splitSome.js

String.prototype.splitSome = function(del, n){
  var a = this.split(del)
    , b = a.slice(n).join(del)

  a.length = n
  a[a.length] = b
  return a
};

"one,two,three,four,five".splitSome(',', 3)
// ->  ['one', 'two', 'three', 'four,five']