How can I force clients to refresh JavaScript files?

As far as I know a common solution is to add a ?<version> to the script's src link.

For instance:

<script type="text/javascript" src="myfile.js?1500"></script>

I assume at this point that there isn't a better way than find-replace to increment these "version numbers" in all of the script tags?

You might have a version control system do that for you? Most version control systems have a way to automatically inject the revision number on check-in for instance.

It would look something like this:

<script type="text/javascript" src="myfile.js?$$REVISION$$"></script>

Of course, there are always better solutions like this one.

Appending the current time to the URL is indeed a common solution. However, you can also manage this at the web server level, if you want to. The server can be configured to send different HTTP headers for javascript files.

For example, to force the file to be cached for no longer than 1 day, you would send:

Cache-Control: max-age=86400, must-revalidate

For beta, if you want to force the user to always get the latest, you would use:

Cache-Control: no-cache, must-revalidate

Google Page-Speed: Don't include a query string in the URL for static resources. Most proxies, most notably Squid up through version 3.0, do not cache resources with a "?" in their URL even if a Cache-control: public header is present in the response. To enable proxy caching for these resources, remove query strings from references to static resources, and instead encode the parameters into the file names themselves.

In this case, you can include the version into URL ex: http://abc.com/v1.2/script.js and use apache mod_rewrite to redirect the link to http://abc.com/script.js. When you change the version, client browser will update the new file.

This usage has been deprected: https://developer.mozilla.org/en-US/docs/Web/HTML/Using_the_application_cache

This answer is only 6 years late, but I don't see this answer in many places... HTML5 has introduced Application Cache which is used to solve this problem. I was finding that new server code I was writing was crashing old javascript stored in people's browsers, so I wanted to find a way to expire their javascript. Use a manifest file that looks like this:

CACHE MANIFEST
# Aug 14, 2014
/mycode.js

NETWORK:
*

and generate this file with a new time stamp every time you want users to update their cache. As a side note, if you add this, the browser will not reload (even when a user refreshes the page) until the manifest tells it to.

How about adding the filesize as a load parameter?

<script type='text/javascript' src='path/to/file/mylibrary.js?filever=<?=filesize('path/to/file/mylibrary.js')?>'></script>

So every time you update the file the "filever" parameter changes.

How about when you update the file and your update results in the same file size? what are the odds?