[greasemonkey-users] extending built-in objects

[T Z]

I'm trying to extend the built-in "URL" object and add a custom getter:

    class Foo extends URL {
        constructor(s) {
            super(s);
        }

        get a() { return 1; }
        set a(x) { return; }
    }

    let f = new Foo("http://abc.com";);
    console.log(f.a);
    console.log(f instanceof URL);
    console.log(f instanceof Foo);

The output is:
    undefined
    true
    false <----- why!?

The code works fine if pasted into the console window (it prints 1, true, 
true meaning it successfully extended the URL object).

It also works fine to extend *other *objects. For example,

    class Foo extends Date {
        constructor(s) {
            super(s);
        }

        get a() { return 1; }
        set a(x) { return; }
    }

    let f = new Foo("http://abc.com";);
    console.log(f.a);
    console.log(f instanceof Date);
    console.log(f instanceof Foo);

prints
    1
    true
    true

What is going on? Why can I extend Date but not URL? Why does it work in a 
console but not in a user.js script? I'm particularly puzzled that using 
the new keyword on the Foo class creates an object for which instanceof Foo 
returns false.

Note that a gross work-around is to not extend URL at all but to just have 
Foo's constructor create an instance property which is a URL. This works:
    class Foo {
        constructor(s) {
            this.url = new URL(s);
        }

        get a() { return 1; }
        set a(x) { return; }
    }

    let f = new Foo("http://abc.com";);
    console.log(f.url.hostname);
    console.log(f.a);

-- 
You received this message because you are subscribed to the Google Groups 
"greasemonkey-users" group.
To unsubscribe from this group and stop receiving emails from it, send an email 
to greasemonkey-users+unsubscr...@googlegroups.com.
To view this discussion on the web visit 
https://groups.google.com/d/msgid/greasemonkey-users/511ba8e3-f9bf-4c7e-9d1a-79cd79748dcf%40googlegroups.com.