Testing if a method returns an Promise

angularjs, promise, unit-testing

Solution

Looking at line 248 in the source for $q (https://github.com/angular/angular.js/blob/master/src/ng/q.js#L248) there isn't really a check you can do thats definite. This would be your best bet

var deferred = method();

if(angular.isObject(deferred) && 
   angular.isObject(deferred.promise) && 
   deferred.promise.then instanceof Function && 
   deferred.promise["catch"] instanceof Function && 
   deferred.promise["finally"] instanceof Function){
   //This is a simple Promise
}

If the promise was actually a function where you could use `new Promise()` then you would be able to use `promise instanceof Promise`, however it's an object so it doesn't have any special identifiers, the only things you can test are their properties.

EDIT:

To test for a "`HttpPromise`" then you can add on checks for `error` and `success` which are defined in the `$http` service (https://github.com/angular/angular.js/blob/master/src/ng/http.js#L726):

var promise = $http(...);

if(angular.isObject(promise) && 
   promise.then instanceof Function && 
   promise["catch"] instanceof Function && 
   promise["finally"] instanceof Function && 
   promise.error instanceof Function && 
   promise.success instanceof Function){
   //This is a HttpPromise
}

EXTRA:

If you notice $http doesn't actually return `deferred`, it returns the straight promise, if you follow the calls it's actually returns `$q.when(...)` with a couple of functions added to it. You can see that `$q.when` doesn't return `deferred`, rather it returns `$q.deferred().promise`, so in turn `$http(...)` would never be `$q.deferred()`

Also, if you were to run the test you had posted I would expect you to get this error:

`TypeError: Expecting a function in instanceof check, but got #<Object>`

Problem

Original source

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