Allow trailing comma in argument list

[back2dos]

While it's not particularly important, I would think it would be rather comfortable to allow trailing commas in argument lists. It's allowed for array and object literals, so I think it would be rather consistent and I don't see it causing any problems.

[Simn]

Is this allowed in ECMA?

[back2dos]

Nope. But ECMA and Haxe have rather different approaches to optionality of delimiters anyway ;)
Trailing commas in arrays and objects are not allowed in ECMA either.

But I do quite appreciate them, because if you have this:

[
    "foo",
    "bar",
    "baz",
]

You can comment out/transpose any entries without breaking syntax.

[rockswang]

Agreed, the trailing comma in array definition is valid in Java & Haxe.
If it's proved there's no any side-effect I'd like the similiar tolerance also apply to function argument list.

[ncannasse]

I'm not sure about the potential side effects, since we're in a ( ) block instead of a { } , I guess this have more chances of causing some issues such as letting pass some code while the user didn't meant it. I vote that we don't touch this now.

[back2dos]

After the parser encounters a comma in the argument list, it expects an expression. If the next token is a closing parenthesis instead, it could just act as though the comma had not existed. There is simply no room for unintended side effects.

[Aurel300]

I would like to see this, both in the argument lists and function calls.

function a(
  arg1:X,
  arg2:Y,
):Z { ... }

a(
  arg1,
  arg2,
);

It is just very convenient to be able to reorder or easily add/delete lines in examples like the ones above. It's also nice when you want to enable/disable arguments to functions with conditional compilation. And finally it would be consistent with our array and map literals now.

(Just realised I'm basically restating @back2dos 's points x)

[Simn]

I kinda hate how that trailing comma looks, but I acknowledge that it's very useful for copy & pasting.

[dpomier]

Could the code editor automatically add/remove the trailing comma when commenting out/transposing? It would reconcile nice-looking comma with very easy manipulations.

[back2dos]

Closing in favor of #10159