Q1658 How do I automatically insert a character after every Nth character in a string?

<script language="JavaScript"><!--
function insertNthChar(string,chr,nth) {
  var output = '';
  for (var i=0; i<string.length; i++) {
    if (i>0 && i%4 == 0)
      output += chr;
    output += string.charAt(i);
  }

  return output;
}

alert(insertNthChar('erty1f213dfs1111', '-', 4));
//--></script>