What does appending "?v=1" to CSS and JavaScript URLs in link and script tags do?

Question

These are usually to make sure that the browser gets a new version when the site gets updated with a new version, e.g. as part of our build process we'd have something like this:

/Resources/Combined.css?v=x.x.x.buildnumber

Since this changes with every new code push, the client's forced to grab a new version, just because of the querystring. Look at this page (at the time of this answer) for example:

<link ... href="http://sstatic.net/stackoverflow/all.css?v=c298c7f8233d">

I think instead of a revision number the SO team went with a file hash, which is an even better approach, even with a new release, the browsers only forced to grab a new version when the file actually changes.

Both of these approaches allow you to set the cache header to something ridiculously long, say 20 years...yet when it changes, you don't have to worry about that cache header, the browser sees a different querystring and treats it as a different, new file.

This makes sure you are getting the latest version from of the css or js file from the server.

And later you can append "?v=2" if you have a newer version and "?v=3", "?v=4" and so on.

Note that you can use any querystring, 'v' is not a must for example:

"?blah=1" will work as well.

And

"?xyz=1002" will work.

And this is a common technique because browsers are now caching js and css files better and longer.

The hash solution is nice but not really human readable when you want to know what version of file is sitting in your local web folder. The solution is to date/time stamp your version so you can easily compare it against your server file.

For example, if your .js or .css file is dated 2011-02-08 15:55:30 (last modification) then the version should equal to .js?v=20110208155530

Should be easy to read properties of any file in any language. In ASP.Net it's really easy...

".js?v=" + File.GetLastWriteTime(HttpContext.Current.Request.PhysicalApplicationPath + filename).ToString("yyMMddHHHmmss");

Of coz get it nicely refactored into properties/functions first and off you go. No more excuses.

Good luck, Art.

In order to answer you questions;

"?v=1" this is written only beacuse to download a fresh copy of the css and js files instead of using from the cache of the browser.

If you mention this query string parameter at the end of your stylesheet or the js file then it forces the browser to download a new file, Due to which the recent changes in the .css and .js files are made effetive in your browser.

If you dont use this versioning then you may need to clear the cache of refresh the page in order to view the recent changes in those files.

Here is an article that explains this thing How and Why to make versioning of CSS and JS files

Javascript files are often cached by the browser for a lot longer than you might expect.

This can often result in unexpected behaviour when you release a new version of your JS file.

Therefore, it is common practice to add a QueryString parameter to the URL for the javascript file. That way, the browser caches the Javascript file with v=1. When you release a new version of your javascript file you change the url's to v=2 and the browser will be forced to download a new copy.