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 traversing the prototype chain for Proxy.prototype, since all relevant operations are effectively backed by the underlying target.

Is it possible to test if an arbitrary object is a Proxy?

Answer 1

In my current project I also needed a way of defining if something was already a Proxy, mainly because I didn't want to start a proxy on a proxy. For this I simply added a getter to my handler, which would return true if the requested variable was "__Proxy":

function _observe(obj) {    if (obj.__isProxy === undefined) {      var ret = new Proxy(obj || {}, {        set: (target, key, value) => {          /// act on the change          return true;        },        get: (target, key) => {          if (key !== "__isProxy") {            return target[key];          }            return true;        }      });      return ret;    }      return obj;  }

Might not be the best solution, but I think it's an elegant solution, which also doesn't pop up when serializing.

Answer 2

In Node.js 10 you can use util.types.isProxy.

For example:

const target = {};
const proxy = new Proxy(target, {});
util.types.isProxy(target);  // Returns false
util.types.isProxy(proxy);  // Returns true

Answer 3

Create a new symbol:

let isProxy = Symbol("isProxy")

Inside the get method of your proxy handler you can check if the key is your symbol and then return true:

get(target, key)
{
    if (key === isProxy)
        return true;

    // normal get handler code here
}

You can then check if an object is one of your proxies by using the following code:

if (myObject[isProxy]) ...