Hmm, just tested with 2v09.134 Linux and go this output when testing the code above.

=Promise: {  }
resolved
Uncaught Error: Unhandled promise rejection: undefined
> 

Thanks! Not sure what happened there then

Any hint how to fix this?

looking at what changed in the PR that supposedly fixed this is probably a good start:

f1d7afd

I guess what's interesting is 'rejected' is now not printed. However the rejection is caught by the OS - so maybe the 'unhandled promise' code doesn't check the state of "done"?

Wow, there have been many changes in jswrap_promise.c since f1d7afd.

Looks like is could be the section

Just added some prints

  jsiConsolePrintf("L121 resolve: %d\n",resolve);
  if (resolve) 
    jsvObjectSetChild(promise, JS_PROMISE_RESOLVED_NAME, data);
  if (fn) {
    _jswrap_promise_resolve_or_reject(promise, data, fn);
    jsvUnLock(fn);
  } else {
    if (!resolve) {
      jsiConsolePrintf("L129 resolve: %d\n",resolve);
      jsExceptionHere(JSET_ERROR, "Unhandled promise rejection: %v", data);
    }
  }

and that's the output:

>L121 resolve: 1
resolved
L121 resolve: 1
>L121 resolve: 0
L129 resolve: 0
Uncaught Error: Unhandled promise rejection: undefined
> 

Hope this helps you to fix it, because I have no clue.

What about making jsExceptionHere(JSET_ERROR, "Unhandled promise rejection: %v", data); dependent on data?

  
  } else {
    if (!resolve && data)
      jsExceptionHere(JSET_ERROR, "L141 Unhandled promise rejection:  %v", data);
  }

Result of test with this change:

test_promises.out.txt

I'm not sure that helps? try:

const p = new Promise(function(resolve, reject) {
  setTimeout(resolve, 100);
  setTimeout(reject, 200, "Oh no");
}).then(function() {
    console.log('resolved');
}).catch(function() {
    console.log('rejected');
});

Just tested - works perfect - many thanks!