How can I make the browser see CSS and Javascript changes?

I found that if you append the last modified timestamp of the file onto the end of the URL the browser will request the files when it is modified. For example in PHP:

function urlmtime($url) {
   $parsed_url = parse_url($url);
   $path = $parsed_url['path'];

   if ($path[0] == "/") {
       $filename = $_SERVER['DOCUMENT_ROOT'] . "/" . $path;
   } else {
       $filename = $path;
   }

   if (!file_exists($filename)) {
       // If not a file then use the current time
       $lastModified = date('YmdHis');
   } else {
       $lastModified = date('YmdHis', filemtime($filename));
   }

   if (strpos($url, '?') === false) {
       $url .= '?ts=' . $lastModified;
   } else {
       $url .= '&ts=' . $lastModified;
   }

   return $url;
}

function include_css($css_url, $media='all') {
   // According to Yahoo, using link allows for progressive 
   // rendering in IE where as @import url($css_url) does not
   echo '<link rel="stylesheet" type="text/css" media="' .
        $media . '" href="' . urlmtime($css_url) . '">'."\n";
}

function include_javascript($javascript_url) {
   echo '<script type="text/javascript" src="' . urlmtime($javascript_url) .
        '"></script>'."\n";

hi grom, i have tried this method. however, i found out that once i include parameter the browser did not cache the css file. It loads the css file each time I revisit the page without caching it. I detected this by keep on making changes in css file to see whether css file is cached by the browser. I am using chrome. May I know how to make the browser cache the css file and load only when changes in parameter? Thx.

@benmsia, have you looked at what HTTP headers are being returned when requesting the CSS file?

Some solutions I have seen involve adding a version number to the end of the file in the form of a query string.

<script type="text/javascript" src="funkycode.js?v1">

You could use the SVN revision number to automate this for you by including the word LastChangedRevision in your html file after where v1 appears above. You must also setup your repository to do this.

I hope this further clarifies my answer?

Firefox also allows pressing CTRL + R to reload everything on a particular page.

In my opinion, it is better to make the version number part of the file itself e.g. myscript.1.2.3.js. You can set your webserver to cache this file forever, and just add a new js file when you have a new version.

When you release a new version of your CSS or JS libraries, cause the following to occur:

1. modify the filename to include a unique version string
2. modify the HTML files which reference the library to point at the versioned file

(this is usually a pretty simple matter for a release script)

Now you can set the Expires for the CSS/JS to be years in the future. Whenever you change the content, if the referencing HTML points to a new URI, browsers will no longer use the old cached copy.

This causes the caching behavior you want without requiring anything of the user.

I was also wondering how to do this, when I found grom's answer. Thanks for the code.

I struggled with understanding how the code was supposed to be used. (I don't use a version control system.) In summary, you include the timestamp (ts) when you call the stylesheet. You're not planning on changing the stylesheet often:

<?php 
    include ('grom_file.php');
    // timestamp on the filename has to be updated manually
    include_css('_stylesheets/style.css?ts=20080912162813', 'all');
?>

?? the include_css function will append the last modified timestamp on the file onto the end of the url. No version control required