REACT CLASS COMPONENTS /// COMPONENT LIFECYCLE /// THIS.SETSTATE /// RENDER() /// BEFORE HOOKS /// COMPONENT LIFECYCLE /// THIS.SETSTATE /// RENDER() ///

React Class Components

Explore the legacy architecture. Learn how state and lifecycles were managed before Hooks changed the React ecosystem.

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React Classes

The Legacy Architecture.

A.D.A:Before Hooks were introduced in React 16.8, Class Components were the only way to manage state and lifecycle methods in React. They are still widely used in legacy codebases.


Skill Matrix

UNLOCK NODES BY LEARNING CORE CLASS CONCEPTS.

Concept: Class Setup

A class component must extend React.Component and must have a render() function returning JSX.

System Check

What is the ONLY mandatory method inside a React Class Component?

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React Class Components

Author

Pascual Vila

Frontend Instructor // Code Syllabus

Before the introduction of Hooks in 2019, React relied exclusively on JavaScript classes to handle state and lifecycle events. Understanding them is crucial for maintaining older enterprise applications.

Syntax & Setup

A class component must extend React.Component and must contain a render() method. The render method is pure and simply returns the JSX to display.

State Management

State is initialized inside the class constructor(). We must call super(props) first. Then, we set the initial state object on this.state. Crucially, we never mutate this object directly later; we rely entirely on this.setState(), which tells React to re-render the component.

The Lifecycle

Classes grant us access to "Lifecycle Hooks" (not to be confused with React Hooks). The most common are:

  • componentDidMount: Runs once immediately after the component is inserted into the DOM.
  • componentDidUpdate: Runs immediately after updating occurs (props or state changes).
  • componentWillUnmount: Runs right before a component is removed. Used for cleanup.

View Full Legacy Context+

Class components introduced the heavy usage of the `this` keyword in JavaScript, leading to common bugs related to event handler binding. Developers either had to explicitly bind methods in the constructor `this.handleClick = this.handleClick.bind(this)` or adopt Class Properties syntax with arrow functions to lexicaly capture `this`.

React Class Glossary

Class Component

A JavaScript class that extends React.Component and contains a render method.

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constructor()

A special method for creating and initializing an object created within a class. In React, used for initial state and binding.

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super(props)

Required in the constructor if you want to access `this.props`. It calls the constructor of the parent class (React.Component).

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this.state

An object containing data specific to the component that may change over time.

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this.setState()

The only safe way to modify state. It schedules an update to a component's state object and tells React to re-render.

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componentDidMount()

A lifecycle method invoked immediately after a component is mounted (inserted into the tree). Best for API calls.

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