Say I have a fairly hefty JavaScript file, packed down to roughly 100kb or so. By file I mean it’s an external file that would be linked in via
The best place for it is just before you need it and no sooner.
Also, depending on your users' physical location, using a service like Amazon's S3 service may help users download it from a server physically closer to them than your server.
Is your js script a commonly used lib like jQuery or prototype? If so, there are a number of companies, like Google and Yahoo, that have tools to provide these files for you on a distributed network.