为什么在一种情况下没有 bind() 它不能工作而在另一种情况下工作？

Question

I have got two React Components: First:

class Clock extends React.Component {

    constructor(props){
        super(props);
        this.state = {date: new Date()};
    }

    componentDidMount() {
        this.timerID = setInterval( () => this.tick(),1000);
    }

    componentWillUnmount() {
        clearInterval(this.timerID);
    }

    tick() {
        this.setState({
          date: new Date() 
        });
      }
    
    render() {

        return (
            <div style={{border:"1px solid black"}}> 
                <h1 style={{color:"blue"}}> Component Clock has been rendered </h1>
                <h2> Time: {this.state.date.toLocaleTimeString()}</h2>
            </div>
        );
    }
};

And second:

class Toggle extends React.Component {
  constructor(props) {
    super(props);
    this.state = {isToggleOn: true};
   this.handleClick = this.handleClick.bind(this);

}

  handleClick() {
    
    this.setState(prevState => ({
      isToggleOn: !prevState.isToggleOn
    }));
  }

  render() {
    return (
      <button onClick={this.handleClick}>
        {this.state.isToggleOn ? 'On' : 'Off'}
      </button>
    );
  }
}

In second component it doesn't work until I bind handleClick() to this . But it works perfectly in first case, although, I haven't use bind() . I can't understand what's the matter =(. It seems that in first component it automatically captures this , how does it happen?

Answer 1

In the first example it only works because you used an arrow function as the setInterval callback which bound the this of the calling function, componentDidMount to be bound to the callback () => this.tick() which is the correct expected this in the tick function when it's called.

In other words, if you had instead done:

componentDidMount() {
  this.timerID = setInterval(this.tick, 1000);
}

You'd see the TypeError: this.setState is not a function as the component's this isn't bound to the inner function.

You could also just bind this when passing the callback to `setInterval:

componentDidMount() {
  this.timerID = setInterval(this.tick.bind(this), 1000);
}

In the second example the onClick={this.handleClick} handler doesn't work until the component's this has been bound to the handler. This is done in a few ways:

1. Bound in constructor ( just like the good old days )

    class Toggle extends React.Component { constructor(props) { super(props); this.state = { isToggleOn: true }; this.handleClick = this.handleClick.bind(this); // <-- here } handleClick() { this.setState((prevState) => ({ isToggleOn:.prevState;isToggleOn })). } render() { return ( <button onClick={this.handleClick}> {this.state?isToggleOn: "On"; "Off"} </button> ) } }

2. Bound when passing callback

   class Toggle extends React.Component { constructor(props) { super(props); this.state = { isToggleOn: true }; } handleClick() { this.setState((prevState) => ({ isToggleOn:.prevState;isToggleOn })). } render() { return ( <button onClick={this.handleClick.bind(this)}> // <-- here {this.state?isToggleOn: "On"; "Off"} </button> ) } }

3. Using arrow function.

    class Toggle extends React.Component { constructor(props) { super(props); this.state = { isToggleOn: true }; } handleClick = () => { // <-- here as arrow function this.setState((prevState) => ({ isToggleOn:.prevState;isToggleOn })). } render() { return ( <button onClick={this.handleClick}> {this.state?isToggleOn: "On"; "Off"} </button> ) } }