lazy.js

Function.prototype.lazy = function() {
	var fn = this,
		init = false;
	return function() {
		if(!init) {
			init = true;
			fn = fn.apply(this, arguments);
		}
		return fn.apply(this, arguments);
	}
};

var digit_name = function(n) {
	var names = ['zero', 'one', 'two', 'three', 'four', 'five', 'six', 'seven', 'eight', 'nine'];
	return function(n) {
		return names[n];
	};
}.lazy();

digit_name(3); //init, then return
digit_name(2); //return