alter_native

TIL : Today I Learned - Series four

User21000000

Today I learned that the best abstraction for a simple cache is likely a closure. I was trying to figure out a generalized way to cache things like bitcoin's blockheight and the exchange rate.

```js
function cachedFetcher(fetcher, { cacheExpiry, forceRefreshThreshold }) {
  let cache = null;

  return async function cachedFetch(...args) {
    const now = Date.now();

    async function fetchAndCache() {
      const result = await fetcher(...args);
      cache = { data: result, createdAt: now };
      return result;
    }

    if (cache) {
      const age = now - cache.createdAt;

      if (age < cacheExpiry) {
        return cache.data;
      } else if (age < forceRefreshThreshold) {
        fetchAndCache().catch(console.error);
        return cache.data;
      }
    }

    return await fetchAndCache();
  };
}

const getBlockHeight = createCachedFetcher(fetchBlockHeight, {
  cacheExpiry: 60 * 1000,
  forceRefreshThreshold: 10 * 60 * 1000,
})
```