React Lead interview questions with solutions

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 Here are some advanced React interview questions often asked to React leads or senior-level developers, along with their explanations and solutions:

1. What are the main features of React?

Answer:

React is a JavaScript library used for building user interfaces, especially for single-page applications (SPAs). Its main features include:

  • Component-Based Architecture: Everything in React is a component, which can be either a class or a function.
  • Virtual DOM: React uses a virtual DOM to optimize updates to the actual DOM, enhancing performance.
  • Declarative UI: React allows developers to describe what the UI should look like, and React takes care of updating the DOM.
  • JSX Syntax: JSX is a syntax extension that allows HTML-like syntax in JavaScript files, making the code more readable and maintainable.
  • Unidirectional Data Flow: Data flows in one direction in React, from parent components to child components via props.
  • Hooks: React hooks like useState, useEffect, etc., allow you to use state and lifecycle methods in functional components.

2. What are React Hooks, and how do they differ from class components?

Answer:

React hooks are functions that let you "hook into" React state and lifecycle features from functional components.

  • useState: Allows you to add state to functional components.
  • useEffect: Allows you to perform side effects in functional components (replaces lifecycle methods like componentDidMount, componentDidUpdate, and componentWillUnmount).
  • useContext: Allows you to access the context API in functional components.
  • useMemo and useCallback: For optimizing performance by memoizing expensive calculations or callbacks.

In class components, state and lifecycle methods are handled with this.state, this.setState(), and lifecycle methods like componentDidMount. With hooks, you can manage state and side effects more concisely in functional components.

3. Explain the difference between useEffect and useLayoutEffect.

Answer:

  • useEffect: This hook runs asynchronously after the component has rendered and committed to the DOM. It’s ideal for operations like fetching data, subscribing to external APIs, and manipulating the DOM.

  • useLayoutEffect: This hook runs synchronously after all DOM mutations but before the browser has painted. It’s useful for DOM reads or layout changes (e.g., measuring DOM elements before rendering).

In most cases, you should prefer useEffect because it's non-blocking, but if you need to perform operations that require the DOM to be immediately available (like measuring DOM nodes), use useLayoutEffect.

4. What is the Context API in React, and how do you use it?

Answer:

The Context API allows you to share data across your component tree without having to pass props down manually through every level.

How to use Context:

  1. Create a Context:


    const MyContext = React.createContext();

  2. Provide a Context Value: Wrap your component tree with a Provider and pass the data you want to share as a value.


    function App() {
  const user = { name: 'John' };
  return (
    <MyContext.Provider value={user}>
      <ComponentA />
    </MyContext.Provider>
  );
}

  3. Consume the Context Value: You can use useContext hook to access the shared data.


    function ComponentB() {
  const user = useContext(MyContext);
  return <div>Hello, {user.name}</div>;
}

5. Explain React’s reconciliation algorithm.

Answer:

React’s reconciliation algorithm (also known as the diffing algorithm) is responsible for efficiently updating the DOM when the state of a component changes. React does this by comparing the new virtual DOM with the previous one and determining the minimal set of changes needed to update the actual DOM.

  • Key concepts:
    • Virtual DOM: A lightweight in-memory representation of the actual DOM.
    • Keys: React uses keys to identify which elements in a list are added, removed, or changed.
    • Fiber: The new reconciliation algorithm introduced in React 16, allowing React to break down updates into units of work and prioritize them accordingly.

React compares the current virtual DOM and the previous virtual DOM tree, calculates the differences (diff), and applies the necessary updates to the actual DOM, making the process more efficient.

6. What are Pure Components in React?

Answer:

A PureComponent is a component that only re-renders when its props or state change. This optimization is achieved by performing a shallow comparison of the component’s props and state.

  • PureComponent performs a shallow comparison of the props and state in shouldComponentUpdate(). If the props and state haven't changed, React skips rendering the component.

class MyComponent extends React.PureComponent {
  render() {
    return <div>{this.props.name}</div>;
  }
}

Difference from regular components: In a regular Component, React re-renders on every state or prop change. PureComponent optimizes this by doing a shallow comparison.

