AngularJS Defining a helper function for private use in a service

I have an angular service that has one function visible to controllers using that service. i.e. GetData p>

I want my service to be able to use a helper function called modifyData on the service that manipulates the data before returning it to the service, which then sends the data to the controller. I don't want to use a helper function outside of the service or have it outside of the service.

app.service("dataService", function() {
/* HELPER FUNCTION */
var modifyData = function(data) {
    data.modified = true;
    return data;
}

this.getData = function() {
    //Do a http request to get oldData variable
    var newData = modifyData(oldData);
    return newData;
}});

      

        
        
        
      

    

My error from angular is Error: modifyData is undefined.

What am I doing wrong here?

EDIT: here is my actual code, since my example code should work, I could simplify it.

rabApp.service("reviewService", 
          ["$http", "$q", "beerService", "userService", 
          function($http, $q, beerService, userService) {

/* HELPER FUNCTIONS */
/* Builds a complete review object from a user object, beer object, and partial review object */
var buildReviewObj = function(reviewObj, switchClass) {
    var deferred = $q.defer();

    if(switchClass) {
        reviewObj.dirClass = "left";
    } else {
        reviewObj.dirClass = "right";
    }

    //Make sure the review picture is valid and if not use default one
    if(reviewObj.image.length <= 0) {
        reviewObj.image = "images/default-beer-pic.jpg"; 
    }

    //Make sure the review style is present if not put a default in
    if(reviewObj.style.length <= 0) {
        reviewObj.style = "Unknown"; 
    }

    //Add user data
    userService.getUser(reviewObj.author_id).success(function(data, status) {
        //Check if we have a matched user
        if(data.status === "success") {
            var userObj = data.data;

            //Make sure the profile picture is valid and if not use default one
            if(userObj.profile_pic.length <= 0) {
                userObj.profile_pic = "images/default-profile-pic.jpg"; 
            }

            reviewObj.author = userObj;
        } else {
            deferred.reject("Bad User object returned");
        }
    });

    //Add beer data
    beerService.getBeer(reviewObj.beer_id).success(function(data, status) {
        //Check if we have a matched beer
        if(data.status === "success") {
            var beerObj = data.data;
            reviewObj.beer = beerObj;
        } else {
            deferred.reject("Bad Beer object returned");
        }
    });
    deferred.resolve(reviewObj);
}

/* Gets an array of review objects from the backend
 * @return Array of Review objects
 */
this.getReviews = function() {
    var deferred = $q.defer();
    $http({
        method: "GET", 
        url: "/includes/services/reviews.php",
        params: { a : "getReviews",
                  limit : "10" }
    }).success(function(data, status) {
        //switchClass variable alternates the review to display left or right
        var switchClass = true;

        //Notify review controller that we are loading reviews
        deferred.notify("loading");

        //Check if we have reviews
        if(data.status === "success") {
            var reviews = [];
            //Add beer and user data to each review
            data.data.forEach(function(reviewObj, index) {
                buildReviewObj(reviewObj, switchClass).then(function(data) {
                    switchClass = !switchClass;
                    reviews.push(reviewObj);
                });
            });
            //Notify review controller that we are done
            deferred.notify("finished");
            //Review array built successfully return reviews
            deferred.resolve(reviews);
        } else {
            //Couldn't get reviews return false
            deferred.reject("Couldn't access reviews.php back end service");   
        }
    }).error(function() {
        //Couldn't get reviews return false
        deferred.reject("Couldn't access reviews.php back end service"); 
    });
    return deferred.promise;
}}]);