spritN.js

String.prototype.splitN = function (n) {
  var a = [];

  var begin = 0;
  var i = 0;
  for (; i < this.length; i++) {
    if (i !== 0 && i % n == 0) {
      a.push(this.substring(begin, i));
      begin = i;
    }
  }
  a.push(this.substring(begin));

  return a;
}

console.log("abcdefg".splitN(0).toString() === ['abcdefg'].toString());
console.log("abcdefg".splitN(1).toString() === ['a', 'b', 'c', 'd', 'e', 'f', 'g'].toString());
console.log("abcdefg".splitN(2).toString() === ['ab', 'cd', 'ef', 'g'].toString());
console.log("abcdefg".splitN(10).toString() === ['abcdefg'].toString());