asyncStorage.js

var asyncStorage = (function () {

  var indexedDB = window.indexedDB ||
                  window.mozIndexedDB ||
                  window.webkitIndexedDB ||
                  window.msIndexedDB;

  var storeName = 'asyncStorage';
  var db;

  function wrap(req) {
    return new Promise(function (resolve, reject) {
      req.onsuccess = function (e) {
        resolve(req.result);
      };
      req.onerror = function (e) {
        reject(req.errorCode);
      };
    });
  }

  function getItem (key) {
    var transaction = db.transaction([storeName]);
    var store = transaction.objectStore(storeName);
    return new Promise(function (resolve, reject) {
      wrap(store.get(key)).then(function(row) {
        resolve(row ? row.value : undefined);
      }, reject);
    });
  }

  function setItem (key, value) {
    var transaction = db.transaction([storeName], 'readwrite');
    var store = transaction.objectStore(storeName);
    return wrap(store.put({ 'key': key, 'value': value }));
  }

  function removeItem (key) {
    var transaction = db.transaction([storeName], 'readwrite');
    var store = transaction.objectStore(storeName);
    return wrap(store.delete(key));
  }

  var exports = {
    'getItem': getItem,
    'setItem': setItem,
    'removeItem': removeItem
  };

  function init () {
    return new Promise(function (resolve, reject) {
      if (!indexedDB) {
        reject('No indexedDB implementation found!');
      }
      var req = indexedDB.open(storeName, 1);
      req.onsuccess = function (e) {
        db = req.result;
        resolve(exports);
      };
      req.onupgradeneeded = function (e) {
        db = req.result;
        db.createObjectStore(storeName, { keyPath: 'key' });
        resolve(exports);
      };
      req.onerror = reject;
    });
  }

  return init();

})();