A localstorage-backed cache wrapper for window.fetch

fetchJson.js

const fetchJson = (function () {
  localStorage.cachedFetchResponses = localStorage.cachedFetchResponses || '{}';

  let cache = localStorage.cachedFetchResponses;

  const setCachedData = (key, data) => {
    const parsedCache = JSON.parse(cache);
    parsedCache[key] = data;
    
    try {
      localStorage.cachedFetchResponses = JSON.stringify(parsedCache);
    } catch (e) {
      console.error(e);
      console.warn('Oh no something went wrong!  Resetting localstorage cache...');
      localStorage.cachedFetchResponses = '{}';
    }
    
    cache = localStorage.cachedFetchResponses;

    return data;
  };

  const getCachedData = key => JSON.parse(cache)[key];

  return (url) => {
    if (getCachedData(url)) {
      return new Promise(
        (resolve, rej) => resolve(getCachedData(url))
      );
    } else {
      return fetch(url).then(
        res => res.json()
      ).then(
        json => setCachedData(url, json)
      ); 
    }
  };
})();