Near as I can tell: it's a bit messy having all your js intertwingled with the html. Much like the once-dreaded embedded font tags - for which stylesheets were a wonderful invention. They pulled everything out of the way and kept them in one place , which cleaned things up a lot.
What are the down sides?
Well - lets say there's some magic JS doing something with the Foo element on your page... only there's a bug in it and it needs fixing ASAP. But all I know is which part of the page is being updated - I have no idea where the JS is located. The KS-functionality appears like magic from.... somewhere.
Lets complicate things (like real life) and say I'm new to the project, and the previous developer quit and is now living on a beach somewhere in a non-extradition country... so I can't ask them.
How do I actually *find* the JS that is unobtrusively messing with my html element?
It's ok if there's just one JS file that does a few things, or if you have a JS file per HTML page or some other sensible way of finding out which JS belongs on what page... but if you're coming onto the typical legacy project, you've got dozens of JS files with uncommented JS code for adding, removing, showing, hiding, dragging, dropping elements willy nilly - some of which are no longer used and who have useful, non-unique names like "move_stuff" or "display_items".
Sure, I can grep across the project for the ID of the html-element in question... unless the JS is operating on it via the DOM (eg every second child element of a td from a table that's inside the third div from the left...). But it's a little awkward.
There's also one other problem. Let's say I *don't* happen to know there's any JS operating on this element, and I update the html without even realising there's some accompanying JS to update (remember, I'm new to the team, and the former developer wasn't much for commenting).
Now the html and JS are out of synch. The JS is almost certainly broken, because I've just moved that html element into the second div from the left and nested it inside another table... and I will never know it was supposed to "display_items" then "move_stuff"... until Joe Random User suddenly notices that he can't submit the end-of-year accounting because the submit button no longer magically appears when he's finished filling out and validating the form (or whatever).
Clearly there are some benefits of the JS being tucked away out of sight (it looks prettier, which makes the html easier to maintain) but there are also clearly some disadvantages (it makes the JS itself difficult to maintain - two separate files mean a synchronisation-problem).
So, what's the solution?
As ever in design for computers, there's a balance/tradeoff involved, and your decision will probably involve a sightly different balance to what we do.
Personally, I like keeping the JS alongside the HTML it operates on. Sure, don't have it entangled in the html, but you *can* have your JS near the html that it relates to and still have it take into account the aspects of unobtrusive JS. ie the JS still adds to your HTML elements by using ids and DOM-paths... but the actual JS is either in the same template as the HTML (at top of bottom of the file), or is in a partial that's included into that template - so you always know it's there.
I'm sure it's far from the "perfect" solution... but it works for me, and makes it less likely that I suffer from the two problems outlined above, ie that I:
What solutions do you use?