Attempt to describe a self-hosted version of WindowProxy

WindowProxy.js

// Reusing parts of https://gist.github.com/4279162

function WindowProxyHandler(target){
    this.target = target;
}

WindowProxyHandler.prototype = {
    get: function (_, name) {
        return this.target[name];
    },
    set: function (_, name, value) {
        return this.target[name] = value;
    },
    getOwnPropertyDescriptor: function (_, name) {
        return Object.getOwnPropertyDescriptor(this.target, name);
    },
    defineProperty: function(_, name, desc){
        if(desc.configurable === false)
            throw new TypeError('Nope');
        else
            return Object.defineProperty(this.target, name, desc);
    },
    isExtensible: function () {
        return true;
    },
    isSealed: function () {
        return false;
    },
    isFrozen: function () {
        return false;
    },
    preventExtensions: function(){
        throw new TypeError('Nope');
    },
    freeze: function(){
        throw new TypeError('Nope');
    },
    seal: function(){
        throw new TypeError('Nope');
    }
};   

function retargetableProxy(target, dummyTarget) {
    var handler = new WindowProxyHandler(target);
    
    return {
        proxy: new Proxy(dummyTarget, handler),
        setTarget: function(newTarget){
            handler.target = newTarget;
        }
    };
}


function Window(){/* create window instances */}

var dummyWindowProxyTarget = {
    get location(){},
    set location(url){},
    get document(){}
    // ...
};

function WindowProxy(){
    var {proxy, setTarget} = retargetableProxy(new Window(), dummyWindowProxyTarget);
    
    return proxy;
}