REACT LIFE CYCLE

Class components can define functions that will execute at certain points during the component’s lifecycle. Using them can give a finer level of control over components.

There are a total of seven lifecycle methods: componentWillMount, componentDidMount, componentWillReceiveProps, shouldComponentUpdate, componentWillUpdate, componentDidUpdate, and componentWillUnmount.

componentWillMount

componentWillMount is called before the component is rendered for the first time. You can use this to call setState before the initial render.

class Scorecard extends Component {
  componentWillMount() {
    this.setState({score: 0});
  }
  render() {
    const {playerName} = this.props;
    // `this.state` defaults to null, but since it'll be set in
    // `componentWillMount`, it can safely be destructured.
    const {score} = this.state;
    const message = `Current Score: ${score}`;
    return (
      <div>
        <h1>{playerName}</h1>
        <div>{message}</div>
      </div>
    );
  }
}

Calling setState will usually trigger a re-render, but calling it in componentWillMount won't (since it hasn't rendered in the first place).

componentDidMount

componentDidMount is called after the component is rendered for the first time. This can be used to start an async operation as soon as the component shows up.

class Scorecard extends Component {
  // Other functions omitted for brevity.
  componentDidMount() {
    // You'd probably want to send an AJAX call or something,
    // but let's say they get 1000 points after the first second.
    setTimeout(() => this.setState({score: 1000}), 1000);
  }
}

componentDidMount won't be called in server rendering.

componentWillReceiveProps

componentWillReceiveProps is called when the component receives new props, but before it renders. You can call setState here without causing another re-render, since there’s already one pending.

class Scorecard extends Component {
  // Other functions omitted for brevity.
  componentWillReceiveProps(nextProps) {
    // `nextProps` is the new props, while `this.props` are the old ones.
    const {playerName} = this.props;
    // It is entirely possible that the new `playerName` is the same as the old one.
    if (nextProps.playerName !== playerName) {
      // Changing your name resets the score to zero.
      this.setState({score: 0});
    }
  }
}

shouldComponentUpdate

shouldComponentUpdate is called after props or state are changed (and after componentWillReceiveProps), but before it renders. It’s unique among lifecycle functions in that it is expected to return a boolean value. If false, render will not be called. This can be very useful for skipping unnecessary renders and save some CPU.

class Scorecard extends Component {
  // Other functions omitted for brevity.
  shouldComponentUpdate(nextProps, nextState) {
    // Same as `componentWillReceiveProps`, `nextProps` is the
    // new props and `this.props` is the old.
    const {playerName} = this.props;
    // Ditto for `nextState` and `this.state`.
    const {score} = this.state;
    // Only `playerName` and `score` affect the display.
    // If something else changes, re-rendering would be a waste.
    return !(nextProps.playerName === playerName && nextState.score === score);
  }
}

componentWillUpdate

componentWillUpdate is called right before the component renders and right after shouldComponentUpdate. It can’t call setState.

class Scorecard extends Component {
  // Other functions omitted for brevity.
  componentWillUpdate(nextProps, nextState) {
    const {playerName} = this.props;
    // If `playerName` changes, log a message.
    if (nextProps.playerName !== playerName) {
      // Note that even though `componentWillReceiveProps` called `setState`, `this.state` is still the original value.
      const {score} = this.state;
      console.log(`${playerName} is now ${nextProps.playerName}.  His score of ${score} is forfeit.`);
    }
  }
}

componentDidUpdate

componentDidUpdate is called after render is finished. As with the other update functions, it’ll have both the new and old versions of props and state, but with the previous versions as the parameters, instead.

class Scorecard extends Component {
  // Other functions omitted for brevity.
  componentDidUpdate(prevProps, prevState) {
    const {playerName} = this.props;
    // You guessed it, `prevProps` are the props as they used to be and `this.props` are what they are now.
    // Ditto for `prevState` and `this.state`.
    if (prevProps.playerName !== playerName) {
      const {score} = prevState;
      console.log(`${playerName} used to be ${prevProps.playerName}.  His former score was ${score}.`);
    }
  }
}

componentWillUnmount

Lastly, componentWillUnmount is called before the component is removed. Use it to say your goodbyes.

class Scorecard extends Component {
  // Other functions omitted for brevity.
  componentWillUnmount() {
    console.log('Sayonara!');
  }
}