bug#8576: 23.2; js-mode doesn't support multi-line variable declarations

[Stefan Monnier <monnier@IRO.UMontreal.CA>]

> const a = 5,
>     b = 6;

Yup, that'd be a bug.

>>>> The only solution I can think of would be to go back up after
>>>> typing a closing brace followed by a comma and indent the
>>>> preceeding lines.
>>>> Some modes do that, but it would require drastic changes to js.el.
>>>> I'd rather get this patch in first and see if the issue annoys me
>>>> (or anyone else) enough to work further on this.
>>> I don't think this will be much of a problem.
>> Probably not, but I fear it's going to be a lot of work - at least
>> when I do it.
> I meant that nobody will care, most likely. But the implementation
> shouldn't be too hard either - add a function to `post-command-hook'
> which would check if the command is `self-insert-command', check if we
> inserted a comma, skip backward to the previous non-whitespace char, see
> if it's } or ], etc.

Rather than a post-command-hook, that would be a post-self-insert-hook.
And that would need to be conditional on some js-electric-foo variable
(or better yet, be made to work with electric-indent-mode).

Note that looking forward during indentation, while occasionally
annoying, is not that big of a problem in practice: contrary to popular
belief, we don't write code quite as linearly as one might think.
We at least as frequently edit code in place.

BTW, rather than a post-self-insert-hook, you could put a special
text-property `js--indent-depends-on-next-line' on the line, and then
when indenting a line, you could check if the previous line has that
property and if so indent both lines.  I'm not claiming it's a better
approach, just an alternative one.


        Stefan