memoize a function with an optional resolver to compute the proper key

memoize.js

const memoize = function(func, resolver) {
  const cache = new Map();

  return (...args) => {
    const key = resolver ? resolver.apply(this, args) : args[0];

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

    const result = func.apply(this, args);
    cache.set(key, result);
    return result;
  }
}