[рони]

Anna_Medvid,

function myMap(callback){
    var arr = [];
    for (var i=0; i<this.length; i++)  {
      arr.push(callback(this[i]))
    }
    return arr;
}
Array.prototype.myMap = myMap;
var a = [1,2,3,4,5], fn = function(b) {
 return b += 3
};
var c = a.myMap(fn)
alert(c);