Get locally instanced ‘this’ for native code prototypes [duplicate]

I’m trying to extend a built-in javascript function via prototype, but when I attempt to refer to this, Window is returned. Given that I don’t really have access to rewrite the implementation, are there any ways to get around this?

Specifically I’m trying to extend URLSearchParams in such a way that it unsets values properly. Something like:

URLSearchParams.prototype.setOrUnset = (key, val) => {
    if(val){
        this.set(key, val)
    } else {
        this.delete(key)
    }
}