I think intrinsic to that deal-breaker is the underlying problem. Memory management in a parallel environment is a tough nut to crack. Leaning on the JVM punt

I think intrinsic to that deal-breaker is the underlying problem. Memory management in a parallel environment is a tough nut to crack. Leaning on the JVM punts on the issue. There's no reason JavaScript couldn't transparently implement parallel map/filter/etc. Other than the fact it is really hard to do. On the other hand a lot of the things JavaScript engines do today are also really hard, so maybe one day it'll happen.