7. What is the use of shouldComponentUpdate in React?

Answer:

The shouldComponentUpdate method allows you to control whether a component should re-render or not. By default, React re-renders every time state or props change. However, implementing shouldComponentUpdate can improve performance by preventing unnecessary renders.

  • It takes the next set of props and state as arguments and returns a boolean (true to update, false to skip the update).
  • It's used in class components.

class MyComponent extends React.Component {
  shouldComponentUpdate(nextProps, nextState) {
    return nextProps.someValue !== this.props.someValue;
  }
}

8. What is React's Error Boundaries feature?

Answer:

Error Boundaries are React components that catch JavaScript errors in their child components, log those errors, and display a fallback UI. This prevents the entire application from crashing when an error occurs.

How to create an Error Boundary:

  1. Create a class component that implements componentDidCatch and renders an error message.

class ErrorBoundary extends React.Component {
  constructor(props) {
    super(props);
    this.state = { hasError: false, errorInfo: null };
  }

  static getDerivedStateFromError(error) {
    return { hasError: true };
  }

  componentDidCatch(error, errorInfo) {
    this.setState({ errorInfo });
    // Log error to an external service, etc.
  }

  render() {
    if (this.state.hasError) {
      return <h1>Something went wrong.</h1>;
    }
    return this.props.children;
  }
}
  1. Wrap your component with the Error Boundary.

<ErrorBoundary>
  <MyComponent />
</ErrorBoundary>

9. What is the difference between useMemo and useCallback?

Answer:

Both useMemo and useCallback are React hooks used for performance optimization by memoizing values and functions, but they differ in their usage:

  • useMemo: It memoizes the result of a computation (a value).


    const memoizedValue = useMemo(() => computeExpensiveValue(a, b), [a, b]);

  • useCallback: It memoizes a function.


    const memoizedCallback = useCallback(() => { doSomething(a, b); }, [a, b]);

Use useMemo when you want to memoize expensive calculations and useCallback when you want to memoize a function that gets passed down to child components to avoid unnecessary re-renders.

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

Answer:

  • Controlled Component: A component where React controls the value of an input field via state. The value is set and updated through the component’s state.


    function ControlledComponent() {
  const [value, setValue] = useState('');
  return <input value={value} onChange={(e) => setValue(e.target.value)} />;
}

  • Uncontrolled Component: A component where React does not manage the state of an input field, and the value is managed by the DOM itself. You typically use refs to interact with uncontrolled components.


    function UncontrolledComponent() {
  const inputRef = useRef();
  return <input ref={inputRef} />;
}

11. What is the purpose of the key prop in React?

Answer:

The key prop is used to identify elements in a list of React components. It helps React identify which items have changed, been added, or been removed, improving performance by minimizing DOM manipulations.

  • Best practice: Use a unique identifier (e.g., id or index) as the key in a list.

const items = ['apple', 'banana', 'cherry'];
return (
  <ul>
    {items.map((item, index) => (
      <li key={index}>{item}</li>
    ))}
  </ul>
);

12. What is the useReducer hook and when would you use it?

Answer:

The useReducer hook is an alternative to useState that is often preferable when managing more complex state logic involving multiple sub-values or when the next state depends on the previous one.

Syntax:


const [state, dispatch] = useReducer(reducer, initialState);
  • reducer is a function that takes the current state and an action and returns the new state.
  • dispatch is a function that dispatches actions to update the state.

Example:


const initialState = { count: 0 };
function reducer(state, action) {
  switch (action.type) {
    case 'increment':
      return { count: state.count + 1 };
    case 'decrement':
      return { count: state.count - 1 };
    default:
      return state;
  }
}

function Counter() {
  const [state, dispatch] = useReducer(reducer, initialState);

  return (
    <div>
      <p>Count: {state.count}</p>
      <button onClick={() => dispatch({ type: 'increment' })}>Increment</button>
      <button onClick={() => dispatch({ type: 'decrement' })}>Decrement</button>
    </div>
  );
}

useReducer is especially useful for handling state logic that is more complex or involves multiple values, or when working with state updates that depend on the previous state.

These are some essential React lead interview questions and answers that demonstrate a solid understanding of React, its core concepts, and its performance optimization techniques. Make sure you’re prepared to explain each of these concepts clearly during your interview.

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