Javascript starts with simple callbacks. Those indeed have no stack, only a shallow list of local variables as state. Then async/await is modelled as a relatively straightforward syntactic sugar on top of that: An await call is still just a callback behind the scenes; if one async function awaits another async function, you get something that looks like a stack, but is really just a chain of callbacks.

In contrast, Python starts with coroutines, which do have a stack, then models async/await by surrounding them with a scheduling runtime. Unfortunately, for async code to be useful, you still need support for callbacks, so you end up with both: an await call represents a mix of suspended coroutine stacks and callback chains, which can be much more complicated to reason about.