A CodePen by Craig Patik (http://cdpn.io/mtHBd)

insertAfterEveryFromRight.js

var insertAfterEveryFromRight = function _insertAfterEveryFromRight(str, insert, every) {
  var result = '';
  
  str.split('').reverse().forEach(function(s, idx) {
    if (idx !== 0 && idx % every === 0) {
      result += insert;
    }
    result += s;
  });
  
  return result.split('').reverse().join('');
};

/*
Examples:

insertAfterEveryFromRight("1234567", ",", 3); // "1,234,567"
insertAfterEveryFromRight("51713", "/", 2); // "5/17/13"
*/