javascript - react 倒计时错误

Question

我已经在 React 组件中实现了一个 vanilla js 倒计时，如下所示:

import React, { Component } from 'react';

class CustomCountDown extends Component {
    constructor(props) {
        super(props);

        this.endTime;
        this.msLeft;
        this.time;
        this.hours;
        this.mins;
        this.element;
    }

    twoDigits( n ){
        return (n <= 9 ? "0" + n : n);
    }

    updateTimer() {
        this.msLeft = this.endTime - (+new Date);
        if (this.msLeft < 1000 ) {
            element.innerHTML = "countdown's over!";
        } else {
            this.time = new Date(this.msLeft );
            this.hours = this.time.getUTCHours();
            this.mins = this.time.getUTCMinutes();
            this.element.innerHTML = (this.hours ? this.hours + ':' + this.twoDigits( this.mins ) : this.mins) + ':' + this.twoDigits( this.time.getUTCSeconds() );
            setTimeout( this.updateTimer, this.time.getUTCMilliseconds() + 500 );
        }
    }

    countdown( elementName, minutes, seconds ) {
        this.element = document.getElementById( elementName );
        this.endTime = (+new Date) + 1000 * (60*minutes + seconds) + 500;
        this.updateTimer();
    }

    componentDidMount() {
        this.countdown("count", 1, 30);
    }

    render() {
        return(
            <div id="count">
            </div>
        );
    }
}

export default CustomCountDown;

我不明白为什么会出现以下错误:

Answer 1

当您将 this.updateTimer 传递给 setTimeout 时，您失去了上下文，即 this 不再指向您的组件实例。无论哪种方式，您都需要保持上下文:

setTimeout( this.updateTimer.bind(this), this.time.getUTCMilliseconds() + 500 );

setTimeout( () => this.updateTimer(), this.time.getUTCMilliseconds() + 500 );

作为更好的选择，您可以在构造函数中绑定(bind) updateTimer。这不会在每次调用 updateTimer 时都创建新函数:

constructor(props) {
    // ...

    this.updateTimer = this.updateTimer.bind(this);
}