What is the difference between ontransfer and onmessage for Workers?

[GeTechG]

It seems that onmessage can accept anything, I am wondering from the technical point of view why there was such a separation and are there any problems if I will communicate by usual onmessage, postMessage?

[ialex32x]

For now , it's better to post only pure JS values as messages. (JS objects of godot Object & Variant are not supported)

The underlying mechanism onmessage used is serializer/deserializer in different JS runtimes, it's challenging to implement it compatible between all JS runtimes.
When using web.impl or jsc.impl, the only way to serialize/deserialize is JSON.
And, it's impossible to serialize custom JS objects in quickjs.

EDIT: JSWorker has not been implemented in web.impl and jsc.impl.