Cache helper which uses the localStorage. Promise polyfill maybe required!

Cache.js

window.Cache = {
  /**
   * Fetch a object from the cache, if it does not exist, create it.
   *
   * @param {string} key Key to find / store the item in the localStorage.
   * @param {function} [fetchPromise] Promise to fetch new data
   *
   * @example
   * Cache.fetch("result", (resolve, reject) => {
   *   resolve(5 * 10);
   * }).then((result) => {
   *   console.log(result);
   * });
   *
   * @returns {Promise} Returns a promise with the result
   */
  fetch: function(key, fetchPromise) {
    return new Promise((resolve, reject) => {
      // check if it's already chached
      var cached = localStorage.getItem(key);
      if (cached) {
        resolve(JSON.parse(cached));
      }

      // fetch new item
      else if (fetchPromise) {
        new Promise(fetchPromise).then((result) => {
          localStorage.setItem(key, JSON.stringify(result));
          resolve(result);
        })
        .catch((error) => reject(error));
      }

      // reject with not found
      else {
        reject("KEY NOT FOUND");
      }
    });
  }
}