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