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);