jsx-newline: support for checking newline at the start and end of children

[burtek]

Fixes #3663

[burtek]

@ljharb this makes sense but I wonder if we shouldn't also include option for banning start/end newlines while forcing empty lines between children 🤔

So this might actually become an enum of force | prevent rather than boolean. What do you think, what should I go with?

[burtek]

Or we might go with:

undefined and false = do nothing
true = behaves same as between children (as per prevent)
force = forces an empty line
prevent = prevents empty line

And it will still take allowMultilines into account

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.74%. Comparing base (3730edb) to head (1b3be40).
⚠️ Report is 205 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3678      +/-   ##
==========================================
+ Coverage   97.65%   97.74%   +0.08%     
==========================================
  Files         132      132              
  Lines        9397     9405       +8     
  Branches     3445     3446       +1     
==========================================
+ Hits         9177     9193      +16     
+ Misses        220      212       -8     

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[ljharb]

prevent: true or false obv need to continue working.

I like the idea of effectively deprecating the boolean config, and instead allowing an enum of 'ignore' (false), 'siblings' (true), 'adjacent' (lexically adjacent, ie everything) - that way if someone wants different options we can easily add them later, but for now we're only adding adjacent to cover the likely reading of the existing docs. Thoughts?

[burtek]

Kinda feel like deprecating (while still supporting) old config, but making new config be:

{
    between: 'force' | 'force-one' | 'prevent' | 'ignore' // between children
    adjecent: 'force' | 'force-one' | 'prevent' | 'ignore' // before first child and after last child
}

force - must be at least 1 empty line
force-one - must be exactly 1 empty line
prevent - must be 0 empty lines
ignore - ignore case

Obviously if both are ignore then rule does nothing.

Those properties might probably be called something else (better?).

[ljharb]

"between" seems weird for a parent/child relationship. Are you just trying to think of possible combinations, or do you have use cases?

A linter saying "adjacent" usually means lexically, not like "siblings" in terms of the AST graph.

[burtek]

So I want to distinguish all four options I saw in different projects:

<parent>
    <childA />
    <childB />
</parent>

<parent>
    <childA />
    
    <childB />
</parent>

<parent>

    <childA />
    
    <childB />
    
</parent>

<parent>

    <childA />
    <childB />
    
</parent>

Easiest way would be allow configuring the rule for newlines between siblings separately from newlines before first and after last sibling. between and adjecent are names that came to my mind, but as non-native speaker I'm not sure what those names should actually be.

And the choice between force, force-one, prevent and ignore is just all the possible options that came to my mind, based on current config scheme

[ljharb]

OK, let's aim for those 4 options, with an eye for future extensibility.

That means two keys, one for siblings and one for directly inside of a parent, each of which a "must have a newline" value, and a "must not have a newline" value. I suppose we could also have "ignore", too.

What about betweenSiblings and aroundChildren?

(allowMultilines would apply to all permutations)

[burtek]

Sounds good