Top 50 React.Js frequently asked questions and answers?

 Here’s a list of 50 frequently asked React.js questions along with concise answers to help you prepare for interviews:

Basic Questions

1. What is React?

   • React is a JavaScript library for building user interfaces, particularly single-page applications. It allows developers to create reusable UI components.

2. What are components in React?

   • Components are the building blocks of a React application. They can be functional or class-based and represent a part of the UI.

3. What is JSX?

   • JSX (JavaScript XML) is a syntax extension that allows you to write HTML-like code in JavaScript. It makes writing React components more intuitive.

4. What is the difference between a functional component and a class component?

   • Functional components are simpler and are written as JavaScript functions, while class components are ES6 classes that can hold state and lifecycle methods.

5. What is state in React?

   • State is a built-in object that holds data or information about the component. It allows components to create dynamic and interactive UIs.

6. What is props in React?

   • Props (short for properties) are read-only attributes passed to components from their parent component. They allow data to flow through the component hierarchy.

7. What are lifecycle methods in React?

   • Lifecycle methods are hooks that allow you to run code at specific points in a component's life, such as componentDidMount, componentDidUpdate, and componentWillUnmount.

8. What is the virtual DOM?

   • The virtual DOM is an in-memory representation of the real DOM. React uses it to optimize updates by re-rendering only the parts of the UI that changed.

9. What is the purpose of keys in React?

   • Keys help React identify which items in a list have changed, are added, or are removed. They should be unique among siblings to ensure efficient updates.

10. What is the use of useEffect?

    • useEffect is a Hook that allows you to perform side effects in functional components, such as data fetching, subscriptions, or manually changing the DOM.

Intermediate Questions

11. What are hooks in React?

    • Hooks are functions that let you use state and other React features in functional components. Common hooks include useState, useEffect, and useContext.

12. What is the difference between useState and useReducer?

    • useState is used for managing simple state, while useReducer is better for complex state logic and when state depends on previous states.

13. What is context in React?

    • Context provides a way to share values between components without having to explicitly pass props down the tree. It is useful for global data like themes or authentication.

14. How do you handle forms in React?

    • Forms in React can be controlled or uncontrolled. Controlled forms use component state to manage input values, while uncontrolled forms access the DOM directly.

15. What is the difference between controlled and uncontrolled components?

    • Controlled components derive their input values from React state, while uncontrolled components store their own state internally in the DOM.

16. What is React Router?

    • React Router is a library for handling routing in React applications. It allows you to create single-page applications with navigation between different views.

17. How do you optimize performance in a React app?

    • Performance can be optimized using techniques like memoization with React.memo, code-splitting with React.lazy, and using useCallback and useMemo.

18. What are fragments in React?

    • Fragments are used to group multiple elements without adding extra nodes to the DOM. You can use <Fragment> or shorthand <>...</>.

19. What is the significance of shouldComponentUpdate?

    • shouldComponentUpdate is a lifecycle method that allows you to optimize rendering by preventing unnecessary updates based on state or props changes.

20. What are error boundaries in React?

    • Error boundaries are components that catch JavaScript errors in their child component tree, log those errors, and display a fallback UI.

Advanced Questions

21. What is the significance of useCallback and useMemo?

    • useCallback memoizes a function to prevent unnecessary re-creation on each render, while useMemo memoizes a value, improving performance for expensive calculations.

22. What is server-side rendering (SSR) in React?

    • SSR is a technique where the server renders the React components to HTML and sends it to the client, improving SEO and initial loading speed.

23. What is static site generation (SSG) in React?

    • SSG pre-renders pages at build time, serving static HTML files to clients. It's often used with frameworks like Next.js.

24. What is React.memo?

    • React.memo is a higher-order component that memoizes functional components, preventing unnecessary re-renders if the props have not changed.

25. How do you implement lazy loading in React?

    • Lazy loading can be implemented using React.lazy and Suspense to load components only when they are needed, improving performance.

26. What is the use of useLayoutEffect?

    • useLayoutEffect is similar to useEffect, but it runs synchronously after all DOM mutations. It is useful for measuring DOM elements or synchronously re-rendering.

