How to test if an object is a Proxy?

Question

I would like to test if a JavaScript object is a Proxy. The trivial approach

if (obj instanceof Proxy) ...

doesn\'t work here, nor does tra

Answer 1

In fact, there is workaround for determine if object is proxy, which is based on several assumptions. Firstly, Proxy determination can be easily solved for node.js environment via C++ extensions or privileged web-page in browser, when page can launch unsecure extensions. Secondly, Proxy is relative new functionality, so it does not exist in old browsers - so solution works only in modern browsers.

JS engine can't clone functions (Since they have bindings to activation context and some other reasons), but Proxy object by definition consists of wrapper handlers. So to determine if object is proxy, it's enough to initiate force object cloning. In can be done via postMessage function.

If object is Proxy, it will failed to copy even it does not contain any functions. For example, Edge and Chrome produces following errors while try to post Proxy object: [object DOMException]: {code: 25, message: "DataCloneError", name: "DataCloneError"} and Failed to execute 'postMessage' on 'Window': [object Object] could not be cloned..