Proxy bug?

[tylerlong]

const util = require('util')

const a = { b: 1 }

const p = new Proxy(a, { get: (target, prop, receiver) => {
  if(prop === util.inspect.custom) {
    return () => 'Hello world'
  }
  return target[prop]
}})

console.log(p)

Code above will output Hello world in node 10.x

It will output { b: 1 } in node 13.x

I am not sure this is by design a breaking change or a bug.

[tylerlong]

I've found something: #26241

[devsnek]

(wrong button)

I recently ran into this problem too. For people actually using proxies to do useful things, this is really annoying. I ended up putting a custom inspect function on the proxy target, but that is not always possible if you don't own the proxy target.

[Trott]

@BridgeAR

[BridgeAR]

This is working as intended. Inspecting something should be side effect free. It was "by chance" that the specific trap was triggered with that key in Node.js 10.x. Browsers do not trigger any traps either and it caused all kinds of difficulties for people inspecting proxies.

Using the proxy target is exactly how it should be done in such cases. That way it is always explicit and does not rely on a implementation detail.

[BridgeAR]

@tylerlong you can achieve what you want by writing:

const util = require('util')

const a = { b: 1 }

const p = new Proxy(a, { get: (target, prop, receiver) => {
  // Do things
  return target[prop]
}})
p[util.inspect.custom] = () => 'Hello world!'

console.log(p)

[BridgeAR]

@tylerlong @devsnek I hope the example above helps :) if there are any other use cases that you want to look into, please let me know.

[devsnek]

I realize this was an intentional design choice, but it's very annoying if your proxy is being used to say, mock language features. I know that my proxies are not doing things that would be considered side effects to the consumers of them, and yet when they log them they see the wrong thing. Perhaps we can have a way to tag proxies or something?

[bnoordhuis]

Perhaps we can have a way to tag proxies or something?

That reintroduces the problem that buggy proxies cause all kinds of hard to debug issues.