如何在Angular单元测试中检查对象是否为promise？

Question

I want to have an assert that checks if the returned object from calling a method is a promise.

Is it enough to check if it exists and has a then method?
Or is there a better way of doing this since having a then method doesn't guarantee the object is a $q deferred promise?

EDIT:
The question is not duplicate of " Any way to know if a variable is an angularjs promise? ".
The solution for that question is to ensure that a promise is returned using $q.when() .
I'm asking how to know that an object is a promise, not how to make it one if it isn't.

Thanks.

Answer 1

As far as I can tell, there is no way to do this directly. When constructing objects, i normally do it like this:

function Derp(){
   this.constructor = Derp;
}
Derp.prototype.method = function(){};

So that I can:

expect( new Derp() instanceof Derp ).toBe(true);

but in both $q.promise and $q itself, there are no constructor or prototype properties except for the constructor property in their __proto__ (in chrome) objects; and even those just point to JavaScripts' fundamental Object.

So in order to test that something is a promise, you have to make the assumption that the object you're returning is a promise based on its' other properties:

expect(typeof result.when).toBe('function');
expect(typeof result.then).toBe('function');

I don't like it either, but that's the only way I can think of to do this.