Envelope. js is a simple, event driven jQuery plugin for in app messaging designed to work with or without popular UI frameworks. You can use Envelope.js along with popular UI frameworks like Twitter Bootstrap
and jQuery UI to generate events such as 'sucess', 'error', etc.
One issue with using callbacks to show user alerts is that they are not very reuseable. Using an evented approach allows you to decouple the action of showing a message to your users with the function or callback that generates it.