memoize.js

function id(x) {
  return x;
}

// Memoize a function -- creates a new function that will cache the results of
// the first return by serializing inputs as a key.
// 
// * `fn`: the function to be memoized.
// * `hash`: a function to create the cache key.
// * `out`: a function to process memoized data on the way out. Useful if you need
//   to return an object copy instead of a reference to first return value.
function memoize(fn, hash, out) {
  var memos = {};
  hash = hash || id;
  out = out || id;

  return function() {
    var key = hash.apply(null, arguments);

    var x = memos[key] != null ?
      memos[key] : memos[key] = fn.apply(null, arguments);

    return out(x);
  };
}