This two year old gist from Isaac Schlueter, author of NPM, has been a focal point for this debate. He describes a rationale for putting commas first, beginning with these simple words:
Note how the errors pop out in the comma-first style.
He's absolutely right. And that, right there, is why I think it's a bad style choice.
I want punctuation to disappear when it's not doing something important. Commas that separate array elements or object properties that are already on separate lines are not doing anything important. So putting commas first may save me from a few comma-related errors every day. (Which, of course, JSHINT will catch promptly, along with all the other errors I make every day). But I will spend ten or twelve hours a day, every day, looking at commas instead of variable names or object definitions. I will have to visually parse the punctuation for every single line I read, instead of not thinking about it.
I agree that putting commas first makes errors more apparent. I disagree that an entire aesthetic should be defined by this desire.
There are some arguments made over the course of this two-year-old (and still going strong) gist about the sorts of errors that automated syntax checking tools will not catch, also justifying this. For example, this is valid:
var x = [ ["asdf", "foo", "bar"], ["baz", "blerg", "boof"] ["quux", "antimatter"] ];
The parser will interpret the last element as a property indexer, which is wrong.
Yes. You can come up with of situations where the error introduced by a missing comma would resemble valid code and not be caught by JSHINT. A comma-first syntax would probably have helped you avoid that problem.
I am not sure such a situation has ever come up in practice, though. This is OCD. This is carrying a bottle of antiseptic and spraying every surface before you touch it, because it (maybe) will prevent you from getting a cold once in a while. The cure is much worse than the disease!
Avoiding syntax errors is important. But is it more important than making code that can be read and understood with the least amount of effort? I don't think so. If comma-first adds even 1% to the amount of mental energy I need to read and understand some code, it's not worth it.
One final note. I'm not a purist at all. Use multiple var statements (but only before your first line of functional code, of course!) Use side effects. Don't use semicolons. I think the goal should be to write code that is the most easily understood, and a lot of "non-purist" techniques can be used to create terse code that's more readable than its longform version. I feel like a lot of the rhetoric on this topic has been about a debate over purism. It shouldn't be.
I have a tendency to use semicolons rigorously, but that's because I program in C# half the time and it's hard to get my brain to move easily back and forth between the two mindsets. But I completely understand the rationale often cited for using ASI: semicolons are visual clutter. Yup, they sure are! And I think that same rationale should apply doubly so for putting commas at the beginning of the line instead of the end. At least when punctuation is at the end of a line, our "left to right" brains can easily dismiss it. When it's at the beginning, you can't avoid it.