function withDatabase(f) {
    if (db) {
      f();
    } else {
      var openreq = indexedDB.open(DBNAME, DBVERSION);
      openreq.onerror = function withStoreOnError() {
        console.error('asyncStorage: can\'t open database:',
            openreq.error.name);
      };
      openreq.onupgradeneeded = function withStoreOnUpgradeNeeded() {
        // First time setup: create an empty object store
        openreq.result.createObjectStore(STORENAME);
      };
      openreq.onsuccess = function withStoreOnSuccess() {
        db = openreq.result;
        f();
      };
    }
  }
This file defines an asynchronous version of the localStorage API, backed by
an IndexedDB database.  It creates a global asyncStorage object that has
methods like the localStorage object.

To store a value use setItem:

  asyncStorage.setItem('key', 'value');

If you want confirmation that the value has been stored, pass a callback
function as the third argument:

 asyncStorage.setItem('key', 'newvalue', function() {
   console.log('new value stored');
 });

To read a value, call getItem(), but note that you must supply a callback
function that the value will be passed to asynchronously:

 asyncStorage.getItem('key', function(value) {
   console.log('The value of key is:', value);
 });

Note that unlike localStorage, asyncStorage does not allow you to store and
retrieve values by setting and querying properties directly. You cannot just
write asyncStorage.key; you have to explicitly call setItem() or getItem().

removeItem(), clear(), length(), and key() are like the same-named methods of
localStorage, but, like getItem() and setItem() they take a callback
argument.

The asynchronous nature of getItem() makes it tricky to retrieve multiple
values. But unlike localStorage, asyncStorage does not require the values you
store to be strings.  So if you need to save multiple values and want to
retrieve them together, in a single asynchronous operation, just group the
values into a single object. The properties of this object may not include
DOM elements, but they may include things like Blobs and typed arrays.

Unit tests are in apps/gallery/test/unit/asyncStorage_test.js