条件判断循环叠加num处理数据

Question

如果workpieceFlow中的workpieces_id === workpieceList.id，就把workpieceFlow中的num取出来相加，最后赋值给workpieceList的realnum属性，请问一下大家有什么方法推荐么？我是加了一个全局变量。。。

var realNum = 0;

_.each($scope.workpieceFlow, function(item){

item.workpieces_id === $scope.workpieceList.id && (realNum += item.num);

});

$scope.workpieceList.realNum = realNum;