JAVASCRIPT

lifecycle methods react

Every component in React goes through a lifecycle of events. I like to think of them as going through a cycle of birth, growth, and death.

Mounting – Birth of your component
Update – Growth of your component
Unmount – Death of your component

reactjs lifecycle class components

import React from 'react';
import ReactDOM from 'react-dom';
 
class Test extends React.Component {
    constructor(props)
    {
        super(props);
        this.state = { hello : "World!" };
    }
 
    componentWillMount()
    {
        console.log("componentWillMount()");
    }
 
    componentDidMount()
    {
        console.log("componentDidMount()");
    }
 
    changeState()
    {
        this.setState({ hello : "Geek!" });
    }
 
    render()
    {
        return (
            <div>
            <h1>GeeksForGeeks.org, Hello{ this.state.hello }</h1>
            <h2>
            <a onClick={this.changeState.bind(this)}>Press Here!</a>
            </h2>
            </div>);
    }
 
    shouldComponentUpdate(nextProps, nextState)
    {
        console.log("shouldComponentUpdate()");
        return true;
    }
 
    componentWillUpdate()
    {
        console.log("componentWillUpdate()");
    }
 
    componentDidUpdate()
    {
        console.log("componentDidUpdate()");
    }
}
 
ReactDOM.render(
    <Test />,
    document.getElementById('root'));

Implementing state lifecycle in react class component

class Example extends React.Component {
  constructor(props) {
    super(props);
    this.state = {
      count: 0
    };
  }

  componentDidMount() {
    document.title = `You clicked ${this.state.count} times`;
  }
  componentDidUpdate() {
    document.title = `You clicked ${this.state.count} times`;
  }

  render() {
    return (
      <div>
        <p>You clicked {this.state.count} times</p>
        <button onClick={() => this.setState({ count: this.state.count + 1 })}>
          Click me
        </button>
      </div>
    );
  }
}

react lifecycle

Initialization = setup props & state

// Lifecycle Phase 
1. mounting // Born
2. update   // Growth
3. unmounting //Dead

// lifecycle method
mounting = constructor->render->componentDidMount
update = render->componentDidUpdate
unmounting = componentWillUnmount

React Component Lifecycle

What is the order that Class components run when React initialize and mounts 
the components.

1. Regardless of React Components, constructors are always ran first 
	1. In constructor, you will just initialize the state in constructor
2. Render runs second, which determines what to show. This is the template 
   of the HTML and dictates the UI is going to be.
3. Third one that runs is componentDidMount, React runs the constructor, 
   runs the render and mounts the initial state and mounts on. Then it 
   will runs the code in componentDidMount();
   
- When setState() is called, the render() method will be called again and 
  re-renders.
	- Every subsequent child with a different props will also be re-rendered.