Comment by connicpu

You can rely on node preventing data races, but those are distinct from race conditions. A race condition in the logic of your code can happen any time two "threads" of execution (in this case a thread could be considered a chain of asynchronous callbacks) interleave their operations. It's possible for one of them to do something with a resource the other was using unless you use some kind of synchronization to prevent the other from using the resource until the first thread is done with it. For example, two callback chains could start using the same database connection object. Perhaps one chain was in the middle of setting up a transaction when it needed to wait for some other async resource to load, and the other chain comes in and does something with it. Now it's in an unintended state because the object was allowed to be used by two different "threads" of callback chains.