vm: accessor properties get converted to data properties inside the vm

[domenic]

Example:

'use strict';
const vm = require('vm');

const x = {};
Object.defineProperty(x, 'prop', {
  get() { return 'foo'; }
});
const o = vm.createContext(x);

const code = 'Object.getOwnPropertyDescriptor(this, "prop")';
const res = vm.runInContext(code, o, 'test');

console.log(res);

gives

$ iojs test.js
Object {
  value: 'foo',
  writable: true,
  enumerable: false,
  configurable: false }

In 659dadd I fixed it to return the correct data descriptor. But it appears the accessor descriptor is just broken.

As far as I can tell this is a shortcoming of the V8 API. The named properties handler and the GlobalPropertyQueryCallback always returns data descriptors. This is probably because that is all that is required by Web IDL for the browser use case, where the named property handler is meant to be used for things like window.idOrName or window.forms.foo, which are always data descriptors.

I think Chrome also has this problem. Even for non-named properties, things like window.top are data descriptors. I'm asking related questions on blink-dev.

Original discovery: jsdom/jsdom#1208, where this is affecting Facebook's use in jest.

/cc @nodejs/v8

[nicolashenry]

I have an issue that may be related :

function Window() {
    this.Window = Window;
    this.console = console;
}

var w = new Window();

var vm = require("vm");

vm.createContext(w);

vm.runInContext('console.log(this instanceof Window);', w);

gives

false

[jasnell]

@domenic @ofrobots ... I see this is still occurring v4.9 in master. Any updates on this?

[brettz9]

I don't know about other contexts, but the problem only appears to occur for me when the accessor is on the vm's global.

This code (which adds an intervening parent object to two lines of code) produces the expected result showing the getter in the accessor:

'use strict';
const vm = require('vm');

const x = {};
Object.defineProperty(x, 'prop', {
  get() { return 'foo'; }
});
const o = vm.createContext({parent: x});

const code = 'Object.getOwnPropertyDescriptor(parent, "prop")';
const res = vm.runInContext(code, o, 'test');

console.log(res);

which logs:

{ get: [Function: get],
  set: undefined,
  enumerable: false,
  configurable: false }

[Trott]

Should this remain open? (I'm guessing the answer is "yes" but I'd be happy to find out I'm wrong.)

[TimothyGu]

@Trott Yes, @domenic's error case is still valid.

[TimothyGu]

In fact, we should take another look at this issue now that v8/v8@b0a7738 has been committed.