js sum 函数
实现：
Array.prototype.Sum = function (fn) { return eval(this.map(x => Number(typeof fn == "function" ? fn(x) : x)).join("+")) || 0 }
用法示例：
var arr1 = [{name:'xx',value:1},{name:'xx',value:2},{name:'xx',value:3}] var arr2 = [1,2,3,4,5,6,7,8,9,10] arr1.Sum(x => x.value) //返回 6 arr1.Sum(x => x.name) //返回 0 arr2.sum() //返回 55