Async caching with WeakMap and promises

cache.js

export function createAsyncCache(fn) {
  const cache = new WeakMap();

  return {
    get(key) {
      if (cache.has(key)) {
        return cache.get(key);
      }

      const promise = Promise.resolve(fn(key));
      cache.set(key, promise);
      return promise;
    }
  };
}