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

As a partial solution, adequate and appropriate for the given example, we can easily distinguish between jQuery.Deferred promises and Angular promises by checking for the presence of specific methods. For instance, Angular's $q promises have the method catch to handle errors, while jQuery.Deferred's promises have the method fail.

function promiseIsAngularOrJQuery(promise) {
    if (typeof promise.fail === 'function') {
        return 'jquery';
    } else if (typeof promise.catch === 'function') {
        return 'angular';
    } else {
        throw new Error("this can't be either type of promise!");
    }
}

However, using this technique to distinguish between more different types of promises, or between promises and non-promises, could get very messy. Different implementations often use use many of the same method names. It could probably be made to work, but I'm not going down that road.

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There is an alternative that should be able to reliably identify $q promises under reasonable conditions: operating on trusted objects in a non-hostile environment, with only a single version of Angular in use. However, some might see it as too "hacky" for serious use.

If you convert a function to a string using the String() function, the result is the source code for that function. We just need to use this to compare the .then method on a potential promise object to the .then method of a known $q promise object:

function isAngularPromise(value) {
    if (typeof value.then !== 'function') {
        return false;
    }
    var promiseThenSrc = String($q.defer().promise.then);
    var valueThenSrc = String(value.then);
    return promiseThenSrc === valueThenSrc;
}