Use NODE_ENV to detect Jest or Vitest

[cruessler]

What:

Currently, when @emotion/react is used in a Vitest environment that has globals: false (which is Vitest’s default), it emits the following warning when included more than once:

You are loading @emotion/react when it is already loaded. Running multiple instances may cause problems. This can happen if multiple versions are used, or if multiple builds of the same version are used.

This warning is suppressed in Jest environments and in Vitest environments that have globals: true by checking for the presence of a global jest or vi, respectively. This test does not work, though, when globals: false because in that case, there is no global vi. (This was also documented in the PR that extended the check to also take vi into account, but the comment was modified for the final version of the PR that got merged.)

Why:

If I understand the existing code correctly, the warning is supposed to be suppressed irregardless of whether globals: true or globals: false.

How:

This PR changes the test to check for process.env.NODE_ENV === 'test' instead. According to the docs, this is a reliable way to detect both Jest and Vitest.

The PR for Jest that introduced the check in its first form has a comment that voices concerns about accessing process.env when using Webpack as a bundler. Since the code surrounding the check uses process.env.NODE_ENV !== 'production' anyway, I felt it’s safe to use NODE_ENV when checking for a test environment.

Checklist:

I’m not entirely sure which of the checklist’s items apply to this PR and would greatly appreciate any guidance! I ticked off “Documentation” as I added a comment to the line I changed, but I don’t know whether that is sufficient.

• Documentation
• Tests
• Code complete
• Changeset

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