previousstate react code example

Example 1: prev props

componentDidUpdate(prevProps) {
  // Utilisation classique (pensez bien à comparer les props) :
  if (this.props.userID !== prevProps.userID) {
    this.fetchData(this.props.userID);
  }
}

Example 2: lifecycles if reactjs

class Clock extends React.Component {
  constructor(props) {
    super(props);
    this.state = {date: new Date()};
  }

  componentDidMount() {  }
  componentWillUnmount() {  }
  render() {
    return (
      <div>
        <h1>Hello, world!</h1>
        <h2>It is {this.state.date.toLocaleTimeString()}.</h2>
      </div>
    );
  }
}