Determine what is being dragged from dragenter & dragover events

Question

I'm trying to use the HTML5 draggable API (though I realize it has its problems). So far, the only showstopper I've encountered is that I can't figure out a way to determine what is being dragged when a dragover or dragenter event fires:

el.addEventListener('dragenter', function(e) {   // what is the draggable element? }); 

I realize I could assume that it's the last element to fire a dragstart event, but... multitouch. I've also tried using e.dataTransfer.setData from the dragstart to attach a unique identifier, but apparently that data is inaccessible from dragover/dragenter:

This data will only be available once a drop occurs during the drop event.

So, any ideas?

Update: As of this writing, HTML5 drag-and-drop does not appear to be implemented in any major mobile browser, making the point about multitouch moot in practice. However, I'd like a solution that's guaranteed to work across any implementation of the spec, which does not appear to preclude multiple elements from being dragged simultaneously.

I've posted a working solution below, but it's an ugly hack. I'm still hoping for a better answer.

Answer 1

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.

Answer 2

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.