This is actually an important property of JavaScript that allows it to work without locks (as long as you do not await, your code runs completely synchronously and will not be interrupted). It's not a flaw, but definitely requires awareness.
Gevent code that doesn't spawn more than one native thread (the one that runs the event loop) also has this property - you don't need locks as long as you do not perform IO. In Python's case it can be more tricky, as you might end up yielding to the event loop by indirectly performing IO when logging or something similar.
In JavaScript, the only case, AFAIK, where this can happen is when you await. Nothing else will cause you to yield to the event loop.
