Best Undocumented Feature of Backbone.js

It’s fairly well known that you can override Backbone.sync in order to provide your own backing store.*  However, what isn’t documented but is clearly visible in the source, is that this.sync not only exists but takes priority.  This is pretty vital if you ever want a presentation-model style collection, where adding and removing causes changes to other models in your application.  I’ll warn you that you need to step through the framework code fairly carefully to get this to work, but it’s really useful.

*Another useful tip: if you’re using ASP.NET MVC, add a route like this

routes.MapRoute("Delete", "{controller}/{id}", new { action : "DELETE" }, 
    new { httpMethod = new HttpMethodConstraint("DELETE"))

Because ASP.NET MVC doesn’t seem to be able to cope with just slapping an [HttpDelete] attribute on an Index method.