RFC: [next/jest] Compile in development mode, and leave NODE_ENV blank

[keyz]

Problem

Currently, process.env.NODE_ENV is set to be production in the next/jest environment. I think the behavior isn't desirable — there are legit cases where we need to assert on DEV-only warnings in tests (for instance, make sure React doesn't fire unique "key" warnings).

Solution

This PR changes the behavior: source code gets compiled in development mode such that React and friends don't omit dev mode warnings. The actual NODE_ENV value is left blank. By default, Jest will set NODE_ENV to "test" (see jestjs/jest@3a38ddf). You can override the behavior via explicitly setting the NODE_ENV environment variable.

Test Plan

• run yarn test-unit: process.env.NODE_ENV === "test"
• run NODE_ENV=development yarn test-unit: process.env.NODE_ENV === "development"

Bug

• Related issues linked using fixes #number
• Integration tests added
• Errors have helpful link attached, see contributing.md

Feature

• Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
• Related issues linked using fixes #number
• Integration tests added
• Documentation added
• Telemetry added. In case of a feature if it's used or not.
• Errors have helpful link attached, see contributing.md

Documentation / Examples

• Make sure the linting passes by running yarn lint

[ijjk]

Hi, this should have been partially fixed by #33032 which is available in v12.0.8-canary.18 of Next.js.

That PR changed to not apply the NODE_ENV or typeof window inlining so the default value set in the environment will be available instead. We could make the development mode transpiling configurable potentially but not sure it should be the default.

[keyz]

Ah, great @ijjk I should've searched before posting the PR :)

Question though. In #33032 you still have development set to true -- how exactly does it affect the compilation for React and styled components?

That said, for my use case, #33032 is probably good enough.

[ijjk]

Yeah potentially if it's more desired to test against the development mode for react and styled-components we could allow it to be set as a dev?: boolean param at the top-level next/jest options.

[keyz]

Makes sense, I'll close this PR for now since I don't really need dev mode compilation (i.e. __self and __source) for React in tests, and I'm not familiar with styled components enough to make a call here. Thank you for the response @ijjk!