9. What are React hooks and how do they differ from class component lifecycle methods?

Basic

9. What are React hooks and how do they differ from class component lifecycle methods?

Overview

React Hooks are functions that let you "hook into" React state and lifecycle features from function components. They provide a more direct API to the React concepts you already know: props, state, context, refs, and lifecycle. Unlike class components, hooks don't require you to use this, making code more readable and easier to maintain. The introduction of hooks in React 16.8 marked a significant shift in how components can be written and organized, moving away from class-based components towards functional components with enhanced capabilities.

Key Concepts

  1. State Hooks: Use the useState hook to add local state to function components.
  2. Effect Hooks: Use the useEffect hook to perform side effects in function components, analogous to lifecycle methods in class components.
  3. Custom Hooks: Create your own hooks to share reusable stateful logic between components.

Common Interview Questions

Basic Level

  1. What is a React Hook?
  2. How do you use the useState hook in a function component?

Intermediate Level

  1. How does the useEffect hook differ from class component lifecycle methods like componentDidMount, componentDidUpdate, and componentWillUnmount?

Advanced Level

  1. How can you optimize performance in a component that uses hooks?

Detailed Answers

1. What is a React Hook?

Answer: React Hooks are functions that let function components hook into React's state and lifecycle features. They offer a simpler and more elegant way to use React's features without writing class components, making the code easier to understand and maintain. Hooks can only be used inside function components.

Key Points:
- Hooks allow function components to manage state and side effects.
- They enable code reuse without the complexity of HOCs or render props.
- Hooks are designed to create components that are easier to understand and test.

Example:

// This is not applicable in C# as React and Hooks are JavaScript/TypeScript concepts.

2. How do you use the useState hook in a function component?

Answer: The useState hook lets you add state to functional components. Call it within a component to declare a state variable. useState returns a pair: the current state value and a function that lets you update it.

Key Points:
- useState allows you to add stateful logic to function components.
- It returns an array containing the current state and a function to update it.
- You can use multiple useState calls to add multiple state variables.

Example:

// This is not applicable in C# as React and Hooks are JavaScript/TypeScript concepts.

3. How does the useEffect hook differ from class component lifecycle methods like componentDidMount, componentDidUpdate, and componentWillUnmount?

Answer: The useEffect hook unifies the use of side effects in functional components, replacing several lifecycle methods in class components. It runs after every render, including the first one. You can control when an effect runs by passing a second argument, an array of values that the effect depends on.

Key Points:
- useEffect can replicate the behavior of componentDidMount, componentDidUpdate, and componentWillUnmount through a single API.
- It simplifies the logic by grouping related code into a single effect.
- Dependency array controls the effect execution, optimizing performance by preventing unnecessary runs.

Example:

// This is not applicable in C# as React and Hooks are JavaScript/TypeScript concepts.

4. How can you optimize performance in a component that uses hooks?

Answer: To optimize a component with hooks, use the React.memo for functional components to prevent unnecessary re-renders. For useEffect, specify a dependency array to ensure the effect runs only when certain values change. Additionally, the useCallback and useMemo hooks can be used to memoize functions and values, preventing unnecessary calculations on every render.

Key Points:
- React.memo prevents re-renders if props haven’t changed.
- Dependency arrays in useEffect limit the execution of effects only when specific values change.
- useCallback and useMemo help in memoizing functions and values to avoid unnecessary recalculations.

Example:

// This is not applicable in C# as React and Hooks are JavaScript/TypeScript concepts.
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