What are the differences and/or advantages, if any, of using commas when declaring a group of variables rather than semicolons.
var foo = 'bar', bar = 'foo';
var foo = 'bar';
var bar = 'foo';
No performance benefit, just a matter of personal choice and style.
The first version is just more succinct.
In terms of the amount of data going over the wire, of course less is better, however you would need a hell of a lot of removed
var declarations in order to see a real impact.
Minification has been mentioned as something that the first example will help with for better minification, however, as Daniel Vassallo points out in the comments, a good minifier will automatically do that for you anyways, so in that respect no impact whatsoever.