handler = {
    get: function(target, property, receiver) {
        let targetObj = target[property];
        if(typeof targetObj == "function") {
          return (...args) => target[property].apply(target,args)
        } else {
            return targetObj;
        }
    }
}
windowProxyObject = new Proxy(window, handler);

Example:
windowProxyObject.document.getElementsByTagName('HTML');