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What is React Component Lifecycle ?

Posted on May 28, 2021December 2, 2021 By christo No Comments on What is React Component Lifecycle ?

React Component has LifeCycle through which we can monitor or manipulate React Component during its phases

React three main phases are

  • Mounting
  • Updating
  • Unmounting

What is Mounting in React Component phase ?

Mounting in React Component phase is that phase in which we put Elements in DOM

React Mounting Methods ?

React has four inbuilt Mounting methods

  • constructor()
  • getDerivedStateFromProps()
  • render()
  • componentDidMount()

in following four mounting method render() method is required while other we use on the behalf of need

constructor()

constructor method is called when React Component initialize

Example:


class Color extends React.Component {
  constructor(props) {
    super(props);
    this.state = {favoritecolor: "blue"};
  }
  render() {
    return (
      <h1>My Favorite Color is {this.state.favoritecolor}</h1>
    );
  }
}

ReactDOM.render(<Color />, document.getElementById('root'));

What is getDerivedStateFromProps in React ?

in mountijng phase getDerivedStateFromProps is called before render() method

getDerivedStateFromProps change the State properties by Props

getDerivedStateFromProps has two parameters one is state and another is props

Example


class Color extends React.Component {
  constructor(props) {
    super(props);
    this.state = {favoritecolor: "red"};
  }
  static getDerivedStateFromProps(props, state) {
    return {favoritecolor: props.favcol };
  }
  render() {
    return (
      <h1>My Favorite Color is {this.state.favoritecolor}</h1>
    );
  }
}

ReactDOM.render(<Color favcol="blue"/>, document.getElementById('root'));

render

render() is that method through that HTML output take place in DOM

Example


class Color extends React.Component {
  render() {
    return (
      <h1>my color is </h1>
    );
  }
}

ReactDOM.render(<Color/>, document.getElementById('root'));

What is componentDidMount in React ?

React Component lifecycle in which componentDidMount method is called after render method call means after HTML output in DOM

Example


class Color extends React.Component {
  constructor(props) {
    super(props);
    this.state = {favoritecolor: "red"};
  }
  componentDidMount() {
    setTimeout(() => {
      this.setState({favoritecolor: "blue"})
    }, 1000)
  }
  render() {
    return (
      <h1>My Favorite Color is {this.state.favoritecolor}</h1>
    );
  }
}

ReactDOM.render(<Color/>, document.getElementById('root'));

React Updating lifecycle after mounting

React has five inbuilt method that is called while updating DOM

  • getDerivedStateFromProps()
  • shouldComponentUpdate()
  • render()
  • getSnapshotBeforeUpdate()
  • componentDidUpdate()

getDerivedStateFromProps()

getDerivedStateFromProps() method is called just after update

Example:


class Color extends React.Component {
  constructor(props) {
    super(props);
    this.state = {favoritecolor: "red"};
  }
  static getDerivedStateFromProps(props, state) {
    return {favoritecolor: props.favcol };
  }
  changeColor = () => {
    this.setState({favoritecolor: "blue"});
  }
  render() {
    return (
      <div>
      <h1>My Favorite Color is {this.state.favoritecolor}</h1>
      <button type="button" onClick={this.changeColor}>Change color</button>
      </div>
    );
  }
}

ReactDOM.render(<Color favcol="yellow"/>, document.getElementById('root'));

No update will take place due to getDerivedStateFromProps() method

shouldComponentUpdate method

default value of shouldComponentUpdate is true. if shouldComponentUpdate will return false then no change will atke place

Example of shouldComponentUpdate with return false


class Color extends React.Component {
  constructor(props) {
    super(props);
    this.state = {favoritecolor: "red"};
  }
  shouldComponentUpdate() {
    return false;
  }
  changeColor = () => {
    this.setState({favoritecolor: "blue"});
  }
  render() {
    return (
      <div>
      <h1>My Favorite Color is {this.state.favoritecolor}</h1>
      <button type="button" onClick={this.changeColor}>Change color</button>
      </div>
    );
  }
}

ReactDOM.render(<Color />, document.getElementById('root'));


