If your data is composite (i.e. a JavaScript object or array) and you have several Promise-based flows of execution (technically not a virtual thread by most definitions of the term, but still a scheduled flow of execution) that end up reading/writing sub-parts of your piece of data in unpredictable order. That's basically a data race, in JavaScript. I've had to debug some of those.

As a side-note, some forms of data races (for perhaps a slightly looser definition of the term) can even happen without explicit asynchronicity, just with callbacks/synchronous events.