var o = {
       forEach: function (callback) {
         // alert(this.length);
         for (var i = , len = this.length; i < len; i++) {
           callback && callback(this[i], i, this);
         }
       },
       get length(){
         var sum=;
         for(var n in this) {
           sum+=;
         }
         return sum;
       }
     };
     Object.defineProperty(o,"length",{enumerable:false});
     Object.defineProperty(o,"forEach",{enumerable:false});
     o[] = ;
     o[] = ;
     o[] = ;
     o.forEach(function(v,i,arr){
       arr[i]=v+;
       console.log(arr[i]+"callback");
     });

var data = [1,2,3,4,5] ;
// 计算数组的和
var sum = 0 ; 
data.forEach(function(value){sum += value; }); // 这里的value 分别代指 data[0~4];
console.log( sum ) // 15
// 每个数组元素自加1
data.forEach(function(v, i, a){ a[i] = v + 1; }) // v 分别代指 data[0~4]; a 代指data;
map() ;

var a = [1,2,3];
b = a.map(function(x) { return x * x; });
filter() 

var getNum = function (a, b, k) {
return a.filter(function (v) {return b.indexOf(v) > -1;})[k-1];
}
var A = [3,4,5,6,7,8,9];
var B = [12,10,8,6];
console.log(getNum(A, B, 1))
console.log(getNum(A, B, 2));
every() some() ;