[arrow-parens] Add a "when-brace" option

[nfroidure]

Our team would like to adopt the AirBnb best practice for arrow functions parens but it appears the current options of this rule doesn't fit it. Since we want to rely on automated coding style rules only and found out it would be useful for others too, here is the PR that fulfill it.

See https://github.com/airbnb/javascript#arrows--one-arg-parens
Discussion: SimpliField/eslint-config-simplifield#14

• The version of ESLint you are using

2.11.1
https://github.com/SimpliField/eslint-config-simplifield/blob/master/package.json#L20

• The rule you want to change

'arrow-parens'

• The code you want to be flagged as incorrect

a => { a } // (1)
(a) => a // (2)

• What happens when the rule is applied to the code without your change

with 'as-needed' (1) is valid

with 'always' (2) is valid

[platinumazure]

In order to facilitate acceptance of this issue and pull request, could you please edit your original post to answer the questions raised in the Proposing a Rule Change documentation? Thanks!

[nfroidure]

@platinumazure done, missed that part.

[ilyavolodin]

Since this is part of AirBnB styleguide, I think we should consider adding an option for this. I don't like proposed name, however. Don't really have any ideas on how to name it though. I'll 👍 this

[nfroidure]

Agree on the naming, not convinced by it too but nothing better came to me ;).

[ilyavolodin]

@eslint/eslint-team We need some more +1s here. And suggestion for option name.

[kaicataldo]

👍

[mikesherov]

👍

[IanVS]

How about breaking this up a bit, and adding an option object with keys like one-arg and with-braces? The values for these could be always, or as-needed. I don't love this and I'd prefer to make them booleans, but I'm not sure how to do that in a way that makes sense. Hopefully this comment might spark a better idea.

The upshot would be making this a bit more flexible and still accommodating the AirBnB style mentioned by the OP. ({'one-arg': 'as-needed', 'with-braces': 'always'})

[nzakas]

Do we have a champion for this?

I'd suggest we look at a configuration like:

arrow-parens: [ "error", "as-needed", {
    requireForBlockBody: true
}]

(Generally I think "block" is a more appropriate description, but could also be requireForBracedBody.)

[IanVS]

@nzakas, I think we would also need requireForMultipleArguments, to match AirBnB style, but I like that strategy.

[nzakas]

@IanVS hmmm, I'm not sure I understand. You can't have multiple parameters for an arrow function without parentheses (it's a syntax error). Can you explain a bit more?

[IanVS]

Doh, of course. I was just reading the styleguide way too literally and trying to invert it without thinking about it:

If your function takes a single argument and doesn’t use braces, omit the parentheses.

[kaicataldo]

I can champion this!

[alberto]

👍 to requireForBlockBody

[nzakas]

Okay @kaicataldo, the power is yours!

[kaicataldo]

Thanks! Have we decided that requireForBlockBody is the config option name we want? Sounds good to me, just want to make sure.

[nzakas]

Yup!