Jest globals differ from Node globals

[thomashuston]

Do you want to request a feature or report a bug?
Bug

What is the current behavior?
After making a request with Node's http package, checking if one of the response headers is an instanceof Array fails because the Array class used inside http seems to differ from the one available in Jest's VM.

I specifically came across this when trying to use node-fetch in Jest to verify that cookies are set on particular HTTP responses. The set-cookie header hits this condition and fails to pass in Jest https://github.com/bitinn/node-fetch/blob/master/lib/headers.js#L38

This sounds like the same behavior reported in #2048; re-opening per our discussion there.

If the current behavior is a bug, please provide the steps to reproduce and either a repl.it demo through https://repl.it/languages/jest or a minimal repository on GitHub that we can yarn install and yarn test.
https://github.com/thomas-huston-zocdoc/jest-fetch-array-bug

What is the expected behavior?
The global Array class instance in Jest should match that of Node's packages so type checks behave as expected.

I've submitted a PR to node-fetch switching from instanceof Array to Array.isArray to address the immediate issue, but the Jest behavior still seems unexpected and it took quite a while to track down.

Please provide your exact Jest configuration and mention your Jest, node, yarn/npm version and operating system.
I am using the default Jest configuration (I have not changed any settings in my package.json).
Jest - 18.1.0
Node - 6.9.1 (also tested in 4.7.0 and saw the same error)
npm - 3.10.8
OS - Mac OS X 10.11.6

[suchipi]

This is likely due to the behavior of vm; see nodejs/node-v0.x-archive#1277

Does Jest do anything to try to avoid this right now?

[PlasmaPower]

I came across the exact scenario. It's very hard to diagnose. I also came across it with Error objects. Writing wrapper workarounds for this is getting annoying.

From the linked nodejs issue:

Yes, Array.isArray() is the best way to test if something is an array.

However, Error.isError is not a function.

[suchipi]

Jest team- should the node (and maybe jsdom) environment(s) be changed to put things like Error, Array, etc from the running context into the vm context? I believe that would solve this issue.

Alternatively, maybe babel-jest could transform instanceof calls against global bindings such that they work across contexts.

[PlasmaPower]

I don't like the babel-jest idea, if something like that is implemented it should be its own plugin. Other than that, I agree.

[cpojer]

We can't pull in the data structures from the parent context because we want to sandbox every test. If you guys could enumerate the places where these foreign objects are coming from, we can wrap those places and emit the correct instances. For example, if setTimeout throws an error, then we can wrap that and re-throw with an Error from the vm context.

[suchipi]

Is there any risk to the sandboxing added other than "if someone messes with these objects directly, it will affect other tests"? Or is there something inherent in the way the contexts are set up that would make this dangerous passively? Just trying to understand. I'd guess that instanceof Error checks are more likely than Error.foo = "bar" type stuff.

[cpojer]

It's one of the guarantees of Jest that two tests cannot conflict with each other, so we cannot change it. The question is where you are getting your Error and Arrays from that are causing trouble.

[PlasmaPower]

They come from node native libraries like fs or http.

[cpojer]

Ah, hmm, that's a good point. It works for primitives but not that well for errors or arrays :(

[suchipi]

What if jest transformed instanceof Array and instanceof Error specifically into something like instanceof jest.__parentContextArray and instanceof jest.__parentContextError?

[cpojer]

meh, I'm not sure I love that :(

[suchipi]

We could override Symbol.hasInstance on the globals in the child context to also check their parent context if the first check fails... But Symbol.hasInstance only works in node 6.10.0+ or babel. Can't remember; does jest use babel everywhere by default?

[cpojer]

I'm ok if this feature only works in newer versions of node. It seems much cleaner to me; assuming it doesn't have negative performance implications.