React Js MCQs with answers Page - 9

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📊 React Js
Q. Which React feature helps break the UI into independent pieces?
  • (A) Fragments
  • (B) Components
  • (C) Hooks
  • (D) Keys
💬 Discuss
✅ Correct Answer: (B) Components

Explanation: React uses components to encapsulate and reuse UI pieces.

📊 React Js
Q. What is a React Fragment used for?
  • (A) To return multiple elements without a wrapper
  • (B) To create reusable logic
  • (C) To import multiple files
  • (D) To define state
💬 Discuss
✅ Correct Answer: (A) To return multiple elements without a wrapper

Explanation: Fragments allow grouping elements without adding extra nodes to the DOM.

📊 React Js
Q. Which of the following is true about useRef?
  • (A) It causes re-render on change
  • (B) It preserves a mutable reference
  • (C) It creates a new state
  • (D) It is used for performance optimization only
💬 Discuss
✅ Correct Answer: (B) It preserves a mutable reference

Explanation: useRef keeps a persistent mutable value that does not cause re-render when updated.

📊 React Js
Q. What is the difference between useMemo and useCallback?
  • (A) useMemo memoizes functions, useCallback memoizes values
  • (B) useCallback is used for memoizing values
  • (C) useMemo memoizes values, useCallback memoizes functions
  • (D) They do the same thing
💬 Discuss
✅ Correct Answer: (C) useMemo memoizes values, useCallback memoizes functions

Explanation: useMemo is for expensive value computations; useCallback is for memoizing functions.

📊 React Js
Q. Which one is a correct way to create a functional component?
  • (A) function MyComponent() {}
  • (B) class MyComponent extends Component {}
  • (C) component MyComponent = () => {}
  • (D) def MyComponent() {}
💬 Discuss
✅ Correct Answer: (A) function MyComponent() {}

Explanation: Functional components are created using JavaScript functions.

📊 React Js
Q. What is the correct syntax to pass a function as a prop?
  • (A) <MyComp func='doSomething' />
  • (B) <MyComp func={doSomething} />
  • (C) <MyComp func={doSomething()} />
  • (D) <MyComp func=doSomething />
💬 Discuss
✅ Correct Answer: (B) <MyComp func={doSomething} />

Explanation: Functions are passed as props using curly braces without invoking them.

📊 React Js
Q. Which hook allows you to share logic between components?
  • (A) useShared
  • (B) useGlobal
  • (C) Custom Hooks
  • (D) useEffect
💬 Discuss
✅ Correct Answer: (C) Custom Hooks

Explanation: Custom hooks let you reuse stateful logic across components.

📊 React Js
Q. Which attribute is required when using map to render lists?
  • (A) ref
  • (B) name
  • (C) key
  • (D) id
💬 Discuss
✅ Correct Answer: (C) key

Explanation: The key prop helps React identify individual items in lists efficiently.

📊 React Js
Q. What is the default behavior of React's `useEffect` without a dependency array?
  • (A) It runs only once
  • (B) It runs on every render
  • (C) It runs only on component unmount
  • (D) It never runs
💬 Discuss
✅ Correct Answer: (B) It runs on every render

Explanation: Without a dependency array, useEffect runs after every render.

📊 React Js
Q. What does `ReactDOM.render()` do?
  • (A) It defines a React component
  • (B) It adds props to components
  • (C) It renders a React element into the DOM
  • (D) It creates a virtual DOM
💬 Discuss
✅ Correct Answer: (C) It renders a React element into the DOM

Explanation: `ReactDOM.render()` mounts a React component to a specific DOM node.

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