What are the differences between functional and class components in React js?

In React.js, there are two main types of components: functional components and class components. Here are the key differences between them:

• Syntax and Definition:

  • Functional Components: Functional components are simple JavaScript functions that accept props as arguments and return React elements describing what should appear on the screen. They are also known as stateless components or presentational components. Example:

  function Welcome(props) {
    return <h1>Hello, {props.name}</h1>;
  }
  

  • Class Components: Class components are ES6 classes that extend from React.Component and must include a render() method which returns a React element. They can also hold and manage their own state. Example:

  class Welcome extends React.Component {
    render() {
      return <h1>Hello, {this.props.name}</h1>;
    }
  }
  

• State Management:

  • Functional Components: Functional components don't have their own state. They are pure functions that only accept props and return JSX.
  • Class Components: Class components can have their own state managed by React. You can initialize state in the constructor and update it using setState() method.

• Lifecycle Methods:

  • Functional Components: Functional components cannot have lifecycle methods like componentDidMount, componentDidUpdate, etc.
  • Class Components: Class components can have lifecycle methods which allow you to hook into different phases of a component's lifecycle.

• Readability and Maintainability:

  • Functional Components: Functional components are generally simpler and more concise. They focus solely on rendering UI based on input props.
  • Class Components: Class components can become more complex and harder to read, especially as the component grows in size and features.

• Performance:

  • Functional Components: Functional components are typically faster to render than class components because they are just plain JavaScript functions.
  • Class Components: Class components might have a slight performance overhead due to the overhead of creating and managing class instances.

• Hooks:

  • Functional Components: Functional components can use React Hooks, which are functions that let you use state and other React features without writing a class.
  • Class Components: Class components do not support hooks. Hooks are exclusive to functional components and cannot be used within class components.

Feature	Functional Components	Class Components
Syntax	Defined as JavaScript functions	Defined as ES6 classes
State	Cannot use state directly	Can use state with this.state
State Management	Use of Hooks (useState, useEffect, etc.)	Use of this.setState() method
Lifecycle Methods	Can use useEffect() Hook for lifecycle events	Can use lifecycle methods (componentDidMount, etc.)
Code Readability	Generally more concise and readable	More verbose syntax
Performance	Generally better performance with Hooks	Might have performance overhead due to class instantiation
Context	Can use useContext() Hook for context	Can access context via this.context
Refs	Can use useRef() Hook for refs	Can use the ref attribute
Code Structure	Functional, often with less boilerplate	Class-based, might require more boilerplate

In recent versions of React, functional components with hooks have become more prevalent due to their simplicity, performance, and flexibility in managing component logic. However, class components are still widely used, especially in legacy codebases.