Top Interview Questions on React Hooks: A Comprehensive Guide

React Hooks have revolutionized the way we write and manage stateful components in React applications. They allow developers to use state and other React features without writing a class. If you’re preparing for a job interview focused on React, understanding Hooks is crucial. In this article, we’ll cover key interview questions related to React Hooks, complete with explanations and examples to help you ace your interview.

What are React Hooks?

React Hooks are functions that let developers “hook into” React state and lifecycle features from function components. Introduced in React 16.8, hooks provide a more straightforward API for managing component state and lifecycle events, promoting cleaner code and better reusability.

Common Types of Hooks

• useState: Allows you to add state to functional components.
• useEffect: Enables you to perform side effects in function components, such as data fetching and subscriptions.
• useContext: Makes it easier to subscribe to React context without using Class components.
• Custom Hooks: You can create your own hooks to abstract logic and state management for reuse.

Interview Questions on React Hooks

1. What is the purpose of the useState Hook?

The useState Hook is used to declare state variables in functional components. It returns an array with two elements: the current state value and a function to update that state.

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

In this example, count is the state variable initialized to 0, and setCount is the function used to update the count.

2. How does useEffect work?

The useEffect Hook is used to perform side effects in a function component. It executes a function after the render is committed to the DOM, which is useful for tasks like data fetching, subscriptions, or manually changing the DOM.

Here’s an example:

useEffect(() => {
    fetchData();
}, []); // Empty dependency array means it runs once after the initial render

The above code calls fetchData when the component mounts. If you want to rerun the effect when specific variables change, you can pass them in the dependency array.

3. Can you explain the rules of Hooks?

There are two primary rules of Hooks:

• Only call Hooks at the top level: You should only call Hooks inside React function components or from your own custom Hooks. This ensures that the Hooks are called in the same order on every render.
• Only call Hooks from React functions: You should not call Hooks from regular JavaScript functions. Instead, you should use them inside functional components or other Hooks.

4. What is the difference between useEffect and useLayoutEffect?

The useEffect Hook runs after the paint is committed to the DOM, while useLayoutEffect runs synchronously after all DOM mutations but before the browser has a chance to paint. This makes useLayoutEffect useful for reading layout from the DOM and synchronously re-rendering to avoid flickers.

useLayoutEffect(() => {
    // Code to read layout and synchronously re-render
    const width = elementRef.current.offsetWidth;
}, []);

5. What are custom Hooks in React?

Custom Hooks are JavaScript functions that start with “use” and allow you to reuse stateful logic across components. They can call built-in hooks and encapsulate reusable stateful logic, improving code modularity and reducing duplication.

Here’s a simple example:

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

This custom hook provides a counter functionality that can be reused across multiple components.

6. Can you use multiple useState Hooks in a single component?

Absolutely! You can use multiple useState calls in one component to manage different pieces of state:

const [name, setName] = useState('');
const [age, setAge] = useState(0);

This allows you to manage independent pieces of state effectively within the same functional component.

7. How do hooks work with context APIs?

The useContext Hook allows you to access the context value in your functional components without wrapping them in a traditional context consumer:

const value = useContext(MyContext);

This provides a cleaner and more concise way to consume context compared to the older Context API method.

Best Practices for Using React Hooks

• Keep the logic reusable: Create custom hooks to encapsulate and reuse logic across components.
• Dependency Arrays: Always manage your dependency arrays carefully to avoid unintended infinite loops.
• Use useEffect wisely: Use useEffect only to handle side effects, and separate them based on their purposes.
• Group Update State Calls: If you have multiple state updates based on the same event, it’s better to group them into a single setState call.

Conclusion

Understanding React Hooks is crucial for modern React development, and mastering them can significantly enhance your coding skill set as a developer. Prepare well for your interviews by studying these questions and practicing your answers, so you can demonstrate your knowledge clearly.

By familiarizing yourself with how hooks work and applying best practices, you can pave the way for writing cleaner code, enhancing reusability, and creating efficient components in your React applications. Happy coding!