browserslist returns default config for unrecognised environments

[sgomes]

Background

browserslist conveniently allows for keeping different lists, through different environments. These can be defined in any one of the locations that browserslist searches, such as package.json.

They can then be used by doing e.g.:

browserslist(undefined, {env:'my-env'});

Description of the issue

If the provided environment (my-env, in the example above) isn't defined everywhere, browserslist will return the default list of browsers, rather than providing any sort of indication that the list is missing. This can lead to hard-to-find bugs in client code if for some reason the browserslist configuration isn't present, such as not having been copied to a production environment.

How to reproduce

• Run browserslist(undefined, {env:'missing-env'}); without having missing-env defined anywhere
• Observe the response

Expected result

An error or an empty list.

Observed result

The default browserslist configuration. In my particular version:

[
  'and_chr 84',        'chrome 87',
  'chrome 86',         'chrome 85',
  'chrome 84',         'chrome 83',
  'edge 84',           'edge 83',
  'firefox 81',        'firefox 80',
  'firefox 79',        'firefox 78',
  'ios_saf 13.4-13.5', 'ios_saf 13.3',
  'opera 69',          'opera 68',
  'safari 13.1',       'safari 13'
]

Next steps

I'm more than happy to work on a PR to fix this issue, but it would likely be best to reach a consensus first on what the correct behaviour should be:

• The current behaviour
• Throwing an error
• Returning an empty list

Thank you!

[ai]

Yeap. Let’s throw an error in the next major release.

[sgomes]

Thank you for the super quick reply, @ai! 👍

Shall I prepare a PR, or is this something you'd prefer to handle yourself?

[ai]

Yeap, PR will be very helpful.

But be ready, that it could take a few months before the next major update.

It is hard to make major releases of Browserslist, since we need to force all tools to switch to new versions (to avoid the case, when different tools parse Browserslist config differently because of different Browserslist version).

[sgomes]

No worries at all, I'm happy to prepare the PR and have it sit in limbo until it's time for the next major release 🙂

Thanks again, @ai!