Synchronous AngularJS $http Call - angularjs

Question

I have an service in my Angular app that is responsible for authorizing the user and returning the auth token back.
However, due to the async nature of $http, I cannot properly isolate the logic of my service. Instead I must return a promise and push the logic of interpreting the response to the caller of the service (single responsibility red flag).
Given this, it's made me reconsider whether I am thinking about this correctly. I come from a Java world where the majority of interactions are synchronous, so something isn't sitting well with me as I try to architect a clean design with single responsibility.
What am I missing here?
UPDATE
I realize the below will not work as intended, but that's my thought of how I'd like it to work at least:
app.service('AuthenticationService', ['$http', '$httpParamSerializerJQLike', function($http, $httpParamSerializerJQLike)
{
this.authServerBaseURL = "...";
this.clientId = "...";
this.authenticate = function(username, password)
{
var config =
{
headers:
{
"Content-Type" : 'application/x-www-form-urlencoded',
"Authorization" : "Basic " + btoa(this.clientId + ":")
}
}
var body = $httpParamSerializerJQLike(
{
grant_type : "password",
username : username,
password : password
});
return $http.post(this.authServerBaseURL + '/oauth/token', body, config).
success(function(data, status, headers, config)
{
return data.access_token;
}).
error(function(data, status, headers, config)
{
return false;
});
}
}]);

Answer 1

Update after you added code: Your thought process can work, see below. However, $http docs say not to use .success and .error. If you instead use .then, as in my examples below, it will work.
Assuming your code is something similar to this:
// AuthService
this.authenticate = function() {
return $http.post('http://example.com', body, config);
}
// Using it:
AuthService.authenticate().then(function(data) {
var token = data.access_token;
});
You can move the knowledge about how the data is extracted to the service like this:
// AuthService
this.authenticate = function() {
return $http.post('http://example.com', body, config).then(function(data) {
return data.access_token;
});
}
// Using it:
AuthService.authenticate().then(function(token) {
var token = token;
});
what happens here is that you make a new promise by calling .then on the $http promise, which is what is returned. The promises are chained, so the $http promise will resolve this new promise, which then resolves itself with the extracted token.