localForage is a handy library that improves the offline experience of your web app by using asynchronous storage (via IndexedDB where available) but with a simple, localStorage-like API.

localForage uses IndexedDB primarily, but includes a localStorage-backed fallback store for browsers with no IndexedDB storage. A WebSQL driver is in the works.

Callbacks

Because localForage uses async storage, it has an async API. It's otherwise exactly the same as the localStorage API.

// In localStorage, we would do:
localStorage.setItem('key', JSON.stringify('value'));
doSomethingElse();

// With localForage, we use callbacks:
localForage.setItem('key', 'value', doSomethingElse);

Similarly, please don't expect a return value from calls to localForage.getItem(). Instead, use a callback:

// Synchronous; slower!
var value = JSON.parse(localStorage.getItem('key'));
alert(value);

// Async, fast, and non-blocking!
localForage.getItem('key', alert);

Also of note is that localForage will automatically convert the values you get and set to JSON if you happen to be using localStorage as a backend. You don't have to pollute your code with JSON.stringify() and JSON.parse calls!

If you try to read a key that hasn't been stored yet, null will be returned in the callback.

Promises

Promises are pretty cool! If you'd rather use promises than callbacks, localForage supports that too:

function doSomethingElse(value) {
    console.log(value);
}

// With localForage, we allow promises:
localForage.setItem('key', 'value').done(doSomethingElse);

Forcing localStorage

For development, it can be easier to use the slower--but easier to debug--localStorage backend. Because localStorage can easily be inspected from the console, we allow for this with a simple global variable assignment: window._FORCE_LOCALSTORAGE = true;. If this is set to any truthy value, localStorage will be used regardless of backend.

Backbone.js

localForage includes a Backbone.js storage library that you can use to store your Backbone collections offline with only a few lines of really simple code.

Of course, Backbone.js is entirely optional and you can use localForage without it!