I Survived A React JS Interview

20 Important React JS Questions for Beginners (with Options)
Description:
Are you new to React JS? Test your knowledge with these 20 fundamental React JS questions designed for beginners! Each question comes with multiple-choice options to help you understand core concepts like components, state, props, hooks, and more. Perfect for interview preparation or self-assessment.
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Answer Key with Explanations
1. What is React JS?
✅ B) A JavaScript library for building user interfaces
React is a front-end library (not a framework) used to build interactive UIs.
2. Which command is used to create a new React app?
✅ B) npx create-react-app my-app
npx runs create-react-app without installing it globally.
3. What is JSX?
✅ A) A syntax extension for JavaScript
JSX allows HTML-like syntax inside JavaScript
4. What is used to pass data to a component from outside?
✅ B) props
Props (short for properties) pass data from parent to child components.
5. Which hook is used to manage state in functional components?
✅ B) useState
Example: const [count, setCount] = useState(0);
6. What does useEffect do?
✅ C) Performs side effects in functional components
Used for API calls, subscriptions, or DOM manipulations after render.
7. How do you render a component conditionally?
✅ B) Using ternary operators or &&
8. What is the virtual DOM?
✅ B) A lightweight representation of the real DOM
React uses it to optimize updates for better performance.
9. Which method is called before a component is removed from the DOM?
✅ B) componentWillUnmount
Used for cleanup (e.g., removing event listeners).
10. How do you update the state in a class component?
✅ B) this.setState({ key: value })
Never modify state directly (e.g., this.state = newState is wrong).
11. What is the purpose of keys in React lists?
✅ B) To uniquely identify elements
Keys help React track which items changed
12. Which tool helps in inspecting React components?
✅ B) React Developer Tools
A browser extension for debugging React component hierarchies.
13. What is the correct way to handle events in React?
✅ B) onClick={handleClick}
React uses camelCase event names (not HTML's lowercase).
14. What is the default port for a React development server?
✅ A) 3000
Runs on http://localhost:3000 by default.
15. Which lifecycle method runs after a component renders?
✅ A) componentDidMount
Used for side effects like API calls after the first render.
16. What is the purpose of useRef?
✅ B) To access DOM elements directly
17. How can you optimize React performance?
✅ A) Using React.memo
Memoizes components to prevent unnecessary re-renders.
18. What does prop drilling mean?
✅ A) Passing props through multiple layers
A common issue solved by Context API or state management (Redux).
19. Which syntax is correct for importing a component?
✅ A) import MyComponent from './MyComponent'
Uses ES6 module syntax (default export).
20. What is Context API used for?
✅ A) Managing global state without prop drilling
Alternative to Redux for sharing data across components.
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