vm: instanceof test fails when assigning global to vm context

[princejwesley]

• Version: v6.1.0 , (master: v7.0.0-pre)
• Platform: Darwin Princes-MacBook-Pro.local 15.5.0 Darwin Kernel Version 15.5.0: Tue Apr 19 18:36:36 PDT 2016; root:xnu-3248.50.21~8/RELEASE_X86_64 x86_64
• Subsystem: vm

When assigning global variables to vm context, instanceof test for the assigned global variables return false

const vm = require('vm')

let ctx = vm.createContext();

let before = vm.runInContext('[] instanceof Array', ctx);
ctx.Array = Array;
let after = vm.runInContext('[] instanceof Array', ctx);

console.log('before =>', before, 'after =>', after); // before => true after => false

before = vm.runInContext('/vm/ instanceof RegExp', ctx);
ctx.RegExp = RegExp;
after = vm.runInContext('/vm/ instanceof RegExp', ctx);

console.log('before =>', before, 'after =>', after); // before => true after => false

Question

Is this a bug or an expected behaviour?

[cjihrig]

Seems like expected behavior to me. Array references a different thing in each context.

[targos]

Yes, that is expected behavior. You can work around this problem for arrays with Array.isArray that works across contexts.

[rubenstolk]

A helpful workaround is to use a custom isInstanceOf utility method, e.g.:

const isInstanceOf = (o, type) => Object.prototype.toString.call(o) === `[object ${type}]`;

isInstanceOf(new RegExp('hello world', 'g'), 'RegExp'); --> true
isInstanceOf(/hello world/ 'RegExp'); --> true
isInstanceOf([] 'Array'); --> true
isInstanceOf(new Error('Error'), 'Error'); --> true