Turn some of the linter messages into errors

[andreypopp]

I think it would be useful to have some of the linter messages to be errors instead of warnings.

For example:

• no-undef (used undefined variable)

Sometimes I don't pay attention to lint warnings and then only see errors when they happen at runtime.

We could also format lint errors in a similar fashion babel loader reports, including code frame with location.

[gaearon]

Sometimes I don't pay attention to lint warnings

Are there any lint warnings in CRA that you don’t pay attention to?
We tried to err on the side of only picking lint warnings that represent actual errors in 95% cases.

[andreypopp]

Are there any lint warnings in CRA that you don’t pay attention to?

Yes, no-unused-vars for example. It is pretty useful but doesn't prevent my code running so I don't bother fixing it while prototyping. Of course I still want it to be reported as I want to fix it before commit, for example.

[gaearon]

Can you compile a list of which warnings you’d rather see as errors?

[andreypopp]

Can you compile a list of which warnings you’d rather see as errors?

Probably just no-undef for now as I validated it through my workflow. Not sure about others for now.

[gaearon]

Send a PR please?

[gaearon]

This should be fixed in 0.4.2.
Please verify!

[andreypopp]

Works well, thanks!

Btw. love how it is easy to upgrade to the new version—just npm install you get new features!

[gaearon]

🍰