Consider disabling JSDOM by default

[nickserv]

Do you want to request a feature or report a bug?
Default change proposal (semver breaking)

What is the current behavior?
Jest's default testEnvironment is jsdom. I understand the intention of Jest is to follow convention over configuration and make things like DOM testing very easy for beginners with minimal configuration, but there is a large overhead to having jsdom enabled by default (often 3 times as slow to boot per test suite in my experience) and I don't think most users even realize this is configuration.

What is the expected behavior?
I think swapping the default to node would be nicer (of course, not without deprecation warnings, semver major breaking changes, and possibly a helpful error message when the DOM is accessed in the Node env). While I tend to disable jsdom because I never need it, I often forget and I'd rather have the default settings optimize for performance in this case. I think this would keep the testEnvironment settings more consistent with Jest's other performance first defaults, like disabling coverage. It's also worth mentioning that many usages of Jest even in web code don't need jsdom, as shown in Jest's own documentation on react-test-renderer.

[anescobar1991]

I think this could also go a long way to shed the perception that Jest is only for testing React components or that it is in any way tied to testing for web code. As surprising as it is this perception does still exist (at least people don't think it still mocks everything by default).

[nickserv]

I understand this is a potentially controversial and breaking change, would it help if I started a PR or should I wait for a discussion with more collaborators?

[rickhanlonii]

The change should be pretty straightforward so I wouldn't recommend submitting it until we decide this is the direction we want to go

I agree that defaulting to node would be better, but I'm not sure when the timing would be right. This will be a breaking change for the majority of Jest users

[SimenB]

@cpojer @aaronabramov @thymikee thoughts on this?

It's easier to google document is not defined than it is to discover your tests are a bit slower than they need to.

We could also potentially special case errors like document is not defined, window is not defined etc and provide a nice error message about setting environment.

[azz]

I wonder if it possible to "detect" whether the code needs JSDOM or not?

• Presence of imports "react", "jquery", ...
• Use of the document or window global identifiers.

You could have the default "testEnvironment": "auto".

Not so sure on the idea myself, but worth considering?

[SimenB]

we'd have to scan the entire dependency tree before we'd be able to start the test then. The dependency tree we have today is built up by looking at the sourcecode of every single file, and extracting require/import with regexes... Not sure if it'd be reliable enough?

[cpojer]

Auto seems too magical. I think we can just switch the default. CRA can keep setting it to jsdom.

We can, however, add the error messages that you pointed out @SimenB. Known DOM globals (document, window, etc.) should trigger the error and ask people to update their config.

Bonus points for a jest --updateConfig feature that will automatically update the config rather than asking people to copy-paste stuff. That of course only works if the config isn't passed as a JSON encoded string to --config. Would be a cool feature, also for the transformers! cc @thymikee

[SimenB]

I like jest --updateConfig, but I think we should encourage the docblock comment if say less than 25% of the tests require jsdom. Maybe not feasible

[cpojer]

yeah makes sense.

[nickserv]

Node is the simplest built in env, right? Out of curiosity is there a third env?

[thymikee]

@nickmccurdy Jest only provides implementations for node and jsdom test environments.

[rickhanlonii]

So in summary:

Jest 23.x

• Add warning for when there's no explicit environment set, "changing default env to node in Jest 24"
• Detect errors on document or window and recommend switching env to jsdom

Jest 24.0.0

• Switch default to node
• Add jest --updateConfig

[SimenB]

Detect errors on document or window and recommend switching env to jsdom

We can do that now - it'd be a pretty useful error regardless of defaults