If you’re preparing for a ReactJS interview or brushing up on your JavaScript skills, this comprehensive guide will help you excel. Below, we’ll explore the most frequently asked ReactJS interview questions along with detailed answers to boost your confidence. Let’s dive in! 🚀

“Opportunities don’t happen. You create them.”

– Chris Grosser

1. What is React?

React is a JavaScript library developed by Facebook for building user interfaces. It is used to create single-page applications and allows developers to create reusable UI components.

2. What is useMemo?

useMemo is a React Hook that memoizes the result of a function to optimize performance by avoiding unnecessary recalculations. It only recalculates when the dependencies change.

3. What are the features of React?

Key features include:

  • Declarative UI
  • Component-Based Architecture
  • Virtual DOM
  • Unidirectional Data Flow
  • React Hooks

4. What is JSX?

JSX stands for JavaScript XML. It is a syntax extension for JavaScript that allows you to write HTML structures in the same file as JavaScript code.

5. What is DOM?

DOM (Document Object Model) is a programming interface for web documents that represents the page structure in a tree format, allowing scripts to update content, structure, and style.

6. What is Virtual DOM?

The Virtual DOM is a lightweight JavaScript representation of the actual DOM. React uses it to improve performance by minimizing direct DOM manipulations.

7. What is the component life cycle of a React class component?

The lifecycle consists of three phases:

  • Mounting (e.g., constructor, componentDidMount)
  • Updating (e.g., shouldComponentUpdate, componentDidUpdate)
  • Unmounting (e.g., componentWillUnmount)

8. What are fragments in React?

Fragments allow you to group multiple elements without adding extra nodes to the DOM. Syntax: <React.Fragment>...</React.Fragment> or shorthand <>...</>.

9. What are props in React?

Props (short for properties) are read-only data passed from parent to child components to configure or customize behavior.

10. What are synthetic events in React?

Synthetic events are cross-browser wrappers around native events, providing consistent behavior across different browsers.

11. Difference between package.json and package-lock.json?

  • package.json: Contains metadata about the project and a list of dependencies.
  • package-lock.json: Locks dependency versions to ensure consistent installations.

12. Difference between client-side and server-side rendering?

  • Client-Side Rendering: Rendering occurs in the browser, improving interactivity but may increase initial load time.
  • Server-Side Rendering: Rendering occurs on the server, delivering a fully-formed HTML page to the client.

13. What is state in ReactJS?

State is an object that determines the behavior and rendering of a component. It is managed within the component and can be updated using setState or React Hooks like useState.

14. What are props?

Props are inputs to a component that allow data to be passed from a parent to a child.

15. Difference between State and Props?

  • State: Managed within the component; mutable.
  • Props: Passed to the component; immutable.

16. What is props drilling?

Props drilling refers to passing props through multiple levels of components, which can make the code complex and harder to maintain.

17. Disadvantages of props drilling and how to avoid it?

Disadvantages:

  • Increases complexity
  • Reduces readability Avoidance:
  • Use Context API or state management libraries like Redux.

18. What are Pure Components?

Pure Components render only when there are changes in props or state, optimizing performance.

19. What are refs in React?

Refs provide a way to access DOM nodes or React elements directly.

20. What is meant by forward ref?

forwardRef allows you to pass refs through a component to its child.

21. What are Error Boundaries?

Error Boundaries are React components that catch JavaScript errors in child components and render a fallback UI.

22. What are Higher Order Components (HOC)?

HOCs are functions that take a component and return a new component, adding additional behavior or props.

23. Difference between controlled and uncontrolled components?

  • Controlled: State is managed by React.
  • Uncontrolled: State is managed by the DOM.

24. What is useCallback?

useCallback memoizes a callback function to optimize performance by preventing unnecessary re-creations.

25. Difference between useMemo and useCallback?

  • useMemo: Memoizes the result of a computation.
  • useCallback: Memoizes the function itself.

26. What are keys in React?

Keys are unique identifiers used to optimize rendering lists by tracking changes efficiently.

27. What is lazy loading?

Lazy loading delays loading components or resources until they are needed, improving performance.

28. What is Suspense in React?

Suspense is a component that wraps lazy-loaded components, providing fallback UI during loading.

29. What are custom hooks?

Custom Hooks are reusable functions that encapsulate component logic using React Hooks.

30. What is useReducer hook?

useReducer is an alternative to useState for managing more complex state logic.

31. What are Portals in React?

Portals allow rendering children into a DOM node outside the parent hierarchy.

32. What is context in React?

Context provides a way to pass data through the component tree without props drilling.

33. Practical: Example of Context API usage?

const ThemeContext = React.createContext();
function App() {
  return (
    <ThemeContext.Provider value={theme}>
      <ChildComponent />
    </ThemeContext.Provider>
  );
}

34. What is the purpose of the callback function in setState()?

It ensures that code runs after the state update has been completed.

35. Practical: Create a custom hook for increment/decrement counter.

function useCounter(initialValue = 0) {
  const [count, setCount] = useState(initialValue);
  return {
    count,
    increment: () => setCount(count + 1),
    decrement: () => setCount(count - 1),
  };
}

36. Which lifecycle hooks are replaced by useEffect in functional components?

  • componentDidMount
  • componentDidUpdate
  • componentWillUnmount

37. What is Strict Mode in React?

Strict Mode highlights potential problems in an application during development.

38. Different ways to pass data from child to parent in React?

  • Callback functions
  • Context API
  • useRef

39. Practical: Send data from child to parent using callback functions.

function Parent() {
  const handleData = (data) => console.log(data);
  return <Child onSend={handleData} />;
}
function Child({ onSend }) {
  return <button onClick={() => onSend("Hello Parent!")}>Send</button>;
}

40. Practical: Send data from child to parent using useRef.

const childRef = useRef();
<Child ref={childRef} />;

41. How to optimize your React application?

  • Use Pure Components
  • Use React.memo
  • Use code-splitting and lazy loading
  • Avoid props drilling
  • Optimize state management

42. How do you consume a RESTful JSON API in ReactJS?

Using fetch or libraries like axios to make API calls.

43. Different design patterns used in React?

  • Container and Presentation Pattern
  • Higher Order Components
  • Render Props

44. Context API vs Redux?

  • Context API: Simpler state management; suitable for small apps.
  • Redux: Complex state management with middleware for large-scale apps.

45. PropTypes in React: How to validate props?

Using the prop-types package to specify types and required props.

46. What are React Mixins?

Mixins are a way to share functionality between components but are now deprecated in favor of HOCs and hooks.

47. What are the different hooks you’ve used?

Common hooks: useState, useEffect, useContext, useReducer, useMemo, useCallback, useRef.

48. What are render props in React?

A pattern where a component’s children is a function that provides data to render.

49. Types of exports and imports?

  • Default Export: export default Component;
  • Named Export: export const Component;
  • Default Import: import Component from './file';
  • Named Import: import { Component } from './file';

50. Difference between createElement and cloneElement?

  • React.createElement: Creates a new React element.
  • React.cloneElement: Clones an existing element, optionally modifying its props.

Prepare these questions and you’ll be ready to ace your ReactJS interview! 🚀

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