问在sum函数中重复两次

EN

function groupBy(arr, key) {
            var newArr = []
                , types = {}
                , newItem, i, j, cur;
            for (i = 0, j = arr.length; i < j; i++) {
                cur = arr[i];
                if (!(cur[key] in types)) {
                    types[cur[key]] = {
                        type: cur[key]
                        , data: []
                    };
                    newArr.push(types[cur[key]]);
                }
                types[cur[key]].data.push(cur);
            }
            return newArr;
        };
        Array.prototype.sum = function (prop) {
            var total = 0
            for (var i = 0, len = this.length; i < len; i++) {
                total += parseInt(this[i][prop])
            }
            console.log(total);
            return total;
        }
        $scope.totalCreadit = function (st) {
            return st.sum("credits");
        }

 var users = [
            {
               "creditType": "Transfer"
              ,"credits": "50"
            }, {
                "creditType": "Transfer"
              ,"credits": "50"
            }
            , {
                 "creditType": "Bought"
              ,"credits": "50"
            }
            , {
                  "creditType": "Spent"
              ,"credits": "50"
            }
            , {
                 "creditType": "Award"
              ,"credits": "50"
            }
            , {
             "creditType": "Received"
              ,"credits": "50"
            }];
                        $scope.items = groupBy(users, "creditType");