Testing if a method returns an Promise

Question

In angularjs, while testing a service, I want to check if the returned object is an Promise.

Right now I am doing the following -

 obj.testMethod()
        .should.be.instanceOf($q.defer());

Answer 1

Testing if an object is a promise is simple:

return !!obj.then && typeof obj.then === 'function';

That's it. If an object has the then method, it's a promise.

It looks like angular's $q doesn't have anything to differentiate it from other kind of promises.

Answer 2

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>

Answer 3

You could mock $http and have it return some object you create. Then you can check whether your service returns that object.

Answer 4

I was working with callbacks, I needed a CTA on click event to fire a function and determine what type of callback was fired, here is some simple examples.

let fn = function () { alert('A simple function') };

let cta = fn();

console.log(cta instanceof Promise);

// logs false

New Promise Example

let fn = function () {

    return new Promise(function (accept, reject) {

        accept();

        // reject();

    });

};

let cta = fn();

if(cta instanceof Promise) {

    cta.then(function () {

        alert('I was a promise');

    });

}

Axios Example 1

let fn = function () {

   return axios.get('/user');

}

let cta = fn();

if(cta instanceof Promise) {

    cta.then(function () {

        alert('I was a promise from axios');

    });

}

// Triggers alert('I was a promise from axios');

Axios Example 2

let fn = function () {

   return axios.get('/user').then(response => {
       console.log('user response', response.data);
   });

}

let cta = fn();

if(cta instanceof Promise) {

    cta.finally(function () {

        alert('I was a final promise from axios');

    });

}

// Triggers alert('I was a final promise from axios');