They can use SharedArrayBuffers[0] which wrap shared memory or they can pass ArrayBuffers. Node may not have the shared arrays yet, but the browsers do.
Python has multiprocessing.shared_memory[1] since 3.8 to share arrays between processes, which accomplishes the same thing and circumvents the GIL issue.
[0]https://developer.mozilla.org/en-US/docs/Web/JavaScript/Refe...