How can I tell if an object is an Angular $q promise?

Question

I have a pre-existing non-Angular API library in my project. It has a .request method which returns jQuery.Deferred promises. I created a simple Angular service

Answer 1

Generally the better approach is to cast whatever object you do have into an Angular promise.

The concept of assimilating thenables is part of the Promises/A+ specification. Most promise libraries have a way to do this. It's what allows for awesome interop between promise implementations and a uniform API.

For this, $q uses .when :

Wraps an object that might be a value or a (3rd party) then-able promise into a $q promise. This is useful when you are dealing with an object that might or might not be a promise, or if the promise comes from a source that can't be trusted.

It uses the concept of thenables to convert a 'non trusted' promise into a $q promise.

So all you have to do is

var p = $q.when(value); // p is now always a $q promise
                        // if it already was one - no harm