Javascript String splitChars(len)

String.prototype.splitChars = function(len) {
a = this.split('');
res = [];/*ww  w  . ja  v a  2 s.  c o m*/
i = 0;
[].forEach.call(a, function(b, c) {
 if (c % len == 0) {
  i++;
  res.push([])
 }
 res[i - 1].push(b);
});
f = [];
res.forEach(function(d) {
 f.push(d.join(''));
 return true;
});
g =f.join('\n');
return g;
};