Saturday, 5 October 2019
Javascript: Promises + this
Answer
Answer
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?
Answer
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));
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