JavaScript doesn’t actually see a web page in the form of text-based HTML, but rather as a tree structure known as the DOM, or Document Object Model. The order of HTML element attributes in the DOM is not defined (in fact, as Svend comments, they’re not even part of the DOM), so the idea of sorting them at the point where JavaScript runs is irrelevant.
I can only guess what you’re trying to achieve. If you’re trying to do this to improve JavaScript/page performance, most HTML document renderers already presumably put a lot of effort into optimising attribute access, so there’s little to be gained there.
If you’re trying to order attributes to make gzip compression of pages more effective as they’re sent over the wire, understand that JavaScript runs after that point in time. Instead, you may want to look at things that run server-side instead, though it’s probably more trouble than it’s worth.