Executing an array returns weird object.

[KonradLinkowski]

Code like this:

const { VM } = require('vm2')
const vm = new VM()
const res = vm.run('[1, 2, 3]')
console.log(res)

Prints:
[ 1, 2, 3, '0': 1, '1': 2, '2': 3 ]
Is this a bug or an expected behaviour?

The NodeJS's build in VM returns:
[1, 2, 3]
as expected.

[alvarperezwefox]

Hi @patriksimek, I quote you as main maintainer. I've this issue too. This happens with VM and with NodeVM.

In my case, inside the NodeVM i have a snippet that -among other things- does this:

const array = [];
array.push({a: 1, b: 2});
console.log(array);

And prints:

[{a: 1, b: 2}, '0': {a: 1, b: 2}]

The console in the NodeVM setup is set to inherit.

Do you have any clue of what could generate this behavior?

Thank you in advance and sorry to bother.

[KonradLinkowski]

This object turned out to be a Proxy. I believe that somewhere in Decontextify.instance a wrong object is returned.

[OleksiyRudenko]

Produces expected output under node v8.9.3 and node v8.10.0.
The issue is confirmed under node v10.14.2

[bugs181]

Ran into this same problem. Frustrating tracking this bug down, thinking it was in my own code. Eventually decided to try a very minimal example and lo and behold, vm2 was the culprit. It appears the for of loop also does some odd things with arrays.

[KonradLinkowski]

Produces expected output under node v8.9.3 and node v8.10.0.
The issue is confirmed under node v10.14.2

What I can propose is to run a debugger in v.8.10.0 and v10.14.2 and find a line of code where the output is different.

[XmiliaH]

This was caused by using a Proxy.handler.ownKeys() trap. This was an issue in node and seems to be fixed in v12.

[KonradLinkowski]

Do you plan any fallback for older versions of node?

[XmiliaH]

The problem is only on node 10. We could do a fix, but that would require to check the caller of a function every time and to get the caller we would need to generate a stack trace. I think that it is not worth doing this.

[bugs181]

The problem is only on node 10. We could do a fix, but that would require to check the caller of a function every time and to get the caller we would need to generate a stack trace. I think that it is not worth doing this.

What about a config or CLI parameter check instead? The code for that could be pretty minimal.

[mr-shabani]

I add console to vm._context
vm._context.console = console
vm.run("console.log([1,2,3])")
it's working correctly!

[XmiliaH]

This is back for all versions as the fix broke Object.keys. As this is only a issue when logging objects this is acceptable.

[bakso]

This was caused by using a Proxy.handler.ownKeys() trap. This was an issue in node and seems to be fixed in v12.

still same now...

[BridgeAR]

The issue is actually a V8 bug. See nodejs/node#41714 (comment)

[fsuk]

A work arround for converting the proxy object to a js object (a bit hacky):

value = JSON.parse(JSON.stringify(proxy))