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 [email protected].
To view this discussion on the web visit
https://groups.google.com/d/msgid/greasemonkey-users/511ba8e3-f9bf-4c7e-9d1a-79cd79748dcf%40googlegroups.com.
