javascript when to set state code example

Example 1: react lifecycle example

class Test extends React.Component {
  constructor() {
    console.log('Constructor')
    super();
    this.state = {
      count: 0
    };
  }

  componentDidMount() {
    console.log("component did mount");
  }
  componentDidUpdate() {
    console.log("component did update");
  }

  onClick = () => {
    this.setState({ count: this.state.count + 1 });
  };
  render() {
    console.log("render");
    return (
      <div>
        Hello Test
        <button onClick={this.onClick}>
      		{this.state.count}
		</button>
      </div>
    );
  }
}


//--for first time
//Constructor
//render
//component did mount
//--for any update
//render
//component did update

Example 2: react setState

incrementCount() {
  // Note: this will *not* work as intended.
  this.setState({count: this.state.count + 1});
}

handleSomething() {
  // Let's say `this.state.count` starts at 0.
  this.incrementCount();
  this.incrementCount();
  this.incrementCount();
  // When React re-renders the component, `this.state.count` will be 1, but you expected 3.

  // This is because `incrementCount()` function above reads from `this.state.count`,
  // but React doesn't update `this.state.count` until the component is re-rendered.
  // So `incrementCount()` ends up reading `this.state.count` as 0 every time, and sets it to 1.

  // The fix is described below!
}