I’ll try to look into this a bit more, but I figured you’d also want to know about it quickly, hence this somewhat incomplete task.

With a strategically placed var_dump in Peast (and also xdebug.max_nesting_level=1024) I can at least get a column number:

$ curl -s 'https://commons.wikimedia.org/wiki/MediaWiki:Gadget-ACDC.js?action=raw' | php maintenance/run.php jsparse -
/home/lucas/git/wikimedia/mediawiki/vendor/mck89/peast/lib/Peast/Syntax/Exception.php:39:
class Peast\Syntax\Position#56635 (3) {
  protected $line =>
  int(20)
  protected $column =>
  int(78254)
  protected $index =>
  int(78830)
}
- ERROR: Peast\Syntax\Exception: Unexpected: function
Failed.

Which is all the way at the end of the file – just after the comma, apparently:

case"end":return t.stop()}}),t)}))),function(e,r){return t.apply(this,arguments)})(mediaWiki,jQuery)}()}();
//                                  ^

Shortest case:

$ echo '(a=1,2)' | php maintenance/run.php jsparse -
- ERROR: Peast\Syntax\Exception: Unexpected: 2
Failed.

Demo that this is valid (if not exactly idiomatic) JS:

$ echo 'function f(){return 1,(a=2,3);}console.log(f())' | php maintenance/run.php jsparse -
- ERROR: Peast\Syntax\Exception: Unexpected: 3
Failed.
$ echo 'function f(){return 1,(a=2,3);}console.log(f())' | node
3

Impact: not critical IMHO. Based on global search for “webpack”, Portuguese Wikipedia’s Gadget-arquivarPEs.js might also be affected, but probably not much else. (I wouldn’t expect anyone to write this syntax pattern by hand.)

Edit: I forgot I can just check for myself. No, that gadget is not affected. ACDC might be the only one.

$ curl -s 'https://pt.wikipedia.org/wiki/MediaWiki:Gadget-arquivarPEs.js?action=raw' | php maintenance/run.php jsparse -
- OK

I’ve worked around it for now by tweaking the code a bit so that webpack seemingly no longer outputs the problematic syntax construct (gadget diff, though it’s pretty useless, MediaWiki can’t make sense of such a huge diff of minified code ^^). So I guess I should go file a Peast issue next…

Hm, apparently Peast starts being able to parse this in ES8:

> Peast\Peast::ES7('(a=1,2)')->parse()

   Peast\Syntax\Exception  Unexpected: 2.

> Peast\Peast::ES8('(a=1,2)')->parse()
= Peast\Syntax\Node\Program {#6254
    +location: Peast\Syntax\SourceLocation {#6322
      +start: Peast\Syntax\Position {#6366},
      +end: Peast\Syntax\Position {#6359},
    },
  }

Was this actually illegal prior to ES8? (It’s possible I didn’t configure webpack correctly and it just never came up so far.)

Was this actually illegal prior to ES8? (It’s possible I didn’t configure webpack correctly and it just never came up so far.)

As far as I can tell, it should have been valid, so I filed Peast issue #65.

Change 989920 had a related patch set uploaded (by Lucas Werkmeister; author: Lucas Werkmeister):

[mediawiki/vendor@master] Bump mck89/peast to 0.16.0

https://gerrit.wikimedia.org/r/989920

Change 989923 had a related patch set uploaded (by Jforrester; author: Jforrester):

[mediawiki/core@master] Bump mck89/peast to 0.16.0

https://gerrit.wikimedia.org/r/989923

Should be fixed with Peast 0.16, rolling out with the train next week.

Change 989920 merged by jenkins-bot:

[mediawiki/vendor@master] Bump mck89/peast to 0.16.0

https://gerrit.wikimedia.org/r/989920

Change 989923 merged by jenkins-bot:

[mediawiki/core@master] Bump mck89/peast to 0.16.0

https://gerrit.wikimedia.org/r/989923