window:error event interface and cancelling

[GeorgianStan]

MDN URL

https://developer.mozilla.org/en-US/docs/Web/API/Window/error_event

What specific section or headline is this issue about?

event type, usage notes

What information was incorrect, unhelpful, or incomplete?

Event type
Is mentioned that the event type is UIEvent.

Usage notes
In usage notes is mentioned:

Unlike other events, the error event is canceled by returning true from the handler instead of returning false. When canceled, the error won't appear in the console, but the current script will still stop executing.

What did you expect to see?

Event type*
If we run this code we'll see in the console that the error is of type ErrorEvent.
In HTML spec is mentioned the same.

window.addEventListener("error", (event) => {
  console.log(event); // event is of type ErrorEvent
});

const badCode = "const s;";
eval(badCode);

Usage notes

The return statement doesn't seem to have an impact, Regardless of the returned value: return true or return false the error message is displayed in the console. This event it looks like is similar with unhandledrejection and event.preventDefault() must be used, so that the message is not printed to the console.

Do you have any supporting links, references, or citations?

discussions page

[wbamberg]

As far as I can tell: the bit about UIEvent looks just wrong and it should be ErrorEvent.

The bit about cancelling the event is a bit more complicated. I think:

• if you listen for an event using addEventListener, yes, you cancel it using preventDefault(). And that works fine for Window's error event as well.
• but if you listen using the on- event handler property, then most of these can also cancel by returning false. Try this:

<textarea>Type in me!</textarea>

document.querySelector("textarea").onkeydown = (e) => {
	console.log("cancelled!");
	return false;
};

Except that for Window's onerror handler, this is reversed, and you have to return true to cancel:

<button id="error">Error!</button>

window.onerror = () => {
	console.log("cancelled!");
	return true;
};

The spec also discusses this: https://html.spec.whatwg.org/multipage/webappapis.html#eventhandler.
This is so weird that it definitely seems worth documenting, but that page should be clearer that it is only talking about the event handler property (onerror) here.

Does that make sense?

[GeorgianStan]

Therefore, looks like UIEvent should be replaced with ErrorEvent and the documentation should be extended to cover both ways of canceling the event: return true for .onerror and preventDefault() for addEventListener.