It's works!

I have

Array.prototype.sum = function() {
 for (var i = 0, sum = 0; i < this.length; sum += parseInt(this[i++]));
 return sum;
};

Function "sum" is guilty. Why? I do not know.

Thanks! Good job!

Do you have a n in your solution? Could you post your solution? (mark it as spoiler).

Edit: thanks for your feedback! No need then to post your solution...

the problem was 'use strict'; on top of my code

lang JS, passes all basic tests and fails with error:

Random tests
ReferenceError: n is not defined
   at do_ex
        at Object.Test.describe