Promises in Angular.JS

By using Promises we can execute a function asynchronously. we can use promises to specify what to do when a operation is success or failed. We have $q built in keyword in angular js for promises. let's create a example :

.service('drawHomeCardService', function(apiCallFactory, $q) {
    this.getTheLogo = function() {
        var deferred = $q.defer();
        apiCallFactory.getData().success(function(response) {
            var logo         =  response[2].businessInfo.logoFilePath;
            var logoFilePath = 'https://oyokeydev.s3.amazonaws.com/' + logo;
            deferred.resolve(logoFilePath);
        }).error(function(response) {
            console.log('Something went wrong');
            deferred.reject('Something went wrong');
        });
        return deferred.promise;
    }
})

$q.defer();

here defer function create a object, by which there is a delay in result until a time.

We call object from factory which is our apiCallFactory and we get the our image from api as below :
 

.factory('apiCallFactory', function($http) {
    var apiCall = {};

    apiCall.getData = function() {
        return $http.get('http://appqa.oyokey.com:9080/api/v1/getdomainkeytag?apiKey=f92ecd00-d3ed-11e2-a6db-e9a269320bf6&domain=evontech.com');
    };
    return apiCall;
})

Now if our call is success in response then we get the image and we return deferred.resolve();. By deferred.resolve we determine whether a deferred object has been resolved and if our call is failed in response, control comes in error function where we return deferred.reject();

Now in calling function, if response is success our first function which is in then is called and control comes here and on failed control comes in error function.

drawHomeCardService.getTheLogo().then(function(logoFile) {
        $scope.logoFile = logoFile;
        console.log('After promise call logoFile==> ' + $scope.logoFile);
    }, function (error) {
        console.log("error " + error)
    });