class Foo{
    constructor(x, y){
        this.x = x;
        this.y = y;
    }
}

function Bar(x, y){
    return new Proxy(new Foo(x, y), {
        get: function(o, p, r){
            console.log(r); // <--
            return o[p];
        }
    });
}

class Buz extends Bar{
    zoo(){
        return "zoo!";
    }
}

var b = new Buz(2,3);
b.x; // <--