Yep, in multithreaded langs you just want to use a ConcurrentMap or whatever the stdlib has for this. The write is very brief because you only write the promise and not eg keep a proper lock on the key until the result has arrived, so no need for a fancier datastructure.

js is single-threaded so no problem there.

Since OP mentioned Mutexes I assumed he's dealing with multithreaded code, but with JS this works as advertised :)