JS is single threaded, but the process lives in an event loop.

You can easily write a JS function that can have a race condition by launching two asynchronous processes (such as writing to local storage in a browser or the file system both operating on the same data).

I had this happen just today while scraping different websites writing to the same files and overwriting each other.

Concurrency is not the same as parallelism. I’ve understood TFA to mean concurrency when it mentions parallelism. This would explain the presence of JS libraries which could test for concurrency problems in asynchronous code.