Document comma-dangle: functions: always-multiline behaviour in promises

1. ESLint Version v3.13.1 (although this is documentation specific, not the linter itself

2. The problem you want to solve. The behaviour of always-multiline comma dangle in promises isn’t clearly documented. As In #7786 @mysticatea described why a trailing comma is forced in some Promise.then situations

3. Your take on the correct solution to problem. Update the documentation for the rule to include a always-multiline section with the rest of the function docs, showing:

-Promise.then(x => x
-  .replace('asdasdsdsda', '123213213')
-  .replace('cxvcxvcxvcx', '546546546')
-  .replace('ewrewrewrew', '877876876'),
-)
+Promise.then(x => (
+  x.replace('asdasdsdsda', '123213213')
+    .replace('cxvcxvcxvcx', '546546546')
+    .replace('ewrewrewrew', '877876876')
+))

Raised this as an issue instead of a PR as I guess it’s a conflict between two rules in the airbnb config, but it might not belong on that page

[not-an-aardvark]

Would a notice about it being es2017 around the functions section make it clearer?

I agree, this seems like it would help a lot. Would you like to make a PR?

Also, keep in mind that the parser configuration (e.g. ecmaVersion: 2016 and the rule configuration (e.g. functions: always-multiline) are independent from each other. The default functions configuration is ignore (i.e. no errors are reported for trailing function commas), but if you’re using a shareable config such as eslint-config-airbnb, it’s possible the config is explicitly enabling functions: always-multiline. To override this, you can use something like

rules: {
  'comma-dangle': ['error', 'always-multiline']
}

[not-an-aardvark]

This is fixed by https://github.com/eslint/eslint/pull/8468.