Javascript: Promises + this

Question

Consider the following code:

foo: function() {
  var self = this;
  var p1 = p2 = someFunctionThatReturnsAPromise();

  Promise.all([p1, p2])
    .then(self.bar);
}

bar: function(promises) {
  var self = this;
  console.log(self);
}

Output:

undefined

But if I do the following instead:

foo: function() {
  var self = this;
  var p1 = p2 = someFunctionThatReturnsAPromise();

  Promise.all([p1, p2])
    .then(function(result) {
      self.bar(result);
    });
}

bar: function(promises) {
  var self = this;
  console.log(self);
}

Output:

{ foo: [Function],
  bar: [Function] }

I don't understand why the first call changes where this points in the bar function. Can someone enlighten me?

Because it's very important how you call a function. Read this: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/this In your first example you "detouched" method from context object, so you lost it. — dfsq, May 19 '16 at 21:28
This has nothing to do with promises, see [How to access the correct `this` / context inside a callback?](http://stackoverflow.com/q/20279484/1048572) for other solutions — Bergi, May 19 '16 at 21:39
@dfsq: I wouldn't even use the term "detouched". There are no methods in JS, functions become them only when being called as object properties. They're unbound otherwise. — Bergi, May 19 '16 at 21:40

trincot · Accepted Answer · 2018-01-16T11:11:20.410

When you pass self.bar to the then method, you pass a function reference. Although it looks like you also specify it should be called on the self object, that is actually not what is happening. The self object is not included in that function reference. The this object's value is determined when a function is called, not when it is defined or passed as argument.

In your second example, self is the this object within the function context, because that is where you call the function from.

Another way to get it working is to force the function's this object to always be self, overriding the above described behaviour. You can achieve that with .bind():

Promise.all([p1, p2])
    .then(self.bar.bind(self));

Javascript: Promises + this

1 Answers1