Not really possible to use global symbols inside vm

[mitar]

So in this issue it is suggested in this comment to pass global variables to the vm context if you want them shared:

vm.runInNewContext("a instanceof Array", {a:[], Array:Array}) === true
vm.runInNewContext("a.constructor === Array", {a:[], Array:Array}) === true

But then arrays created inside fail:

vm.runInNewContext("[] instanceof Array", {a:[], Array:Array}) === false
vm.runInNewContext("[].constructor === Array", {a:[], Array:Array}) === false

So how can one share symbols between vm and outside context, fully? I would like that Array (and Error, buffers and others) all behave in the same way and that I can pass objects from inside to outside without issues.

[bnoordhuis]

That's really more of a V8 question than a node.js question but the reason is efficiency. It's also a fundamental limitation that won't change.

[].__proto__.constructor is the context's native Array function, not the one you passed in. You can override it like this:

vm.runInNewContext(
    'var a = []; a.__proto__ = new Array(); a instanceof Array',
    { Array });  // true

[mitar]

Hm, but there is no way to make outside Array be proto for all arrays created inside?

[bnoordhuis]

No. Array literals are always instances of the native Array constructor. Same with object literals.

[TimothyGu]

This is one reason why Array.isArray exists:

> vm.runInNewContext("Array.isArray([])", { Array })
true
> vm.runInNewContext("Array.isArray([])")
true
> vm.runInNewContext("Array.isArray(a)", { a: [], Array })
true
> vm.runInNewContext("Array.isArray(a)", { a: [] })
true

[mitar]

Yes, but Error.isError does not? And checking for ArrayBuffer?

Also, if I package code which does not expect to be run inside vm, then they do things like using instanceof and it does not work.

[bnoordhuis]

As a rule of thumb you should obtain references to globals from the other context and operate on those:

const cx = vm.createContext();
const { Error: ThatError } = vm.runInContext('this', cx);
const e = vm.runInContext('new Error("boom")', cx);
console.log(e instanceof Error);  // false
console.log(e instanceof ThatError);  // true

[mitar]

vm.runInContext('this', cx) is what did the trick. I tried using cx directly, but it did not yet have all the symbols. Thanks.

But things are still tricky. For example:

const context = vm.createContext({Buffer: Buffer});
vm.runInContext('new Buffer(new ArrayBuffer(10))', context);

Throws an error:

TypeError: First argument must be a string, Buffer, ArrayBuffer, Array, or array-like object.

[bnoordhuis]

You probably need to upgrade your node, that was fixed a while ago.

[mitar]

I used v6.3.0. And you are right, upgrading to v6.10.0 solved the problem. Thanks.