Question

Can I do the following to shave milliseconds of loading for some users?

Fix a suitable version x of jQuery, say x = 1.4.1, and find the most recent version y of jQuery cached by the user. Then,

if (y > x === true) {
     /* Use the cached version y of jQuery */
} else {
     /* Download and use the latest version of jQuery */
}

Also, would this be advantageous?

Answer 1

Caching doesn’t work this way. You won’t know whether or not jQuery is cached in the browser, because it won’t be present in the DOM until you include a script reference to it. If that script reference happens to be to a URL that’s previously been cached in that browser (your odds of this will improve if you’re using a public CDN), then you get the benefits of caching for free.

In short, the only thing you can really do here to optimize “cache hits” is to use a publicly hosted copy of jQuery, like one of these.