How to force the browser to reload cached CSS/JS files?

I have noticed that some browsers (in particular, Firefox and Opera) are very zealous in using cached copies of .css and .js files, even between browser sessions. This leads to a problem when you update one of these files but the user's browser keeps on using the cached copy.

The question is: what is the most elegant way of forcing the user's browser to reload the file when it has changed?

Ideally, the solution would not force the browser to reload the file on every visit to the page. I will post my own solution as an answer, but I am curious if anyone has a better solution and I'll let your votes decide.

Update :

After allowing discussion here for a while, I have found John Millikin and da5id's suggestion to be useful. It turns out there is a term for this: auto-versioning.

I have posted a new answer below which is a combination of my original solution and John's suggestion.

Another idea that was suggested by SCdF would be to append a bogus query string to the file. (Some Python code to automatically use the timestamp as a bogus query string was submitted by pi.). However, there is some discussion as to whether or not the browser would cache a file with a query string. (Remember, we want the browser to cache the file and use it on future visits. We only want it to fetch the file again when it has changed.)

Since it is not clear what happens with a bogus query string, I am not accepting that answer.

Answers:

Answer

Update: Rewritten to incorporate suggestions from John Millikin and da5id. This solution is written in PHP, but should be easily adapted to other languages.

Update 2: Incorporating comments from Nick Johnson that the original .htaccess regex can cause problems with files like json-1.3.js. Solution is to only rewrite if there are exactly 10 digits at the end. (Because 10 digits covers all timestamps from 9/9/2001 to 11/20/2286.)

First, we use the following rewrite rule in .htaccess:

RewriteEngine on
RewriteRule ^(.*)\.[\d]{10}\.(css|js)$ $1.$2 [L]

Now, we write the following PHP function:

/**
 *  Given a file, i.e. /css/base.css, replaces it with a string containing the
 *  file's mtime, i.e. /css/base.1221534296.css.
 *  
 *  @param $file  The file to be loaded.  Must be an absolute path (i.e.
 *                starting with slash).
 */
function auto_version($file)
{
  if(strpos($file, '/') !== 0 || !file_exists($_SERVER['DOCUMENT_ROOT'] . $file))
    return $file;

  $mtime = filemtime($_SERVER['DOCUMENT_ROOT'] . $file);
  return preg_replace('{\\.([^./]+)$}', ".$mtime.\$1", $file);
}

Now, wherever you include your CSS, change it from this:

<link rel="stylesheet" href="/css/base.css" type="text/css" />

To this:

<link rel="stylesheet" href="<?php echo auto_version('/css/base.css'); ?>" type="text/css" />

This way, you never have to modify the link tag again, and the user will always see the latest CSS. The browser will be able to cache the CSS file, but when you make any changes to your CSS the browser will see this as a new URL, so it won't use the cached copy.

This can also work with images, favicons, and JavaScript. Basically anything that is not dynamically generated.

Answer

Simple Client-side Technique

In general, caching is good.. So there are a couple of techniques, depending on whether you're fixing the problem for yourself as you develop a website, or whether you're trying to control cache in a production environment.

General visitors to your website won't have the same experience that you're having when you're developing the site. Since the average visitor comes to the site less frequently (maybe only a few times each month, unless you're a Google or hi5 Networks), then they are less likely to have your files in cache, and that may be enough. If you want to force a new version into the browser, you can always add a query string to the request, and bump up the version number when you make major changes:

<script src="/myJavascript.js?version=4"></script>

This will ensure that everyone gets the new file. It works because the browser looks at the URL of the file to determine whether it has a copy in cache. If your server isn't set up to do anything with the query string, it will be ignored, but the name will look like a new file to the browser.

On the other hand, if you're developing a website, you don't want to change the version number every time you save a change to your development version. That would be tedious.

So while you're developing your site, a good trick would be to automatically generate a query string parameter:

<!-- Development version: -->
<script>document.write('<script src="/myJavascript.js?dev=' + Math.floor(Math.random() * 100) + '"\><\/script>');</script>

Adding a query string to the request is a good way to version a resource, but for a simple website this may be unnecessary. And remember, caching is a good thing.

