Event.preventDefault does not seem to work (async context)

Question

I realise there are a few questions about preventDefault not working. But my question is about the rather straightforward case where you have an async function and an error is thrown:

throw 'error of some kind';

... and then you want to have a "mopping up" function to catch any and all such errors. This then works as expected:

window.addEventListener('unhandledrejection', function (event) {
    console.log( 'unhandled Promise rejection, event: ', event );

    // event.preventDefault();
});

But if I uncomment the event.preventDefault() , I expect to see the throw message not then get printed as an error to console. But it always does get so printed.

I checked to make sure that event is indeed cancelable in this case. It claims to be so.

PS from my experiments it seems to be that

window.onunhandledrejection = event => {
    ...
};

is just another way of implementing this same listener for this same "unhandledrejection" event. Using event.preventDefault() there doesn't seem to work either.

Edit

After a few experiments I seem to find that the only way I can find to prevent this is to swallow the error (using try...catch ) at some point, preferably at the highest async call in the stack (or rather in the sequence of calls of async functions), if that can be determined.

 window.addEventListener('unhandledrejection', event => { console.log(event.cancelable, event.reason, "unhandled rejection") event.preventDefault() event.stopPropagation() event.stopImmediatePropagation() return false }) async function x() { console.log('async') throw "async error" } x() new Promise(res => { console.log('prom') throw "prom error" })

Answer 1

Verified on Firefox 76. Straight Promises don't work either. Looks like a bug. Probably should file an issue with Mozilla's Firefox tracker.
Note: this is not a solution. It's doubtful there is any solution except filing a bug and having it fixed.

 window.onerror = msg => console.log(msg) || false window.addEventListener('unhandledrejection', event => { console.log(event.cancelable, event.reason, "unhandled rejection") event.preventDefault() event.stopPropagation() event.stopImmediatePropagation() return false }) async function x() { console.log('async') throw "async error" } x() new Promise(res => { console.log('prom') throw "prom error" })

Answer 2

Repro on Firefox97. It seems the error logs were printed on the console before triggering the 'unhandledrejection' handler so the event.preventDefault() could not help.