Bump: Add --ext mjs to default file extensions

[cookiengineer]

This is the same issue as #9592 that was closed due to old age (and that the TSC, at the time, decided against its consideration).

1. Now we have 2020, and Browser Modules have shipped in all stable Browser channels, see https://caniuse.com/#feat=es6-module

2. All Browsers (meaning Firefox, Chromium, Edgium, Safari 12+ and all downstream forks, including WebKitGTK / QTWebKit) accept the .mjs ESM module file extension by default and expressively check against the application/javascript header for that specific file extension when being used as <script type="module"> and it can be assumed that this is the defacto standard file extension for ESM modules now.

3. Node.js removed the --experimental flag in its stable release channel.

4. The IETF is working on an RFC to include mjs as the official file extension in the following Draft: https://tools.ietf.org/html/draft-ietf-dispatch-javascript-mjs-05

5. The Python language adapted mjs as the file extension for ESM modules: https://bugs.python.org/issue31715

6. Golang adapted mjs as the file extension for ESM modules: net/http: add built-in mime type for *.mjs golang/go#30547

Given the circumstances and the state that ESM modules are in, right now, and that mjs as a file extension was already pushed out on stable channels of above mentioned implementations - and considering the IETF effort that is put this into an RFC - I think it is safe to say that mjs will be the official file extension soon.

I hope that the file extension mjs can now be reconsidered to be included in the default file extensions that are linted by eslint.

Paste the command you used to run ESLint:

eslint --ext js,mjs ./*; # mjs should not be necessary

What did you expect to happen?

eslint ./*; # should behave the same way as above

Are you willing to submit a pull request to fix this bug?

Yes, of course.

[anikethsaha]

Thanks for the issue.

if I am not wrong, this might need sourceType : module by default then, 🤔 ?

[cookiengineer]

@anikethsaha Yep, mjs are the extension that forces node and the Browser into using ESM import syntax, whereas js files cannot use import and neither export.

[anikethsaha]

yes, it needs ecmaVersion to 2015 by default as well. (breaking)

This might need RFC

let's hear from others.

[eslint-deprecated]

Unfortunately, it looks like there wasn't enough interest from the team
or community to implement this change. While we wish we'd be able to
accommodate everyone's requests, we do need to prioritize. We've found
that issues failing to reach accepted status after 21 days tend to
never be accepted, and as such, we close those issues.
This doesn't mean the idea isn't interesting or useful, just that it's
not something the team can commit to.

Thanks for contributing to ESLint and we appreciate your understanding.