Don’t add a custom prototype to Array.

If you want to add your own prototype to Array. You have to remember that DO NOT USE ‘for (in)’ statement. Because the statement will show not only items in array but also your custom prototypes.

You can test it by below code in chrome console.
Damn.. I struggled with this day long.

Array.prototype.someFunc = function(){
  return 'someFunc';
};
var arr = [1, 2];
for (var k=0, max=arr.length; k<max; ++k){
	console.log(11, arr[k]);
}
for (var i in arr){
	console.log(22, arr[i]);
}

result:
11 1
11 2
22 1
22 2
22 function (){
  return 'someFunc';
}