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

Question

Hi all,

I have been looking at a HTML 5 boilerplate template (from http://html5boilerplate.com/) and noticed the use of "?v=1" in URLs when referring to CSS and Javascript files.

1. What does appending "?v=1" to CSS and Javascript URLs in link and script tags do?
2. Not all Javascript URLs have the "?v=1" (example from the sample below: js/modernizr-1.5.min.js). Is there a reason why this is the case?

Sample from their index.html:

<!-- CSS : implied media="all" -->
<link rel="stylesheet" href="css/style.css?v=1">

<!-- For the less-enabled mobile browsers like Opera Mini -->
<link rel="stylesheet" media="handheld" href="css/handheld.css?v=1">

<!-- All JavaScript at the bottom, except for Modernizr which enables HTML5 elements & feature detects -->
<script src="js/modernizr-1.5.min.js"></script>

<!------ Some lines removed ------>

<script src="js/plugins.js?v=1"></script>
<script src="js/script.js?v=1"></script>

<!--[if lt IE 7 ]>
  <script src="js/dd_belatedpng.js?v=1"></script>
<![endif]-->


<!-- yui profiler and profileviewer - remove for production -->
<script src="js/profiling/yahoo-profiling.min.js?v=1"></script>
<script src="js/profiling/config.js?v=1"></script>
<!-- end profiling code -->

Thanks in advance.

Answer 1

This make sure you get the right last version from the css or js file.

That means that you tell the browser that you updated the file to don't use the cached version.

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 these days because browsers are now caching js and css files better and longer.

Answer 2

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"&gt;

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.

Answer 3

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.

Answer 4

IS there a php code to work on this or you just have to manually include those numbers?