Using the $q service with angular

Question

I still can't understand the role of using the $q service, (what exactly will it add) if you want to create a service that need to call only one API via http , in this situation I don't know why shouldn't I just do the following (without using $q) :

this.getMovie = function(movie) {
  return $http.get('/api/v1/movies/' + movie)
    .then(
    function(response) {
      return {
        title: response.data.title,
        cost: response.data.price
      });
    },
    function(httpError) {
      // translate the error
      throw httpError.status + " : " +
        httpError.data;
    });
};

Answer 1

Very good question and one very few people appreciate the answer to.

Consider this:

this.getMovie = function(movie) {
  return $http.get('/api/v1/movies/' + movie);
};

Great code but these rules apply:

$http will resolve for 2xx responses and will otherwise reject. sometimes we don't want this. we want to reject on resolution and resolve on rejection. This makes sense when you consider a HEAD request to check the existence of something.

A HEAD request to /book/fred returning 200 shows book fred exists. But if my function is testing whether book fred is unique, it is not and so we want to reject on a 200. this is where $q comes in. Now I can do this:

var defer = $q.defer();
$http.head('/book/fred').then(function(response) {
    // 2xx response so reject because its not unique
    defer.reject(response);
}).catch(function(failResponse) {
    defer.resolve(failResponse);
});
return defer.promise;

$q gives me total control of when I reject AND when I resolve.

Also, $q allows me to reject or resolve with any value. So if I am only interested in some of the response I can resolve with just the bit I am interested in.

Finally, I can use $q to turn non-promise based code into a promise.

var a = 5;
var b = 10;
var defer = $q.defer();
var resolve(a+b);
return defer.promise;

Bosh, if I need a promise as my return value then I've got one.

This is also great when mocking for unit tests.