js数组方法forEach,map,filter,every,some实现

 1 Array.prototype.map = function(fun /*, thisp*/)
 2 {
 3   var len = this.length;
 4   if (typeof fun != "function")
 5     throw new TypeError();
 6 
 7   var res = new Array(len);
 8   var thisp = arguments[1];
 9   for (var i = 0; i < len; i++)
10   {
11     if (i in this)
12       res[i] = fun.call(thisp, this[i], i, this);
13   }
14 
15   return res;
16 };
17 
18 Array.prototype.filter = function(fun /*, thisp*/)
19 {
20   var len = this.length;
21   if (typeof fun != "function")
22     throw new TypeError();
23 
24   var res = new Array();
25   var thisp = arguments[1];
26   for (var i = 0; i < len; i++)
27   {
28     if (i in this)
29     {
30       var val = this[i]; // in case fun mutates this
31       if (fun.call(thisp, val, i, this))
32         res.push(val);
33     }
34   }
35 
36   return res;
37 };
38 
39 Array.prototype.some = function(fun /*, thisp*/)
40 {
41   var len = this.length;
42   if (typeof fun != "function")
43     throw new TypeError();
44 
45   var thisp = arguments[1];
46   for (var i = 0; i < len; i++)
47   {
48     if (i in this && fun.call(thisp, this[i], i, this))
49       return true;
50   }
51 
52   return false;
53 };
54 
55 Array.prototype.every = function(fun /*, thisp*/)
56 {
57   var len = this.length;
58   if (typeof fun != "function")
59     throw new TypeError();
60 
61   var thisp = arguments[1];
62   for (var i = 0; i < len; i++)
63   {
64     if (i in this && !fun.call(thisp, this[i], i, this))
65     return false;
66   }
67 
68   return true;
69 };
70 
71 Array.prototype.forEach = function(fun /*, thisp*/)
72 {
73   var len = this.length;
74   if (typeof fun != "function")
75     throw new TypeError();
76 
77   var thisp = arguments[1];
78   for (var i = 0; i < len; i++)
79   {
80     if (i in this)
81       fun.call(thisp, this[i], i, this);
82   }
83 };