1820521 - Instantiated subclass of Xray-wrapped DOM constructor is not an instance of the subclass, but the superclass.

Status: NEW

(Core :: XPConnect, defect)

Description

[Rob Wu [:robwu]]

Attachment: content-script-subclass-dom.zip

Given a XrayWrapper of a DOM constructor, when a class subclasses that, and an instance of that subclass is created, the instance is unexpectedly not an instance of the subclass, but of the native DOM constructor.

STR:

1. Load the attached extension (relevant script pasted below) at about:debugging.
2. Visit example.com
3. Look at the console.

Expected logs:

• "constructor called"
• "instanceof Sub =" true
• "myMethod() called"

Actual logs:

• "constructor called"
• "instanceof Sub =" false
• TypeError: sub.myMethod is not a function

// In an extension content script, CustomEvent is a XrayWrapper of the native constructor.
class Sub extends CustomEvent {
  constructor(name) {
    super(name);
    console.log("constructor called");
  }
  myMethod() {
    console.log("myMethod() called");
  }
}

let sub = new Sub("x"); // logs constructor called
console.log(sub instanceof Sub); // false
sub.myMethod(); // TypeError: sub.myMethod is not a function

Comment 2

[Peter Van der Beken [:peterv]]

I think that the issue here is that in step 3.2 in https://webidl.spec.whatwg.org/#internally-create-a-new-object-implementing-the-interface prototype in this case is a prototype from within the sandbox, so we create the CustomEvent object and use a CCW as its proto (because we're creating the CustomEvent outside of the sandbox and then wrap it inside it). But the Xray for WebIDL objects ignores that prototype chain and simply returns CustomEvent.prototype.

Comment 3

[Peter Van der Beken [:peterv]]

Attachment: WIP: Bug 1820521 - Instantiated subclass of Xray-wrapped DOM constructor is not an instance of the subclass, but the superclass.

Comment 5

[Rob Wu [:robwu]]

Peter, I see that you've attached a WIP patch but without further activity. What is blocking the patch?

Comment 6

[Peter Van der Beken [:peterv]]

Figuring out if it's safe. It's moving away from how Xrays traditionally work, so it's a bit scary.