How to treat `Promise.reject` as a first-class function?

Question

Why can I do this:

> Promise.reject(3);
< Promise {[[PromiseStatus]]: "rejected", [[PromiseValue]]: 3}

But not this:

> var f = Promise.reject;
< undefined

> f(3)
< VM2366:1 Uncaught TypeError: PromiseReject called on non-object
    at reject (<anonymous>)
    at <anonymous>:1:1

Answer 1

The spec defines Promise.reject as follows (emphasis mine):

25.4.4.4 Promise.reject( r )

The reject function returns a new promise rejected with the passed argument.

1. Let C be the this value.
2. If Type(C) is not Object, throw a TypeError exception.
3. Let promiseCapability be ? NewPromiseCapability(C).
4. Perform ? Call(promiseCapability.[[Reject]], undefined, « r »).
5. Return promiseCapability.[[Promise]].

NOTE: The reject function expects its this value to be a constructor function that supports the parameter conventions of the Promise constructor.

As you can tell from this, Promise.reject expects to be called on a Promise constructor (either native promises or other compatible implementation). When treating Promise.reject as a first-class function like that, you're calling it on the global object, which is not a Promise constructor and therefore fails.¹

If you need to use Promise.reject in this way, I'd recommend to bind it first:

var f = Promise.reject.bind(Promise);
f(3); // Promise {[[PromiseStatus]]: "rejected", [[PromiseValue]]: 3}

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¹ I'm not exactly sure why the global object is not considered an object, though, since Promise.reject.call({ }) gives Uncaught TypeError: object is not a constructor.