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

Adding 'support' for instanceof Proxy:

I don't recommend it, but If you want to add support for instanceof, you could do the following before instantiating any Proxies:

(() => {
  var proxyInstances = new WeakSet()
  
  // Optionally save the original in global scope:
  originalProxy = Proxy

  Proxy = new Proxy(Proxy, {
    construct(target, args) {
      var newProxy = new originalProxy(...args)
      proxyInstances.add(newProxy)
      return newProxy
    },
    get(obj, prop) {
      if (prop == Symbol.hasInstance) {
        return (instance) => {
          return proxyInstances.has(instance)
        }
      }
      return Reflect.get(...arguments)
    }
  })
})()

// Demo:

var a = new Proxy({}, {})
console.log(a instanceof Proxy) // true
delete a

var a = new originalProxy({}, {})
console.log(a instanceof Proxy) // false
delete a