Bug 188083 – The instanceof operator does not work as expected for classes that extend CustomEvent or Event

• Component: JavaScriptCore
• Status: RESOLVED CONFIGURATION CHANGED

[Eugene Kashida]

Overview:

class FooEvent extends CustomEvent {
  constructor() {
    super('foo');
  }
};
var foo = new FooEvent();
console.log('This evaluates to "false" which is unexpected', foo instanceof FooEvent);
console.log('This evaluates to "true" which is expected', foo instanceof CustomEvent);
console.log('This evaluates to "true" which is expected', foo instanceof Event);


Expected Results:

The instanceof operator, when given an instance of FooEvent which extends CustomEvent, and the FooEvent constructor, should evaluate to "true".

Build Date & Hardware:

Release 61 (Safari 12.0, WebKit 13606.1.25.1)
Mac OS 10.13.6

[Ryosuke Niwa]

I don't think class inheritance ever worked with Event's.

[Eugene Kashida]

It seems to be working as expected in Firefox and Chrome, and Edge has the following open issue:
https://developer.microsoft.com/en-us/microsoft-edge/platform/issues/11923584/

[Alexey Shvayka]

(In reply to Eugene Kashida from comment #0)
> Expected Results:
> 
> The instanceof operator, when given an instance of FooEvent which extends
> CustomEvent, and the FooEvent constructor, should evaluate to "true".

As of https://trac.webkit.org/changeset/256716, extending CustomEvent (as well as other DOM constructors) works as expected.

[Radar WebKit Bug Importer]

<rdar://problem/60743143>