IndexDB

indexDB.js

const dbPromise = idb.open('post-store', 1, function(db) {
  if (!db.objectStoreNames.contains('posts')) {
    db.createObjectStore('posts', {keyPath: 'id'});
  }
});

// service worker implementation
...
event.respondWith(fetch(event.request)
  .then(res => {
    const clonedRes = res.clone();
    cloneRes.json()
      .then(data => {
        for (let key in data) {
          dbPromise
            .then(db => {
              const tx = db.transaction('posts', 'readwrite');
              const store = tx.objectStore('posts');
              store.put(data[key]);
              return tx.complete;
            })
        }
      });
    return res;
  })               
                 
);

// or set up it's own indexdb file
const dbPromise = idb.open('post-store', 1, function(db) {
  if (!db.objectStoreNames.contains('posts')) {
    db.createObjectStore('posts', {keyPath: 'id'});
  }
});

function writeData(store, data) {
  return dbPromise
    .then(db => {
      const tx = db.transaction(store, 'readwrite');
      const store = tx.objectStore(store);
      store.put(data);
      return tx.complete;
    });
}

function readAllData(store) {
  return dbPromise
    .then(db => {
      const tx = db.transaction(store, 'readonly');
      const store = tx.objectStore(store);
      return store.getAll(); // don't need tx.complete
    });
}

use-indexDB-getAll-Data

if ('indexedDB' in window) {
  readAllData('post')
    .then(data => {
      if (!networkDataReceived) {
        console.log('from cache', data);
        updateUI(data);
      }
    })
}