使用方法调用时 Javascript 类属性返回未定义

Question

所以这是一个番茄钟。当我调用 inter 方法（每 1 秒调用一次倒计时）时，this.work 返回 undefined，我不知道为什么。如果我从类外部调用该属性（例如 t1.work），它已定义，但从倒计时（this.work）内部调用它将不起作用。有谁知道为什么？

class Timer {

  //constructor de la clase, se definen las propiedades  
  constructor(work, rest) {
    this.work = work;
    this.rest = rest;
    this.interval = undefined;
  }

  //countdown method (dhu)
  countdown(){
    if (this.work >= 0) {
      console.log(this.work);
      this.work -= 1;
      return;
    } else {
      console.log(this.rest);
      (this.rest > 0) ? this.rest -= 1 : clearInterval(this.interval);
    }
  }

  //interval to invoque countdown method every second
  inter(){
    this.interval = setInterval(this.countdown, 1000);
  }

}

//Creating an object with the timer class, passing values.
var t1 = new Timer(5, 3);

//Calling the method inside object t1
t1.inter();

一开始我以为是If问题，但尝试做一个简单的console.log(this.work)也没有成功。谢谢

Answer 1

在您的情况下，您没有将 this 绑定到 setInterval 函数内的当前上下文

从 setInterval 调用函数时绑定 this -

this.countdown.bind(this),

class Timer {

  //constructor de la clase, se definen las propiedades  
  constructor(work, rest) {
    this.work = work;
    this.rest = rest;
    this.interval = undefined;
  }

  //countdown method (dhu)
  countdown(){
    if (this.work >= 0) {
      console.log(this.work);
      this.work -= 1;
      return;
    } else {
      console.log(this.rest);
      (this.rest > 0) ? this.rest -= 1 : clearInterval(this.interval);
    }
  }

  //interval to invoque countdown method every second
  inter(){
    this.interval = setInterval(this.countdown.bind(this), 1000);
  }

}

//Creating an object with the timer class, passing values.
var t1 = new Timer(5, 3);

//Calling the method inside object t1
t1.inter();