Essential React Hooks Interview Questions Every Developer Should Know

React has revolutionized the way we build user interfaces by allowing developers to create reusable components. With the introduction of React Hooks in version 16.8, handling state and lifecycle methods became easier, and the functional component paradigm gained immense popularity. As you prepare for an interview, knowing React Hooks is crucial. This article highlights some essential React Hooks interview questions, complete with answers and examples to help you impress your interviewer.

1. What are React Hooks?

React Hooks are functions that let you use state and other React features without writing a class. They allow functional components to manage component lifecycle and side effects, making your code easier to read and test. The most notable Hooks include useState, useEffect, and useContext.

2. Explain the useState Hook.

The useState Hook allows you to add state to a functional component. It returns an array with the current state value and a function to update it.

import React, { useState } from 'react';

function Counter() {
    const [count, setCount] = useState(0);

    return (
        <div>
            <p>Current Count: {count}</p>
            <button onClick={() => setCount(count + 1)}>Increment</button>
        </div>
    );
}

In this example, the state variable count is initialized to 0, and the setCount function updates its value when the button is clicked.

3. What is useEffect and how is it used?

The useEffect Hook manages side effects in functional components, replacing lifecycle methods such as componentDidMount, componentDidUpdate, and componentWillUnmount. It accepts two arguments: a function containing the side-effect code and an optional dependencies array.

import React, { useState, useEffect } from 'react';

function Timer() {
    const [count, setCount] = useState(0);

    useEffect(() => {
        const interval = setInterval(() => setCount(prevCount => prevCount + 1), 1000);
        return () => clearInterval(interval);
    }, []);

    return <p>Timer: {count}</p>
}

This example starts a timer when the component mounts and clears the interval when it unmounts.

4. Can you explain the useContext Hook?

The useContext Hook allows you to access context directly within a functional component, helping to avoid prop drilling. You must first create a context using React.createContext.

import React, { useContext } from 'react';

const MyContext = React.createContext();

function ContextComponent() {
    const value = useContext(MyContext);
    return <p>Context Value: {value}</p>
}

function App() {
    return (
        <MyContext.Provider value="Hello, World!">
            <ContextComponent />
        </MyContext.Provider>
    );
}

In this example, ContextComponent accesses the context provided by App.

5. What are custom hooks?

Custom Hooks allow you to encapsulate logic and state management for reuse across components. They start with “use” and can call other Hooks.

import { useState } from 'react';

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

export default useCounter;

This custom hook, useCounter, allows any component to use counter functionality without duplicating code.

6. How to optimize performance using useMemo and useCallback?

Performance optimization can be achieved using useMemo and useCallback. They help to memoize expensive computations and ensure that functions only update when necessary.

Example of useMemo:

import React, { useMemo } from 'react';

function Fibonacci({ num }) {
    const fib = useMemo(() => {
        const computeFibonacci = (n) => (n < 2 ? n : computeFibonacci(n - 1) + computeFibonacci(n - 2));
        return computeFibonacci(num);
    }, [num]);

    return <p>Fibonacci of {num} is: {fib}</p>
}

Example of useCallback:

import React, { useState, useCallback } from 'react';

function Counter() {
    const [count, setCount] = useState(0);
    
    const increment = useCallback(() => {
        setCount(c => c + 1);
    }, [setCount]);

    return (
        <div>
            <p>Count: {count}</p>
            <button onClick={increment}>Increment</button>
        </div>
    );
}

In these examples, useMemo is used to calculate the Fibonacci number only when the num prop changes, and useCallback prevents the increment function from being recreated unless necessary, optimizing performance in child components.

7. Can you explain lifting state up and how Hooks support it?

Lifting state up is the practice of moving state to the closest common ancestor of components that need to share that state. Hooks support this by allowing you to define state at higher levels and pass it down as props.

import React, { useState } from 'react';

function Parent() {
    const [value, setValue] = useState("");

    return (
        <div>
            <ChildA value={value} setValue={setValue} />
            <ChildB value={value} />
        </div>
    );
}

function ChildA({ value, setValue }) {
    return <input value={value} onChange={(e) => setValue(e.target.value)} />;
}

function ChildB({ value }) {
    return <p>Value from Parent: {value}</p>;
}

This example demonstrates lifting state from ChildA to the Parent, which manages the shared state.

8. What is the difference between component-based state and global state?

Component-based state refers to state scoped within a specific component, while global state is shared across multiple components. While Hooks manage component state easily, for global state management, libraries like Redux or Context API can be used alongside Hooks.

9. How do you handle errors in a functional component using Hooks?

Error boundaries can only be created with class components, but functional components can handle errors using try/catch within useEffect or by employing a custom error boundary pattern.

import React, { useState, useEffect } from 'react';

function ErrorHandlingComponent() {
    const [data, setData] = useState(null);
    const [error, setError] = useState(null);

    useEffect(() => {
        fetch('/api/data')
            .then(response => response.json())
            .then(setData)
            .catch(setError);
    }, []);

    if (error) {
        return <p>Error: {error.message}</p>;
    }

    return data ? <p>Data: {data}</p> : <p>Loading...</p>;
}

10. What patterns can you follow while unit testing components using Hooks?

When unit testing components utilizing Hooks, consider the following patterns:

• Utilize React Testing Library for rendering components
• Mock useEffect side effects
• Test state updates by simulating user interactions

Example:

import { render, fireEvent } from '@testing-library/react';
import Counter from './Counter';

test('increments count on button click', () => {
    const { getByText } = render(<Counter />);
    const button = getByText(/increment/i);
    
    fireEvent.click(button);
    
    expect(getByText(/count: 1/i)).toBeInTheDocument();
});

Wrap-Up

Understanding React Hooks is essential for any modern React developer. By mastering these concepts and the related interview questions discussed above, you’ll be well-prepared for your next job interview. Ensure you practice implementations and explore various use cases of React Hooks. Good luck, and happy coding!