Better yet, support it!

Yeah, is there a particular reason it's not supported?

Curious if you've seen the EnterLeaveEventPlugin.js. It's automatically injected into React by default, so you can use it today. IMHO it's (much) better than onMouseOver. (I suppose we could support onMouseOver just for the sake of completeness, but I thought I'd just make sure you guys were aware of onMouseEnter/onMouseLeave.)

We discussed this for a bit on IRC -- @BenQuinn was just confused that onMouseOver silently didn't work whereas other events did -- perhaps adding more documentation (#171) will fix the problem too but it seems like we might as well add onMouseOver for completeness.

(I agree that enter/leave are much more useful and suggested that over/out are likely missing simply because no one has needed them!)

Agree - I believe there are a few extra allocations (not-pooled) occuring in the event system already today. I'd love to track those down before adding more mouse-move event allocations. If you open the memory profiler in Chrome and move the mouse around, you'll see memory being allocated (and then properly freed of course) but if we use PooledClasses everywhere, we should be flat. That would make me much more comfortable with adding mouseMove to SimpleEventPlugin.js.

Edit: Just checked out SimpleEventPlugin.js - we already have onMouseMove which fires way more frequently than onMouseOver would so it's probably not such a big deal to add onMouseOver - though we really should get rid of all additional allocations in the event path.

Just started to look a little bit into the allocations here. Redefining trapBubbledEvent in ReactEventEmitter with:

function trapBubbledEvent(topLevelType, handlerBaseName, element) {
  EventListener.listen(
    element,
    handlerBaseName,
    function() {}
  );
}

still shows allocations in the Chrome memory timeline view, so I'm not convinced that there's anything we can do here? (Replacing trapBubbledEvent itself with a noop makes the allocations go away. EventListener.listen simply calls addEventListener.)

@jordwalke I am pretty sure we have done all we can here -- at least I think we've tracked down every callsite where PooledClass would have helped us. I updated our recast transforms to log every syntax construct that would cause an obvious allocation and couldn't find anything. I was thinking maybe there was an array slice or a bind() in there, but @spicyj's experiment shows that this is not the case.

(I didn't verify that we don't have more allocations than the empty-function case; I don't know of a good way to do so.)

I might be misinformed, but isn't there quite a difference in use cases between mouseenter and mouseover events?

Here's a theoretical (non-react) example working with some hover states for nested elements:
http://jsfiddle.net/aJM2S/3/

I tried to apply the above concept in react (with nested components) using onMouseEnter instead, but it obviously fails because it's only triggered once on the top-most element. Moving into a child element and back has no effect.

Maybe there's a better approach for this in react altogether?

Maybe there's a better approach for this in react altogether?

@danielstocks If you render the child hierarchy with React, you can listen to onMouseEnter/onMouseLeave on each child element you are interested in instead of using event bubbling. It is cheap.