It's also worth noting that the browser isn't necessarily stingy about keeping files in cache. Browsers have policies for this sort of thing, and they are usually playing by the rules laid down in the HTTP specification. When a browser makes a request to a server, part of the response is an EXPIRES header.. a date which tells the browser how long it should be kept in cache. The next time the browser comes across a request for the same file, it sees that it has a copy in cache and looks to the EXPIRES date to decide whether it should be used.

So believe it or not, it's actually your server that is making that browser cache so persistent. You could adjust your server settings and change the EXPIRES headers, but the little technique I've written above is probably a much simpler way for you to go about it. Since caching is good, you usually want to set that date far into the future (a "Far-future Expires Header"), and use the technique described above to force a change.

If you're interested in more info on HTTP or how these requests are made, a good book is "High Performance Web Sites" by Steve Souders. It's a very good introduction to the subject.

Answer

Google's mod_pagespeed plugin for apache will do auto-versioning for you. It's really slick.

It parses HTML on its way out of the webserver (works with PHP, rails, python, static HTML -- anything) and rewrites links to CSS, JS, image files so they include an id code. It serves up the files at the modified URLs with a very long cache control on them. When the files change, it automatically changes the URLs so the browser has to re-fetch them. It basically just works, without any changes to your code. It'll even minify your code on the way out too.

Answer

Instead of changing the version manually, I would recommend you use an MD5 hash of the actual CSS file.

So your URL would be something like

http://mysite.com/css/[md5_hash_here]/style.css

You could still use the rewrite rule to strip out the hash, but the advantage is that now you can set your cache policy to "cache forever", since if the URL is the same, that means that the file is unchanged.

You can then write a simple shell script that would compute the hash of the file and update your tag (you'd probably want to move it to a separate file for inclusion).

Simply run that script every time CSS changes and you're good. The browser will ONLY reload your files when they are altered. If you make an edit and then undo it, there's no pain in figuring out which version you need to return to in order for your visitors not to re-download.

Answer

Not sure why you guys are taking so much pain to implement this solution.

All you need to do if get the file's modified timestamp and append it as a querystring to the file

In PHP i would do it as:

<link href="mycss.css?v=<?= filemtime('mycss.css') ?>" rel="stylesheet">

filemtime is a PHP function that returns the file modified timestamp.

Answer

You can just put ?foo=1234 at the end of your css / js import, changing 1234 to be whatever you like. Have a look at the SO html source for an example.

The idea there being that the ? parameters are discarded / ignored on the request anyway and you can change that number when you roll out a new version.


Note: There is some argument with regard to exactly how this affects caching. I believe the general gist of it is that GET requests, with or without parameters should be cachable, so the above solution should work.

However, it is down to both the web server to decide if it wants to adhere to that part of the spec and the browser the user uses, as it can just go right ahead and ask for a fresh version anyway.

Answer

I've heard this called "auto versioning". The most common method is to include the static file's mtime somewhere in the URL, and strip it out using rewrite handlers or URL confs:

See also:

Answer

The 30 or so existing answers are great advice for a circa 2008 website. However, when it comes to a modern, single page application (SPA), it might be time to re-think some fundamental assumptions… specifically the idea that it is desirable for the web server to serve only the single, most recent version of a file.

Imagine you're a user that has version M of a SPA loaded into your browser:

  1. Your CD pipeline deploys the new version N of the application onto the server
  2. You navigate within the SPA, which sends an XHR to the server to get /some.template
    • (Your browser hasn't refreshed the page, so you're still running version M)
  3. The server responds with the contents of /some.template — do you want it to return version M or N of the template?

If the format of /some.template changed between versions M and N (or the file was renamed or whatever) you probably don't want version N of the template sent to the browser that's running the old version M of the parser.†

Web apps run into this issue when two conditions are met:

  • Resources are requested asynchronously sometime after the initial page load
  • The app logic assumes things (that may change in future versions) about resource content

Once your app needs to serve up multiple versions in parallel, solving caching and "reloading" becomes trivial:

  1. Install all site files into versioned dirs: /v<release_tag_1>/…files…, /v<release_tag_2>/…files…
  2. Set HTTP headers to let browsers cache files forever
    • (Or better yet, put everything in a CDN)
  3. Update all <script> and <link> tags, etc. to point to that file in one of the versioned dirs

That last step sounds tricky, as it could require calling a URL builder for every URL in your server-side or client-side code. Or you could just make clever use of the <base> tag and change the current version in one place.

