Change notice behavior for trailing-comma esSpecCompliant

[MatthiasKunnen]

PR Checklist

• Addresses an existing issue: fixes Changing notifice behavior for trailing-comma, esSpecCompliant option #329
• That issue was marked as status: accepting prs

Overview

I've changed the notice behavior of esSpecCompliant property for rule trailing-comma. Previously, a notice would be given when the esSpecCompliant was present. Since eslint only supports esSpecCompliant == true no notice should be given in that instance.

After this change, a notice will be given when esSpecCompliant is either undefined or false.

[JoshuaKGoldberg]

👋 @MatthiasKunnen, thanks for reporting the issue and opening a PR!

I'm a little confused. TSLint's trailing-comma docs states:

To align this rule with the ECMAScript specification that is implemented in modern JavaScript VMs, there is a third option esSpecCompliant. Set this option to true to disallow trailing comma on object and array rest and rest parameters.

...but ESLint's comma-dangle docs look like they prefer allowing the trailing comma.

Per the PR template, would you mind filing an issue to discuss the changes? In general it'd be preferable to discuss whether changes should happen in issues, and save PRs for just the implementation changes.
(I can go ahead and do that if you don't have time to soon)

[MatthiasKunnen]

You're absolutely correct about both implementations.

Both TSLint and ESLint implemented their respective rules in a way that did not deal with rest params. Both changed their rules to deal with rest params but they went separate directions. TSLint decided that old versions of typescript should not be broken as they allowed the trailing comma so they added an option. ESLint, however, decided since a trailing comma after a rest param is a syntax error, a breaking change was okay and they since disallow a comma after a rest param (discussion here eslint/eslint#7297).

TSLint actually had a discussion about removing the esSpecCompliant option and disallowing the trailing comma after a rest param which would result in the same behavior as ESLint (see palantir/tslint#4172). That issue got closed before any action could be taken due to TSLint's deprecation. The issue mentions the following:

esSpecCompliant: true is enabled in all the built-in configs, tslint:all, tslint:recommended, and tslint:latest, which I believe should be sufficient for most users. we can change the behavior of the rule to make esSpecCompliant: true the default behavior in v6.0

We can thus assume that esSpecCompliant will be true in nearly all configs that have the trailing-comma rule. Right now, these configs will receive warnings about ESLint's lack of such a feature. This, however, won't be a problem and for that reason this PR changes the conversion in order to default esSpecCompliant to true and warn when it's specifically set to false.

I hope that this explanation is clear as to the purpose of the PR and the underlying reasons. If not I can make an issue.

[JoshuaKGoldberg]

If not I can make an issue.

Yes please!

[JoshuaKGoldberg]

Excellent. Thanks so much for persevering through my lack of understanding! 💪