JavaScript callbackify

callbackify.js

/*
 * callbackify
 *
 * Copyright (c) 2012 "Cowboy" Ben Alman
 * Licensed under the MIT license.
 * http://benalman.com/about/license/
 */

function callbackify(fn, args, done) {
  // If the function arity exceeds args length, it's accepts a "done" callback.
  var async = fn.length > args.length;
  // If the function expects a done callback, add it into the args array.
  if (async) { args = args.slice(); args[fn.length - 1] = done; }
  // Invoke the function, passing all args, getting its result.
  var result = fn.apply(this, args);
  // If async, fn will call the done callback.
  if (async) { return; }
  // Otherwise it has to be called explicitly.
  done(result);
}

usage.js

// This function accepts a callback.
function callbackable(a, b, done) {
  done(a + b);
}

// This function returns a value.
function returnable(a, b) {
  return a + b;
}

callbackify(callbackable, [1, 2], console.log.bind(console));
// logs 3

callbackify(returnable, [1, 2], console.log.bind(console));
// logs 3

What about an async function that doesn't specify it's callback in args, and pulls it off the argument stack when run? fn.length isn't fool proof in this sense.