Bug 187822 – unhandledrejection event doesn't fire across realms

• Component: WebCore JavaScript
• Status: NEW

[Timothy Gu]

With this HTML:

<iframe></iframe>
<script>
const frame = frames[0];
frame.onunhandledrejection = () => { alert("Rejected!"); };
frame.Promise.reject();
</script>

The "Rejected!" message is not shown, but it is on Chrome.

----

There is an interesting twist to this, as executing the script inside a frame.setTimeout() (which makes the entry realm [1] to Promise.reject be that of the iframe) would allow the alert to execute. The following code works:

<iframe></iframe>
<script>
const frame = frames[0];
frame.setTimeout(() => {
  frame.onunhandledrejection = () => { alert("Rejected!"); };
  frame.Promise.reject();
});
</script>

[1]: https://html.spec.whatwg.org/multipage/webappapis.html#concept-entry-realm

----

Merely a setTimeout does not fix the problem. The following does not alert the message either:

<iframe></iframe>
<script>
const frame = frames[0];
setTimeout(() => {
  frame.onunhandledrejection = () => { alert("Rejected!"); };
  frame.Promise.reject();
});
</script>

[Timothy Gu]

There is certainly some ambiguity in the HTML spec about what the right global object the event is fired on should be (should it be on frame? or window?) [1]. An enhanced version of reproduction case tries to handle this uncertainty by permitting both possibilities:

<iframe></iframe>
<script>
const frame = frames[0];
window.onunhandledrejection = () => { alert("Rejected!"); };
frame.onunhandledrejection = () => { alert("Rejected!"); };
frame.Promise.reject();
</script>

Yet, even in this case, the message is not shown.

As an additional data point, with this HTML sample, the "Rejected!" message is shown as soon as I try to launch Web Inspector.

[1]: https://github.com/whatwg/html/issues/958