ES6 async memoize, using async/await and a Map to store the cache.

memoize_async.js

function memoize_async(func) {
	const m = async function (key) {
		if (!m.cache.has(key)) m.cache.set(key, await func.apply(this, arguments));
		return m.cache.get(key);
	};
	m.cache = new Map();
	return m;
};