使用 Promise 时，为什么在类方法中未定义“this"? [英] Why is 'this' undefined inside class method when using promises?

问题描述

我有一个 javascript 类，每个方法都返回一个 Q 承诺.我想知道为什么 this 在 method2 和 method3 中没有定义.有没有更正确的方法来编写这段代码?

I have a javascript class, and each method returns a Q promise. I want to know why this is undefined in method2 and method3. Is there a more correct way to write this code?

function MyClass(opts){
  this.options = opts;

  return this.method1()
    .then(this.method2)
    .then(this.method3);
}

MyClass.prototype.method1 = function(){
  // ...q stuff...

  console.log(this.options); // logs "opts" object

  return deferred.promise;
};

MyClass.prototype.method2 = function(method1resolve){
  // ...q stuff...

  console.log(this); // logs undefined

  return deferred.promise;
};

MyClass.prototype.method3 = function(method2resolve){
  // ...q stuff...

  console.log(this); // logs undefined

  return deferred.promise;
};

我可以使用 bind 解决这个问题:

I can fix this by using bind:

function MyClass(opts){
  this.options = opts;

  return this.method1()
    .then(this.method2.bind(this))
    .then(this.method3.bind(this));
}

但不完全确定为什么 bind 是必要的；.then() 是否关闭了 this ?

But not entirely sure why bind is necessary; is .then() killing this off?

推荐答案

this 始终是调用方法的对象.但是，当将方法传递给 then() 时，您并没有调用它！该方法将存储在某个地方并稍后从那里调用.如果你想保留this，你必须这样做:

this is always the object the method is called on. However, when passing the method to then(), you are not calling it! The method will be stored somewhere and called from there later. If you want to preserve this, you will have to do it like this:

.then(() => this.method2())

或者如果你必须按照 ES6 之前的方式来做，你需要在之前保留 this :

or if you have to do it the pre-ES6 way, you need to preserve this before:

var that = this;
// ...
.then(function() { that.method2() })

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