Determine what is being dragged from dragenter & dragover events

A (very inelegant) solution is to store a selector as a type of data in the dataTransfer object. Here is an example: http://jsfiddle.net/TrevorBurnham/eKHap/

The active lines here are

e.dataTransfer.setData('text/html', 'foo');
e.dataTransfer.setData('draggable', '');

Then in the dragover and dragenter events, e.dataTransfer.types contains the string 'draggable', which is the ID needed to determine which element is being dragged. (Note that browsers apparently require data to be set for a recognized MIME type like text/html as well in order for this to work. Tested in Chrome and Firefox.)

It's an ugly, ugly hack, and if someone can give me a better solution, I'll happily grant them the bounty.

Update: One caveat worth adding is that, in addition to being inelegant, the spec states that all data types will be converted to lower-case ASCII. So be warned that selectors involving capital letters or unicode will break. Jeffery's solution sidesteps this issue.

The short answer to my question turns out to be: No. The WHATWG spec doesn't provide a reference to the element being dragged (called the "source node" in the spec) in the dragenter, dragover, or dragleave events.

Why not? Two reasons:

First, as Jeffery points out in his comment, the WHATWG spec is based on IE5+'s implementation of drag-and-drop, which predated multi-touch devices. (As of this writing, no major multi-touch browser implements HTML drag-and-drop.) In a "single-touch" context, it's easy to store a global reference to the current dragged element on dragstart.

Second, HTML drag-and-drop allows you to drag elements across multiple documents. This is awesome, but it also means that providing a reference to the element being dragged in every dragenter, dragover, or dragleave event wouldn't make sense; you can't reference an element in a different document. It's a strength of the API that those events work the same way whether the drag originated in the same document or a different one.

But the inability to provide serialized information to all drag events, except through dataTransfer.types (as described in my working solution answer), is a glaring omission in the API. I've submitted a proposal for public data in drag events to the WHATWG, and I hope you'll express your support.

I wanted to add a very clear answer here so that it was obvious to everyone who wanders past here. It's been said several times in other answers, but here it is, as clear as I can make it:

dragover DOES NOT HAVE THE RIGHTS to see the data in the drag event.

This information is only available during the DRAG_START and DRAG_END (drop).

The issue is it's not obvious at all and maddening until you happen to read deeply enough on the spec or places like here.

WORK-AROUND:

As a possible work-around I have added special keys to the DataTransfer object and tested those. For example, to improve efficiency I wanted to look up some "drop target" rules when my drag started instead of every time a "drag over" occurred. To do this I added keys identifying each rule onto the dataTransfer object and tested those with "contains".

ev.originalEvent.dataTransfer.types.includes("allow_drop_in_non_folders")

And things like that. To be clear, that "includes" is not a magic bullet and can become a performance concern itself. Take care to understand your usage and scenarios.