homework from javascriptmasterclass.com; paired with http://github.com/chrisdickinson

cache.js

var Cache = function() {
	this.table = {};
};

Cache.prototype = {
	'get': function(key) {
		return this.table[key];
	},
	'set': function(key, value) {
		return this.table[key] = value;
	}
};

var cache = function(argument) {
	var cacheResult;
	if (!this._cacheObject) {
		this._cacheObject = new Cache();
	}
	cacheResult = this._cacheObject.get(argument);
	if (cacheResult === undefined) {
		cacheResult = this._cacheObject.set(argument, this(argument));
	}
	return cacheResult;
};

exports.Cache = Cache;

Function.prototype.cache = cache;

prime.js

var cache = require('./cache');

var isPrime = function(num) {
	num = Math.abs(num);

	if (num.toString().indexOf('.') !== -1) {
		return false;
	}

	if (num < 2) {
		return false;
	}
	for (var i = 2; i < num; i++) {
		if (num % i == 0) {
			return false;
		}
	}
	return true;
};

exports.isPrime = function(num) {
	return isPrime.cache(num);
};

test.js

var cache = require('../cache'),
	prime = require('../prime'),
	vows = require('vows'),
	assert = require('assert');

var suite = vows.describe('cache');

suite.addBatch({
	'Call isPrime': {
		'with a prime number': function() {
			assert.isTrue(prime.isPrime(17));
			assert.isTrue(prime.isPrime('19'));
			assert.isTrue(prime.isPrime(524287));
		},
		'with a non-prime number': function() {
			assert.isFalse(prime.isPrime(54.212));
			assert.isFalse(prime.isPrime(0));
			assert.isFalse(prime.isPrime(-10));
			assert.isFalse(prime.isPrime(1));
			assert.isFalse(prime.isPrime("1"));
			assert.isFalse(prime.isPrime(20));
		}
	}
}).addBatch({
	'Call cache': {
		'with an empty cache': function() {
			var fn = Math.sin,
				rando = Math.random();
			assert.isUndefined(fn._cacheObject);

			var result = fn.cache(rando),
				realResult = fn(rando);

			assert.instanceOf(fn._cacheObject, cache.Cache);
			assert.equal(result, realResult);
		},
		'with a populated cache': function() {
			var fn = Math.sin,
				rando = Math.random(),
				secretRando = Math.random();
			fn.cache(rando);
			fn._cacheObject.set(rando, secretRando);

			assert.equal(fn.cache(rando), secretRando);
			assert.notEqual(fn.cache(rando), fn(rando));
		},
		'with non-truthy value in cache': function() {
			var fn = Math.sin,
				rando = Math.random();

			assert.equal(fn.cache(rando), fn(rando));
			fn._cacheObject.set(rando, false);
			assert.equal(fn.cache(rando), false);
		}
	}
}).run();