SponsorBlockServer/src/utils/createMemoryCache.js

module.exports = function createMemoryCache(memoryFn, cacheTimeMs) {
  if (isNaN(cacheTimeMs)) cacheTimeMs = 0;

  // holds the promise results
  const cache = new Map();
  // holds the promises that are not fulfilled
  const promiseMemory = new Map();
  return (...args) => {
    // create cacheKey by joining arguments as string
    const cacheKey = args.join('.');
    // check if promising is already running
    if (promiseMemory.has(cacheKey)) {
      return promiseMemory.get(cacheKey);
    }
    else {
      // check if result is in cache
      if (cache.has(cacheKey)) {
        const cacheItem = cache.get(cacheKey);
        const now = Date.now();
        // check if cache is valid
        if (!(cacheItem.cacheTime + cacheTimeMs < now)) {
          return Promise.resolve(cacheItem.result);
        }
      }
      // create new promise
      const promise = new Promise(async (resolve, reject) => {
        resolve((await memoryFn(...args)));
      });
      // store promise reference until fulfilled
      promiseMemory.set(cacheKey, promise);
      return promise.then(result => {
        // store promise result in cache
        cache.set(cacheKey, {
          result,
          cacheTime: Date.now(),
        });
        // remove fulfilled promise from memory
        promiseMemory.delete(cacheKey);
        // return promise result
        return result;
      });
    }
  };
};