Everyone else has committed good comments, but there are a few other considerations to make.
Sometimes the javascript will be hosted on a different domain, and be prone to timeout.
Sometimes that domain may become inacessible, while your site remains accessible. Its not good to have your site completely stack itself in this scenario.
For this reason, "blocking" scripts ( ie: document write inline ) like that present in google's tracker, should be avoided, or at very least, should go as late in the page as possible so the page renders whether or not the domain is timing out requests or not.
If you happen to be serving JS from a broken/malicious server, by intent or by accident, one can halt page rendering simply by having a script that serves that javascript which just calls "sleep(forever)" once its sent all the headers.
Some People Use NoScript
Like the above problem, sometimes the clients environment may block certain script sources, be it the users choosing, or other reasons ( ie: browser security satisfactions, odd antivirus/anti-malware apps ). The most popular and controllable instance of this is NoScript, and I myself paranoidly block some of the popular tracking/advertising services with it ( some proxy servers will do this too ).
However, if a site is not well designed, the failing of one script to load still executes code that was dependant on that script being present, which yeilds errors and stops everything working.
My recommendation is :
- Use Firebug
- Use NoScript and block out everything --> See Site still works
- Enable core site scripts that you cant' do without for anything --> See site still works and firebug doesn't whine.
- Enable 3rd party stuff --> See site still works and firebug doesn't whine.
There are a lot of other complications that can crop up, but satisfying the above 2 should solve most of them. Just assume that, for whatever reason, one or more resources that comprise a page are viable to spontaneously disappear ( they do, all the time ), and you want the page to "survive" this problem as amicably as possible. For the problems that may persist for < 10 seconds, its not so bad, refresh the page and its fixed, but if its a problem that can occur, and severley hamper usability for an hour or more at a time.
In essence, instead of thinking "oh, theres the edge case users that don't have javascript", try thinking more a long the lines of "its really easy to have something go wrong, and have ALL of our users with broken javascript. Ouch! Lets try make it so we dont' really hose ourself when that does happen"
( I've seen IE updates get rolled out and hose javascript for that entire browser until the people whom wrote the scripts find a workaround. Losing all your IE customers is not a good thing )
:set sarcasm
:set ignoreSpelling
:set iq=76
Don't worry, its only a 5% Niché Market
Nobody cares about targeting Niché markets right? All those funny propeller heads running lynx in their geeky stupid linoox cpus, spending all their time on the intarwebs surfing because they have nothing better to do with their life or money? the crazy security paranoid nerds disabling javascript left and right because they don't like it?
Nobody wants them as your primary customer now do they?
Niché markets. Pfft. Who cares!
:set nosarcasm