Some Angular 1 strong typing from http service to controller usage

IUserService.ts

module app.services {

    export interface IUserModel {
        id: number;
        firstName: string;
        lastName: string;
    }

    export interface IFindUserByIdRequestModel {
        id: number;
    }

    export interface IUserService {

        findUserById (model: IFindUserByIdRequestModel): ng.IPromise<IUserModel>;
    }
}

UserController.ts

module app {

    export interface IUserControllerViewModel {

        user: app.services.IUserModel;

        getFullname(): string;
    }

    class UserController implements IUserControllerViewModel {

        user: app.services.IUserModel;

        static $inject = ['UserService'];
        constructor (private UserService: app.services.IUserService) {

        }

        // public by default in TS
        getFullname = (): string => {
            return this.user.firstName + ' ' + this.user.lastName;
        }

        // example of internal logic not exposed from viewmodel interface
        private getUser = (): void => {

            var that = this;

            this.UserService
                .findUserById({
                    id: 1
                })
                .then(successHandler, errorHandler);

            function successHandler (user: app.services.IUserModel) {
                that.user = user;
            }

            function errorHandler (error: ng.IHttpPromiseCallbackArg<any>) {
                that.user = undefined;
            }
        }
    }

    angular
        .module('app')
        .controller('UserCtrl', UserController);
}

UserService.ts

module app.services {

    class UserService implements IUserService {

        // angular dependency injection for minify/uglify-safe
        static $inject = ['$q', '$http'];
        constructor (private $q: ng.IQService,
                     private $http: ng.IHttpService) { }

        findUserById = (model: IFindUserByIdRequestModel): ng.IPromise<IUserModel> => {

            var url: string = 'path-to-api/users/' + model.id;
            var deferred: ng.IDeferred<IUserModel> = this.$q.defer();

            this.$http
                .get('path-to-api/users/')
                .then(successHandler, errorHandler);

            return deferred.promise;

            function successHandler (success: ng.IHttpPromiseCallbackArg<IUserModel>) {
                deferred.resolve(success.data);
            }

            function errorHandler (error: ng.IHttpPromiseCallbackArg<any>) {
                deferred.reject(error);
            }
        }
    }

    angular
        .module('app')
        .service('UserService', UserService);
}