The expiring part is very simple. For just get/set/delete and scheduled garbage collection, here’s an implementation that’s almost precisely equivalent to what KV.js offers (just with sane arguments and return types, fewer bugs, and markedly better performance due to not supporting stuff you certainly don’t need and removing unnecessary abstractions):

  const store = new Map();
  const expiryTimes = new Map();

  function get(key) {
      // (This works even for non-expiring keys because undefined is not less than any number.)
      if (expiryTimes.get(key) < Date.now()) {
          store.delete(key);
      }
      return store.get(key);
  }

  function set(key, value, expiry) {
      store.set(key, value);
      if (expiry) {
          expiryTimes.set(expiry);
      }
  }

  function del(key) {
      store.delete(key);
      expiryTimes.delete(key);
  }

  function flushExpired() {
      const now = Date.now();
      for (const [key, time] of expiryTimes) {
          if (time < now) {
              del(key);
          }
      }
  }

  // Optional: automatically delete expired entries to free memory, once a second since that’s what KV.js does, but that’s probably waaaaay more often than you need.
  setInterval(flushExpired, 1000);

Working directly with store and expiryTimes for more advanced stuff will genuinely regularly be easier than working with the KV.js abstraction.