The then() method returns a Promise. It takes up to two arguments: callback functions for the success and failure cases of the Promise.

gistfile1.txt

authService.login = function(credentials) {
           
    // we need this to return a promise but we also want to hook into the promise.
    return ApiInterface.makeRequest('/api/auth', 'POST', credentials)
    .then( function(response) { // success                   
        Session.setAuthToken(response.data.authToken);
        Session.initDataStore('order-details', credentials);
        $rootScope.$emit(AUTH_EVENTS.loginSuccess);
    })
    .catch( function(error) { // failed                                      
        switch (error.code) {
            case 500: $rootScope.$emit(AUTH_EVENTS.internalServerError); break;
            case 523: $rootScope.$emit(AUTH_EVENTS.vendorUnavailable); break;
            case 422: 
                if (error.errors[0] == 'invalid orderId') {
                    $rootScope.$emit(AUTH_EVENTS.validationErrorOrderId);
                } else if (error.errors[0] == 'The post code must be between 3 and 4 digits.') {
                    $rootScope.$emit(AUTH_EVENTS.validationErrorPostCode);
                } else {
                    $rootScope.$emit(AUTH_EVENTS.loginFailed);
                }                        
                break;                        
        }
    })         

};