Memoization.js

Function.prototype.memoize = function() {
	var fn=this;
	var args = Array.prototype.slice.call(arguments);
	var target = args.length > 0 ? args[0] : null;
	fn._vals = fn._vals || {};
	return function() {
		var call_args = Array.prototype.slice.call(arguments);
		if (fn._vals[call_args] !== undefined) return fn._vals[call_args];
		fn._vals[call_args] = fn.apply(target, arguments);
		return fn._vals[call_args];
	}
}

// This works because it references fib through a closure.
var fib = (function(n) {
	recs++;
	if (n<2) {
		return n;
	} else {
		return fib(n-1)+fib(n-2);
	}
}).memoize();

for (var i = 0; i < 30; i++) print(fib(i));


// This also works, but it invokes the non-memoized fib because arguments.callee references the non-memoized function. E.g. 4356586 recursions are required to calculate the sequence versus 30 in the memoized version.
var fib = (function(n) {
	recs++;
	if (n<2) {
		return n;
	} else {
		return arguments.callee(n-1)+arguments.callee(n-2);
	}
}).memoize();

for (var i = 0; i < 30; i++) print(fib(i));

MemoizationRevisited.js

Function.prototype.memoize = function() {
	var fn;
	eval("fn="+this.toString().replace(/arguments.callee/gm,"arguments.callee.caller"));
	var args = Array.prototype.slice.call(arguments);
	var target = args.length > 0 ? args[0] : null;
	fn._vals = fn._vals || {};
	return function() {
		var call_args = Array.prototype.slice.call(arguments);
		if (fn._vals[call_args] !== undefined) return fn._vals[call_args];
		fn._vals[call_args] = fn.apply(target, arguments);
		return fn._vals[call_args];
	}
}
var recs = 0;
var fib = (function(n) {
	recs++;
	if (n<2) {
		return n;
	} else {
		return arguments.callee(n-1)+arguments.callee(n-2);
	}
}).memoize();

for (var i = 0; i < 30; i++) print(fib(i));
print(recs);