React is able to render custom attributes as described at http://facebook.github.io/react/docs/jsx-gotchas.html:
If you want to use a custom attribut
If anyone is trying to use event.target in React and finding a null value, it is because a SyntheticEvent has replaced the event.target. The SyntheticEvent now holds 'currentTarget', such as in event.currentTarget.getAttribute('data-username').
https://facebook.github.io/react/docs/events.html
It looks like React does this so that it works across more browsers. You can access the old properties through a nativeEvent attribute.
I do not know about React, but in the general case you can pass custom attributes like this:
1) define inside an html-tag a new attribute with data- prefix
data-mydatafield = "asdasdasdaad"
2) get from javascript with
e.target.attributes.getNamedItem("data-mydatafield").value
You can simply use event.target.dataset object . This will give you the object with all data attributes.