What does "return $q.reject(response)" do?

Question

I am a bit confused about what the statement

return $q.reject(response);

does inside the responseError interceptor.

I have gone through this article on webdeveasy and one on angular docs but they haven't helped.

Here is my code (for reference):

(function () {
    "use strict";
    angular.module('xyz').factory('errorResponseInterceptor', ['$q', 'toaster', function ($q, toaster) {
        return {
            ...
            ...
            ...
            responseError: function (response) {
                ...
                ...
                //some logic here
                ...
                ...
                if (response.status == 500) {
                        toaster.error({ title: "", body: response.statusText });
                    return $q.reject(response);//what does this statement does and why do we need to put it here?
                }
                return response;
            }
        };
    }]);
}());

My Question is:

1. Why do we need to write return $q.reject(response)?
2. How does that line affect the angular app (what does it do)?

Answer 1

The $q object in Angular allows us to define a promise and handle the behaviour associated with a promise. An example of how we might do this is:

var q = $q.defer()

//do something
if(success){
  q.resolve()
} else {
  q.reject()
}

return q

What we are doing here is defining a promise object and assigning it to the variable q. Then we perform some operation and use the result of that to determine whether the promise returns successfully or not. Calling .resolve() on the promise object is indicative of the fact that the promise has returned correctly. By the same token calling .reject() on the promise is indicative of the fact that is has failed.

If we consider how we actually use a promise:

SomeFactory.getSomething().then(function(data){
  //gets called if promise resolves
}, function(error){
  //gets called if promise rejected
}

So in the example you have provided, we are checking to see if the response has a 500 error code, and if it does, we are rejecting the promise thus allowing us to handle the error.

In a responseError interceptor you have two choices with regards to what you return. If you return $q.reject(response) you are continuing the error handling chain of events. If you return anything else (generally a new promise or a response) you are indicating that the error has been recovered from and should be treated as a success.

With regards to your two points:

1- You need to write that line to indicate that the response should still be considered an error.

2- The effect on the angular app is that the error callback will be called instead of the success callback.