Add func-style ESLint rule

[ehmicky]

This adds the func-style ESLint rule.

[ehmicky]

Those functions can be safely moved to the top-level scope.

[erezrokah]

Should we enable https://eslint.org/docs/rules/func-style#allowarrowfunctions?

[ehmicky]

That option is always enabled when the rule is set to expression (the default), i.e. this PR allows both styles: const myFunc = function(...){...} and const myFunc = (...) => {...}).

Both styles are almost the same and work IMHO. However, do you think it would it be useful for consistency to enforce one of them over the other?

If so, which ESLint rule would be the best to enforce this? I have struggled with this specific linting problem in my personal projects, but there might be a solution. I have even opened an issue to ESLint repository in April about this :)
However I have tried to enforce the first style (no arrow function). Maybe the second style (arrow function) is easier to enforce. no-restricted-syntax might do the trick.

[erezrokah]

That option is always enabled when the rule is set to expression (the default),

Cool, didn't notice that.

I would also prefer to choose one of the two styles. Don't think it is a blocker for this PR (especially if we don't have built in support for it).

[ehmicky]

Rebased. Ready for another review 👍

It looks like there might be a way to enforce the function declaration style using no-restricted-syntax, but it might be better to experiment this in another PR.

[ehmicky]

This is enforceable with no-restricted-syntax. However, enforcing arrow functions would also require writing:

module.exports = {
  onBuild() {...}
}

As:

module.exports = {
  onBuild: () => {...}
}

Which might be more verbose?

Also, this would not come with autofixing, so I am thinking this specific rule might be more of a nuisance than a help there, and leaving inconsistent style might be more practical. What do you think?

[erezrokah]

Also, this would not come with autofixing, so I am thinking this specific rule might be more of a nuisance than a help there, and leaving inconsistent style might be more practical. What do you think?

Agreed