What are React Hooks?
Hooks allow functional components to use state and lifecycle methods without writing a class.
import { useState } from “react”;
function Counter() {
const [count, setCount] = useState(0);
return <button onClick={() => setCount(count + 1)}>Count: {count}</button>;
}
7. What is useState and useEffect in React?
- useState is used to declare state variables in functional components.
- useEffect is used to handle side effects like fetching data, subscriptions, or updating the DOM.
8. What is a React Router?
React Router is a library for managing navigation in a React application, enabling single-page application behavior with multiple views.
import { BrowserRouter as Router, Route, Switch } from ‘react-router-dom’;
9. What is Redux?
Redux is a state management library that provides a single source of truth for application state and facilitates predictable state updates.
10. What is the difference between Context API and Redux?
| Feature | Context API | Redux |
| Complexity | Simpler | More complex |
| Use Case | Local component-level state management | Global state management |
| Boilerplate | Minimal | Requires actions, reducers, and store |
11. What is a Higher-Order Component (HOC)?
A HOC is a function that takes a component and returns a new component with additional functionality.
const withLogger = (WrappedComponent) => {
return (props) => {
console.log(“Rendered with props: “, props);
return <WrappedComponent {…props} />;
};
};
12. What is the significance of keys in lists?
Keys help React identify which items have changed, helping in efficient re-renders. Each child in a list should have a unique key.
{items.map(item => <li key={item.id}>{item.name}</li>)}
13. What are controlled and uncontrolled components?
- Controlled Components: Form inputs controlled by React state.
- Uncontrolled Components: Form inputs managed by the DOM.
14. What are React Fragments?
Fragments allow grouping multiple elements without adding extra nodes to the DOM.
<>
<h1>Hello</h1>
<p>World</p>
</>
15. What is reconciliation in React?
Reconciliation is the mechanism React applies to efficiently update the DOM by checking the Virtual DOM against the former version.
16. What does lazy loading mean in React?
Lazy loading enables components to be loaded on the fly, enhancing performance.
const LazyComponent = React.lazy(() => import(‘./LazyComponent’));
17. What is React.memo?
React.memo is used to optimize functional components by preventing unnecessary re-renders.
const MemoizedComponent = React.memo(MyComponent);
18. What are error boundaries?
Error boundaries are components that catch JavaScript errors in their child components.
class ErrorBoundary extends React.Component {
componentDidCatch(error, info) {
console.log(“Error: “, error);
}
render() {
return this.props.children;
}
}
19. What is Prop Drilling, and how do you prevent it?
Prop drilling refers to passing props from one component to another for many levels of components. This can be prevented with Context API or state management libraries such as Redux.
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20. How do you improve the performance of a React application?
- Utilize React.memo for memoization.
- Use React.lazy and Suspense for lazy loading.
- Optimize component re-renders with useCallback and useMemo.
- Implement code splitting with dynamic imports.
Conclusion
These ReactJS interview questions range from beginner to expert-level concepts and will assist you in preparing well for your next interview. Practice and create projects to reinforce your understanding!
