Waiting for the consensus on the spec discussion.

Looks like this is a real developer pain point (for example, [1]). Regardless of the spec change, we could print a console message.

[1] https://bugs.chromium.org/p/chromium/issues/detail?id=851123#c4

Just wanted to echo that I'd love to see more specific error messages. I'm working on developing a live-coding environment for worklets and the catch-all error is not helpful..

 Issue 872909  has been merged into this issue.

I'll start fixing this in parallel with the spec discussion.

I'm now trying to send V8's error object (JS_ERROR_TYPE) from a worklet thread to the main thread for resolving a promise returned by addModule() with it. This requires to serialize the error object as SerializedScriptValue. However, looks like v8's object serializer (v8::ValueSerializer::WriteObject() and v8::ValueSerializer::WriteJSReceiver()) called from SerializedScriptValue::Serialize() doesn't support it and returns a null object. We might need to change the V8 side... I'll take a closer look.

+adamk@: Hi Adam. Do you know who is the best person to ask about V8's object serializer? Specifically, I'd like to know whether a JS_ERROR_TYPE object is serializable (see my previous comment#8).

My memory is that, per spec, Error objects aren't cloneable. MDN agrees (https://developer.mozilla.org/en-US/docs/Web/API/Web_Workers_API/Structured_clone_algorithm#Things_that_don't_work_with_structured_clone) but I can't find the precise bit of the HTML spec that knows this.
 
Also CCing jbroman@, who wrote the value serializer.

Quoth the HTML spec[1]:

"Otherwise, if value has any internal slot other than [[Prototype]] or [[Extensible]], then throw a "DataCloneError" DOMException."

Quoth the ECMAScript spec[2]:

"NativeError instances...have an [[ErrorData]] internal slot whose value is undefined"

Thus NativeError objects (i.e., JS_ERROR_TYPE) are not structured serializable.

[1]: https://html.spec.whatwg.org/multipage/structured-data.html#structuredserializeinternal
[2]: http://www.ecma-international.org/ecma-262/6.0/index.html#sec-properties-of-nativeerror-instances

err, linked NativeError, but the same applies to error: http://www.ecma-international.org/ecma-262/6.0/index.html#sec-properties-of-error-instances

adamk@, jbroman@: Thank you for the clarification! Hmm, then we need to find other ways to notify errors on worklets. I'll back to the spec discussion...

I'm still not sure what is the best way to do this. As a stopgap, I'll make a CL to print console messages as c#3.

WIP CL: https://chromium-review.googlesource.com/c/chromium/src/+/1317110