† One way around this is to be aggressive about forcing the browser to reload everything when a new version is released. But for the sake of letting any in-progress operations to complete, it may still be easiest to support at least two versions in parallel: v-current and v-previous.

Answer

Dont use foo.css?version=1! Browsers aren't supposed to cache URLs with GET variables. According to http://www.thinkvitamin.com/features/webapps/serving-javascript-fast, though IE and Firefox ignore this, Opera and Safari don't! Instead, use foo.v1234.css, and use rewrite rules to strip out the version number.

Answer

The RewriteRule needs a small update for js or css files that contain a dot notation versioning at the end. E.g. json-1.3.js.

I added a dot negation class [^.] to the regex so .number. is ignored.

RewriteRule ^(.*)\.[^.][\d]+\.(css|js)$ $1.$2 [L]
Answer

In Laravel (PHP) we can do it in following clear and elegant way (using file modification timestamp):

<script src="{{ asset('/js/your.js?v='.filemtime('js/your.js')) }}"></script>

And similar for CSS

<link rel="stylesheet" href="{{asset('css/your.css?v='.filemtime('css/your.css'))}}">

Example html output (filemtime return time as as a Unix timestamp)

<link rel="stylesheet" href="assets/css/your.css?v=1577772366">
Answer

For ASP.NET 4.5 and greater you can use script bundling.

The request http://localhost/MvcBM_time/bundles/AllMyScripts?v=r0sLDicvP58AIXN_mc3QdyVvVj5euZNzdsa2N1PKvb81 is for the bundle AllMyScripts and contains a query string pair v=r0sLDicvP58AIXN_mc3QdyVvVj5euZNzdsa2N1PKvb81. The query string v has a value token that is a unique identifier used for caching. As long as the bundle doesn't change, the ASP.NET application will request the AllMyScripts bundle using this token. If any file in the bundle changes, the ASP.NET optimization framework will generate a new token, guaranteeing that browser requests for the bundle will get the latest bundle.

There are other benefits to bundling including increased performance on first time page loads with minification.

Answer

Here is a pure JavaScript solution

(function(){

    // Match this timestamp with the release of your code
    var lastVersioning = Date.UTC(2014, 11, 20, 2, 15, 10);

    var lastCacheDateTime = localStorage.getItem('lastCacheDatetime');

    if(lastCacheDateTime){
        if(lastVersioning > lastCacheDateTime){
            var reload = true;
        }
    }

    localStorage.setItem('lastCacheDatetime', Date.now());

    if(reload){
        location.reload(true);
    }

})();

The above will look for the last time the user visited your site. If the last visit was before you released new code, it uses location.reload(true) to force page refresh from server.

I usually have this as the very first script within the <head> so it's evaluated before any other content loads. If a reload needs to occurs, it's hardly noticeable to the user.

I am using local storage to store the last visit timestamp on the browser, but you can add cookies to the mix if you're looking to support older versions of IE.

Answer

Interesting post. Having read all the answers here combined with the fact that I have never had any problems with "bogus" query strings (which I am unsure why everyone is so reluctant to use this) I guess the solution (which removes the need for apache rewrite rules as in the accepted answer) is to compute a short HASH of the CSS file contents (instead of the file datetime) as a bogus querystring.

This would result in the following:

<link rel="stylesheet" href="/css/base.css?[hash-here]" type="text/css" />

Of course the datetime solutions also get the job done in the case of editing a CSS file but I think it is about the css file content and not about the file datetime, so why get these mixed up?

Answer

For my development, I find that chrome has a great solution.

https://developer.chrome.com/devtools/docs/tips-and-tricks#hard-reload

With developer tools open, simply long click the refresh button and let go once you hover over "Empty Cache and Hard Reload".

This is my best friend, and is a super light weight way to get what you want!