No change will take place with return false

Example of shouldComponentUpdate with return true


class Color extends React.Component {
  constructor(props) {
    super(props);
    this.state = {favoritecolor: "red"};
  }
  shouldComponentUpdate() {
    return true;
  }
  changeColor = () => {
    this.setState({favoritecolor: "blue"});
  }
  render() {
    return (
      <div>
      <h1>My Favorite Color is {this.state.favoritecolor}</h1>
      <button type="button" onClick={this.changeColor}>Change color</button>
      </div>
    );
  }
}

ReactDOM.render(<Color />, document.getElementById('root'));

Update will take place and value will be blue after click

render method in Update

After update render() will called to rerender the HTML to the DOM


class Color extends React.Component {
  constructor(props) {
    super(props);
    this.state = {favoritecolor: "red"};
  }
  changeColor = () => {
    this.setState({favoritecolor: "blue"});
  }
  render() {
    return (
      <div>
      <h1>My Favorite Color is {this.state.favoritecolor}</h1>
      <button type="button" onClick={this.changeColor}>Change color</button>
      </div>
    );
  }
}

ReactDOM.render(<Color />, document.getElementById('root'));

getSnapshotBeforeUpdate in Update phase

getSnapshotBeforeUpdate is associate with componentDidUpdate() without componentDidUpdate() association getSnapshotBeforeUpdate will show error

getSnapshotBeforeUpdate has access to State and Props of React before update

componentDidUpdate() has acces to State and Props after Update


class Color extends React.Component {
  constructor(props) {
    super(props);
    this.state = {favoritecolor: "red"};
  }
  componentDidMount() {
    setTimeout(() => {
      this.setState({favoritecolor: "yellow"})
    }, 1000)
  }
  getSnapshotBeforeUpdate(prevProps, prevState) {
    document.getElementById("div1").innerHTML =
    "Before the update, the favorite was " + prevState.favoritecolor;
  }
  componentDidUpdate() {
    document.getElementById("div2").innerHTML =
    "The updated favorite is " + this.state.favoritecolor;
  }
  render() {
    return (
      <div>
        <h1>My Favorite Color is {this.state.favoritecolor}</h1>
        <div id="div1"></div>
        <div id="div2"></div>
      </div>
    );
  }
}

ReactDOM.render(<Color />, document.getElementById('root'));

componentDidUpdate method

componentDidUpdate method is called after update to DOM

Example


class Color extends React.Component {
  constructor(props) {
    super(props);
    this.state = {favoritecolor: "red"};
  }
  componentDidMount() {
    setTimeout(() => {
      this.setState({favoritecolor: "yellow"})
    }, 1000)
  }
  componentDidUpdate() {
    document.getElementById("mydiv").innerHTML =
    "The updated favorite is " + this.state.favoritecolor;
  }
  render() {
    return (
      <div>
      <h1>My Favorite Color is {this.state.favoritecolor}</h1>
      <div id="mydiv"></div>
      </div>
    );
  }
}

ReactDOM.render(<Color />, document.getElementById('root'));


Unmounting lifecycle

Unmounting is the reverse of mounting it means in Unmounting HTML will get remove from DOM

Unmounting has only one inbuilt method

  • componentWillUnmount()

Example


class Color extends React.Component {
  constructor(props) {
    super(props);
    this.state = {show: true};
  }
  delHeader = () => {
    this.setState({show: false});
  }
  render() {
    let myheader;
    if (this.state.show) {
      myheader = <Colorchild  />;
    };
    return (
      <div>
      {myheader}
      <button type="button" onClick={this.delHeader}>Delete Header</button>
      </div>
    );
  }
}

class Colorchild extends React.Component {
  componentWillUnmount() {
    alert("The component named Header is about to be unmounted.");
  }
  render() {
    return (
      <h1>Hello World!</h1>
    );
  }
}

ReactDOM.render(<Color />, document.getElementById('root'));

React.js Tags:componentDidMount, componentDidUpdate, getDerivedStateFromProps, getSnapshotBeforeUpdate, React life cycle, render, shouldComponentUpdate

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