当一个函数中有两个类时会出现这个问题

Question

我在一个函数中有两个类，由于某种原因，第二个函数this没有被正确识别。

类是Emitter和Receiver。

我想弄清楚为什么this这里没有被正确接收。日志包含在下面的代码中：

const chat = (messages) => {
  class Emitter {
    constructor(messages = []) {
      this.messages = messages;
      this.event = () => {};
    }

    setEvent(fn) {
      this.event = fn;
    }

    trigger() {
      this.messages.forEach(message => this.event(message));
    }
  }

  class Receiver {
    constructor() {
      this.messages = [];
      // this prints correctly here
      ---> Receiver { messages: [] }
      console.log('this  ===> ', this)
    }

    ping(message) {
      console.log('this ===>', this)
      // this here prints the following
      this ===> Emitter {
        messages: [ 'Hi', 'Hola', 'Bonjour', 'Hi' ],
        event: [Function: ping] }
          this.messages.push(message);
       }
  }

  const myReceiver = new Receiver();
  const myEmitter = new Emitter(messages);

  myEmitter.setEvent(myReceiver.ping);
  myEmitter.trigger();

  return myReceiver.messages;
};

Answer 1

这this取决于调用它的范围，而不是它定义的范围。 https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/this 在调用 myEmitter.setEvent(myReceiver.ping) 中，只有函数 ping 被传递给 myEmitter，而不是它的作用域我的接收者。如果您想传递 myRevciever 范围，您可以将其绑定到函数调用。

myEmitter.setEvent(myReceiver.ping.bind(myReceiver));