Claude Pache Reporter
	Description • 9 years ago

When `Promise.prototype.then` is applied to a non-promise (e.g., `Promise.prototype.then.call(null)`), it must throw a TypeError.

Currently, it returns a rejected promise with a TypeError as reason.

Reference: Step 2 of:

http://www.ecma-international.org/ecma-262/6.0/#sec-promise.prototype.then

	Claude Pache Reporter
	Comment 1 • 9 years ago

(Dunno whether it should block Bug 856878 or Bug 885333. Arbitrarily picking the oldest of them.)

	Jeff Walden [:Waldo]
	Comment 2 • 9 years ago

Promises are implemented in the DOM right now, not in JS yet.  That'll have to change eventually, but for now you want this filed there.

	Boris Zbarsky [:bzbarsky]
	Comment 3 • 9 years ago

Actually, as long as Promises stay in the DOM this behavior won't change.  Per WebIDL, all Promise-returning methods convert their exceptions to rejected promises, and Promise.prototype.then is certainly a Promise-returning method.

See also bug 911216.

	Tooru Fujisawa [:arai]
	Comment 4 • 8 years ago

Tested on Nightly 50.0a1 (2016-07-17) (with bug 911216 fixed), now it throws an error.

> Promise.prototype.then.call(null)
TypeError: Receiver of Promise.prototype.then call is not a non-null object

> Promise.prototype.then.call({})
TypeError: Promise.prototype.then called on incompatible value

	Claude Pache Reporter
	Comment 5 • 8 years ago

> Tested on Nightly 50.0a1 (2016-07-17) (with bug 911216 fixed), now it throws an error.

So does it in current stable release (without bug 911216 fixed).

According to my tests, the bug has been silently fixed between v44 and v45.

	Claude Pache Reporter
	Comment 6 • 8 years ago

> According to my tests, the bug has been silently fixed between v44 and v45.

My wild guess is that it was incidentally fixed with Bug 1170760.

	Boris Zbarsky [:bzbarsky]
	Comment 7 • 8 years ago

Indeed.  Bug 1170760 changed the return value from "Promise" to "any", so we no longer get the "convert an exception to a Promise" machinery.  I actually wonder whether that's correct in all cases....