Output a warning if React version is missing in settings

[alexzherdev]

Resolves #1789

This message will appear when running some of the tests. Not sure if that should be avoided.

[ljharb]

Thanks, this seems great.

[ljharb]

i think we can use // eslint-disable-next-line no-console here

[kud]

Hello.

I've got a little problem with this PR to be frank since I've got it.

Apparently this PR was done because of this #1789 which is in a way really cool.

However, I'm in the opposite situation and I try to keep react always updated (or at least often).

With this PR, it forces me to keep my .eslintrc.js always synchronised with my react version. And I'm pretty sure my team or I will forget to update it in the same time that we update our React version.

The question is so, is it possible in fact to react the version of the react installed via package.json instead of having the need to specify the version in our eslint config file?

[kud]

Another idea is to add "latest" as react version. :D

[ljharb]

@kud i could see adding a separate option for "detect" (using the "version" as a fallback), which did require('react').version - want to open an issue or PR that adds that?

[ljharb]

(Note that it'd have to use process.cwd(), and the resolve package, to get the proper require path)

[alexzherdev]

I think I got the "detect" option to work. I found something I didn't expect though: when extending airbnb's config (as opposed to the recommended config from the plugin), there is a version there in context.settings even if you don't set it in .eslintrc. I believe it comes from here /~https://github.com/airbnb/javascript/blob/02b4eea3452b0ae6da666b01f9da23f26148f5b6/packages/eslint-config-airbnb/rules/react.js#L464
Is this intended? I found this surprising and non-obvious 🤔

[ljharb]

Yes, absolutely it's intended.

[alexzherdev]

Oh. Is this just the version used at Airbnb?
@ljharb you mentioned elsewhere you would make not specifying the version throw. But then (correct me if I'm wrong) if most of plugin's users use it via extending airbnb, they would never observe that because of this implicit (for them) setting.

[ljharb]

Yes, that is true. However, with that behavior, the airbnb config would probably remove that setting.

With "detect", it'd probably use that, though.