caching function results (single argument function)

fib.js

function fib(x) {
  if (x < 2)
    return 1;

  return fib(x-1) + fib(x-2);
}

memoize.js

if (!Function.prototype.memoize) {
  Function.prototype.memoize = function () {
    var cache = {};
    var func = this;

    return function (x) {
      if (!(x in cache)) {
        cache[x] = func.call(this, x);
      }
      return cache[x];
    };
  };
}