Overview

In React, understanding the differences between functional components and class components is crucial for designing and maintaining applications efficiently. Initially, React was built around class components, which are feature-rich. However, with the introduction of Hooks in React 16.8, functional components have become more powerful, allowing them to manage state and side effects, previously exclusive to class components. Knowing when and how to use each type of component is essential for writing clean, efficient React code.

Key Concepts

1. State Management: How class components use this.setState and functional components use useState.
2. Lifecycle Methods vs Hooks: Class components have lifecycle methods (e.g., componentDidMount), whereas functional components use Hooks (e.g., useEffect) for side effects.
3. Syntax and Boilerplate: The syntax differences and the amount of boilerplate code required for class components compared to functional components.

Common Interview Questions

Basic Level

1. What are the fundamental differences between functional and class components in React?
2. How do you manage state in class components versus functional components?

Intermediate Level

3. How can lifecycle methods in class components be replicated in functional components?

Advanced Level

4. Discuss the performance implications of using functional components with Hooks versus class components.

Detailed Answers

1. What are the fundamental differences between functional and class components in React?

Answer: Functional components are simple JavaScript functions that accept props as an argument and return React elements. They are stateless by default but can use Hooks to manage state and lifecycle events. Class components, on the other hand, are ES6 classes that extend React.Component and can hold and manage state and lifecycle methods internally.

Key Points:
- Functional components are easier to write and understand due to their simplicity and use of plain JavaScript functions.
- Class components provide more features out-of-the-box, such as state management and lifecycle methods, making them more suitable for complex scenarios.
- With the introduction of Hooks, functional components can now perform most tasks previously only possible with class components.

Example:

// This is not applicable in C# as React components are written in JavaScript or TypeScript.
// Please refer to JavaScript examples for functional and class components.

2. How do you manage state in class components versus functional components?

Answer: In class components, state is managed using this.state to initialize and this.setState() to update state. Functional components use the useState Hook to manage state, which returns an array containing the current state value and a function to update it.

Key Points:
- this.setState in class components merges the new state with the previous state, whereas useState in functional components does not automatically merge updates.
- State updates in class components may be asynchronous, and functional components share this behavior when using useState.
- useState allows for a more modular and reusable state logic in functional components.

Example:

// This is not applicable in C# as React components are written in JavaScript or TypeScript.
// For state management in React, see JavaScript or TypeScript examples.

3. How can lifecycle methods in class components be replicated in functional components?

Answer: Lifecycle methods in class components such as componentDidMount, componentDidUpdate, and componentWillUnmount can be replicated in functional components using the useEffect Hook. useEffect can be configured to run when the component mounts, updates, and unmounts, mirroring the behavior of lifecycle methods.

Key Points:
- useEffect without dependencies or with an empty dependency array ([]) acts like componentDidMount.
- useEffect with a dependency array mimics componentDidUpdate, running the effect when dependencies change.
- Returning a cleanup function from useEffect replicates the behavior of componentWillUnmount.

Example:

// This is not applicable in C# as React components are written in JavaScript or TypeScript.
// For lifecycle management in React, see JavaScript or TypeScript examples of useEffect.

4. Discuss the performance implications of using functional components with Hooks versus class components.

Answer: Functional components with Hooks can lead to better performance in some cases, mainly due to their reduced overhead and the potential for more straightforward component trees. However, improper use of Hooks, such as inline function definition and unnecessary re-renders, can negate these benefits. Class components may incur a slight performance penalty due to their larger size and complexity but provide more explicit control over component behavior and optimizations.

Key Points:
- Functional components can be less memory-intensive and faster to mount and update due to their simplicity.
- Misuse of Hooks can lead to performance issues, similar to mismanagement of lifecycle methods in class components.
- React memoization techniques and shouldComponentUpdate can optimize both functional and class components.

Example:

// This is not applicable in C# as React components are written in JavaScript or TypeScript.
// For performance comparison and optimization in React, see JavaScript or TypeScript examples.