getOrdersService.js

//to use call getOrders.sum().then(function(sum){console.log(sum)});
angular.service('getOrders',function ($http,$q) {
  return {
    sum: function () {
      var d = $q.defer();
      //can add try method here
      $http.get('/orders').success(function (data) {
        //assuming data is just json array of orders.
        var orders = data;
        var sumOfOrders = 0;
        orders.forEach(function (order) {
          sumOfOrders += order.value;
        });
        return d.resolve(sumOfOrders);
      }).error(function (res) {
        return d.reject(res);
      });
      return d.promise;
    }
  };
});