Enforce Trailing Commas by Default

[btnwtn]

I propose that trailing commas should be enabled by default. IE8 is a non issue for most, and this will provide better diffing.

const arr = [
  'multi',
  'line'
]

should be formatted as:

const arr = [
  'multi',
  'line',
]

I would also suggest not enabling it for function arguments, considering that this is not in the official spec yet and won't be fully supported without a transpiler like babel. e.g.:

foo(
  arg1,
  arg2,
)

should be formatted as:

foo(
  arg1,
  arg2
)

[Pomax]

I'll represent the other camp: clean, readable code does not need the trailing commas, as commas are separators, not prefix/postfix operators, and there's nothing following that comma. Injecting commas in this case would be a reason for me to not want to use this formatter.

[leeoniya]

arrays often have line-level similarity and for editors that have a "duplicate line" functionality, a trailing comma at the end can yield better ergonomics.

EDIT: but if this is simply for immutable output, then i'd agree with stripping trailing

/ $0.02

[jlongster]

Right now we at least have the option, so you can turn it on. This is a question of if it should be the default. I'm leaning towards not, because it I get the sense that the majority (by a little margin) doesn't use them, but I can let this sit for a while. You can at least turn them on if you want them.

[bnjmnt4n]

I think they should not be enabled by default, due to the trailing commas in function parameters as well. In fact, that should probably also be made configurable similar to recast:

trailingCommas: {
  array: true,
  object: true, 
  function: false 
}

[jamiebuilds]

I get the sense that the majority (by a little margin) doesn't use them

@jlongster That probably has a lot to do with trailing commas being invalid in a number of places which has since changed. I know Airbnb's code style thing changed afterwards @ljharb

[ljharb]

Trailing function commas recently hit stage 4, and haven't been published in the standard yet (which it will be this June - although "published" is irrelevant, only "hit master on github" matters).

The airbnb config requires trailing commas anywhere they're allowed to appear, to ensure consistency and to minimize git diffs. Whether it's the default or not is certainly your choice, but my hunch is that the majority of the airbnb config is going to be very close to what most users use, either now, or eventually.

[mxstbr]

Related article by @nikgraf: https://medium.com/@nikgraf/why-you-should-enforce-dangling-commas-for-multiline-statements-d034c98e36f8#.ur8s3venv

[okonet]

I'm opposed to that. I have tried both styles and thus better ergonomics in the editor is a valid argument but this only matters until I Finish editing it. Same with the diff — if the algorithm can't handle it, algorithm should be improved and not the source code get altered to please the machine.

The formatting of the code should not be dictated by tools but by readability. Tools should follow what is good for humans and not for robots. A trailing comma has no semantic but represents a visual noise.

[ljharb]

Minimal git diffs are good for humans, and consistency is good for readability.

[potch]

I expect formatted code to run in browsers without any additional processing, and would prefer any feature that breaks that not be on by default. This rules out trailing commas in functions.

[ljharb]

Technically that rules out all forms of trailing commas as well as any unquoted reserved words, or variable names with reserved words - either you're hand coding to ES3, you're breaking browsers, or you're using some additional processing ¯\_(ツ)_/¯

[mattbasta]

@potch trailing commas in lists and objects is supported by everything in recent history (IE9 was the last browser affected, and only in compatibility mode; IE8 was the last browser to wholesale shit the bed on trailing commas).

I think, obviously, trailing commas in arg lists is a bad idea for now. I don't think it's reasonable to make it a default until IE11 is dead and gone and ES2017 has settled in all of the evergreen browsers for a few years.

As for why I support trailing commas, I find it prevents many errors when copying and pasting. It's a quality of life thing. If you use the "shift lines" feature in your text editor, you'll know how much of a hassle it is when you've got a missing comma. In particular, if you always use a trailing comma, you'll never have a problem. This is in contrast to, say, leaving off semicolons where you need to remember a handful of rules for when you have to include them.

[potch]

@mattbasta yeah, can't say I super care strongly about arrays and object literals for trailing commas. It feels like a violation of principle of least surprise to see characters added like that, but I'd rather have consistent formatting in an easy-to-use tool than it hew exactly to my preferences.

Just no syntax errors please! I guess I'd be annoyed if I was intentionally writing ES3 to have it ES5ified, but that's a super niche case and the basic config covers it.

[btnwtn]

@potch how do trailing commas in an array or object break browsers? Are we targeting IE8 here? And if so... please reconsider.