27. What is the purpose of the keyExtractor in React Native?

    • keyExtractor is used to provide unique keys for list items in a flat list, ensuring optimal rendering and performance.

28. What are higher-order components (HOCs)?

    • HOCs are functions that take a component and return a new component, allowing for code reuse and enhancing functionality, such as adding authentication or data fetching.

29. How do you handle side effects in React?

    • Side effects can be managed using the useEffect hook, which allows you to perform operations like data fetching, subscriptions, and timers.

30. What is the purpose of useImperativeHandle?

    • useImperativeHandle customizes the instance value that is exposed to parent components when using ref, allowing you to control what the parent can access.

Practical Questions

31. How do you fetch data in a React component?

    • You can fetch data in the useEffect hook, using the Fetch API or libraries like Axios, and update the component state with the response data.

32. Write a simple component using props.

    jsx
    function Greeting(props) {
      return <h1>Hello, {props.name}!</h1>;
    }
    

33. How would you implement a toggle button?

    jsx
    function ToggleButton() {
      const [isOn, setIsOn] = useState(false);
      return (
        <button onClick={() => setIsOn(!isOn)}>
          {isOn ? 'ON' : 'OFF'}
        </button>
      );
    }
    

34. How do you create a simple context?

    jsx
    const MyContext = React.createContext();
    

35. How do you consume context in a component?

    jsx
    const value = useContext(MyContext);
    

36. What are controlled components in forms?

    • Controlled components derive their value from state and notify changes via onChange handlers.

37. How do you pass functions as props?

    • You can pass functions directly as props, just like any other value.

    jsx
    <ChildComponent onClick={handleClick} />
    

38. How would you implement a simple counter?

    jsx
    function Counter() {
      const [count, setCount] = useState(0);
      return (
        <div>
          <p>{count}</p>
          <button onClick={() => setCount(count + 1)}>Increment</button>
        </div>
      );
    }
    

39. How do you use React Router for navigation?

    jsx
    import { BrowserRouter as Router, Route, Switch } from 'react-router-dom';
    
    function App() {
      return (
        <Router>
          <Switch>
            <Route path="/" component={Home} />
            <Route path="/about" component={About} />
          </Switch>
        </Router>
      );
    }
    

40. What is a custom hook in React?

    • A custom hook is a reusable function that uses built-in hooks to encapsulate logic. It allows you to share stateful logic across components.

Miscellaneous Questions

41. How do you implement error handling in a React app?

    • Use error boundaries to catch errors in the component tree and display fallback UI.

42. What is the use of React.StrictMode?

    • React.StrictMode is a wrapper that helps identify potential problems in an application by activating additional checks and warnings.

43. What is the difference between React.Fragment and an array of elements?

    • React.Fragment groups a list of children without adding extra nodes to the DOM, while arrays create multiple elements in the DOM.

44. How do you access the ref of a DOM element?

    jsx
    const inputRef = useRef(null);
    <input ref={inputRef} />;
    

45. What is the purpose of React.lazy?

    • React.lazy allows you to dynamically import components, enabling code-splitting and lazy loading of those components.

46. How do you handle focus on input fields in React?

    • Use refs to directly access and focus on input fields.

    jsx
    inputRef.current.focus();
    

47. What is the significance of the key prop in lists?

    • The key prop helps React identify which items have changed, are added, or are removed, improving performance in rendering lists.

48. How can you create a higher-order component (HOC)?

    jsx
    function withLogger(WrappedComponent) {
      return function EnhancedComponent(props) {
        console.log(props);
        return <WrappedComponent {...props} />;
      };
    }
    

49. What are the common lifecycle methods in class components?

    • Common lifecycle methods include componentDidMount, componentDidUpdate, and componentWillUnmount.

50. What is a use case for useImperativeHandle?

    • useImperativeHandle is useful when you want to expose specific methods or properties from a child component to its parent using refs.

Conclusion

These questions cover a wide range of React topics, from fundamental concepts to practical applications. Preparing for these will give you a solid foundation for React interviews. Good luck!