Last arg function

LastArg.js

var fn  = function(one, two, three, callback) {
  var args = Array.prototype.slice.apply(arguments);
  var lastArg = args.pop()
  if (callback !== lastArg && typeof lastArg == 'function') {
    callback = lastArg;
    arguments[args.length] = undefined; // not -1 bc of pop above
  }
  // Handle undefined values, by setting default value
  // .. Rest of function ..
}