javascript - 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;
}