4 ms·

The single thread limitation is caused by the global interpreter lock[0] that's common to most interpreted languages. Along with JS, it also appears in CPython

• tlrobinson 11y ago
  That's not the reason. As I understand it, the GIL is an implementation detail of certain interpreters that prevents threads from running on multiple OS threads (and thus multiple CPUs). JavaScript has explicitly chosen not to support preemptive multithreading at all (aside from Web Workers which don't share memory), while Ruby and Python do have it, even if it can't fully utilize multicore processors.
• Lerc 11y ago
  That doesn't eliminate the possibility of pre-emption though does it? A theoretical version of setTimeout that triggered immediately suspending the active code and executing the timeout function then resuming (or potentially not). That only requires the interpretor to execute one piece of code at a time. The notions of thread safety would apply to the interpreted code, but not to the interpretor itself.
• vidarh 11y ago
  No, it's not. MRI for example uses OS-level threads from 1.9.x onwards. The GIL prevents multiple of those threads from executing inside the interpreter at any given time, which may sound like it forces a single thread to be running but in practice most typical code will end up spending a lot of time running in C extensions or waiting on kernel space, so it's much less of a hindrance than you might think (it's still not great, but it's far better than it'd be otherwise)