New Rule Proposal: Enforce indentation of ternary expressions on multiple lines

Please describe what the rule should do:

Enforce predictable indentation of ternary expression alternatives on separate lines.

var location = env.development
  ? 'localhost'
  : 'www.api.com'

var location = env.development
    ? 'localhost'
    : 'www.api.com'

Neither multiline-ternary nor operator-linebreak enforce such indentation.

What category of rule is this? (place an “X” next to just one item)

[X] Enforces code style [ ] Warns about a potential error [ ] Suggests an alternate way of doing something [ ] Other (please specify:)

Provide 2-3 code examples that this rule will warn about:

Less than expected indentation:

var location = env.development
? 'localhost'
: 'www.api.com'

Irregular indentation:

var location = env.development
   ? 'localhost'
       : 'www.api.com'

More than expected indentation:

var location = env.development
              ? 'localhost'
              : 'www.api.com'

Why should this rule be included in ESLint (instead of a plugin)?

The rule would fill a gap in core’s enforcement of fixed and predictable indentation.

---

Related: feross/eslint-config-standard#46

cc @feross @dcousens

[not-an-aardvark]

Thanks for the suggestion. I think we should check for this, but in my opinion it would be better if the existing indent rule was in charge of it (either by default or with an opt-in option), rather than having a separate rule just for ternary expressions. I would say not checking ternary expressions is a bug in the indent rule, although we would have to make sure that we don’t cause too large an ecosystem impact by breaking everyone’s builds with a bugfix.

(For what it’s worth, there’s a rewrite of the indent rule in progress, which checks ternary expressions by default along with some other cases that aren’t currently checked.)

[platinumazure]

Agreed with @not-an-aardvark, this absolutely belongs in “indent”.