collatzconjecture.js

const memoize = (fn) => {
	const cache = new Map();
	const memo = (...a) => {
		const key = a.reduce((hash, val) =>
			hash += val === Object(val) ?
				JSON.stringify(val) :
				val, '');
		if (!cache.has(key)) cache.set(key, fn(...a));
		return cache.get(key);
	}
	return Object.defineProperties(memo, {
		length: { value: fn.length },
		_cache: { value: () => cache }
	});
}

const collatz = memoize(num =>
  num !== 1 ?
    [num].concat(collatz(num % 2 === 0 ? num / 2 : (3 * num) + 1)) :
    num);

console.log(collatz(2345))