Answer

Thanks at Kip for his perfect solution!

I extended it to use it as an Zend_view_Helper. Because my client run his page on a virtual host I also extended it for that.

Hope it helps someone else too.

/**
 * Extend filepath with timestamp to force browser to
 * automatically refresh them if they are updated
 *
 * This is based on Kip's version, but now
 * also works on virtual hosts
 * @link http://stackoverflow.com/questions/118884/what-is-an-elegant-way-to-force-browsers-to-reload-cached-css-js-files
 *
 * Usage:
 * - extend your .htaccess file with
 * # Route for My_View_Helper_AutoRefreshRewriter
 * # which extends files with there timestamp so if these
 * # are updated a automatic refresh should occur
 * # RewriteRule ^(.*)\.[^.][\d]+\.(css|js)$ $1.$2 [L]
 * - then use it in your view script like
 * $this->headLink()->appendStylesheet( $this->autoRefreshRewriter($this->cssPath . 'default.css'));
 *
 */
class My_View_Helper_AutoRefreshRewriter extends Zend_View_Helper_Abstract {

    public function autoRefreshRewriter($filePath) {

        if (strpos($filePath, '/') !== 0) {

            // path has no leading '/'
            return $filePath;
        } elseif (file_exists($_SERVER['DOCUMENT_ROOT'] . $filePath)) {

            // file exists under normal path
            // so build path based on this
            $mtime = filemtime($_SERVER['DOCUMENT_ROOT'] . $filePath);
            return preg_replace('{\\.([^./]+)$}', ".$mtime.\$1", $filePath);
        } else {

            // fetch directory of index.php file (file from all others are included)
            // and get only the directory
            $indexFilePath = dirname(current(get_included_files()));

            // check if file exist relativ to index file
            if (file_exists($indexFilePath . $filePath)) {

                // get timestamp based on this relativ path
                $mtime = filemtime($indexFilePath . $filePath);

                // write generated timestamp to path
                // but use old path not the relativ one
                return preg_replace('{\\.([^./]+)$}', ".$mtime.\$1", $filePath);
            } else {

                return $filePath;
            }
        }
    }

}

Cheers and thanks.

Answer

Have not found the client sided DOM approach creating the script node (or css) element dynamically:

<script>
    var node = document.createElement("script"); 
    node.type = "text/javascript";
    node.src = 'test.js?'+Math.floor(Math.random()*999999999);
    document.getElementsByTagName("head")[0].appendChild(node);
</script>
Answer

google chrome has Hard Reload as well as Empty Cache and Hard Reload option.You can click and hold the reload button (In Inspect Mode) to select one .

Answer

You can force a "session-wide caching" if you add the session-id as a spureous parameter of the js/css file:

<link rel="stylesheet" src="myStyles.css?ABCDEF12345sessionID" />
<script language="javascript" src="myCode.js?ABCDEF12345sessionID"></script>

If you want a version-wide caching you could add some code to print the file date or similar. If you're using Java you can use a custom-tag to generate the link in an elegant way.

<link rel="stylesheet" src="myStyles.css?20080922_1020" />
<script language="javascript" src="myCode.js?20080922_1120"></script>
Answer

Say you have a file available at:

/styles/screen.css

your can either append a query parameter with version information onto the URI, e.g.:

/styles/screen.css?v=1234

or you can prepend version information, e.g.:

/v/1234/styles/screen.css

IMHO the second method is better for CSS files because they can refer to images using relative URLs which means that if you specify a background-image like so:

body {
    background-image: url('images/happy.gif');
}

its URL will effectively be:

/v/1234/styles/images/happy.gif

This means that if you update the version number used the server will treat this as a new resource and not use a cached version. If you base your version number on the Subversion/CVS/etc. revision this means that changes to images referenced in CSS files will be noticed. That isn't guaranteed with the first scheme, i.e. the URL images/happy.gif relative to /styles/screen.css?v=1235 is /styles/images/happy.gif which doesn't contain any version information.

I have implemented a caching solution using this technique with Java servlets and simply handle requests to /v/* with a servlet that delegates to the underlying resource (i.e. /styles/screen.css). In development mode I set caching headers that tell the client to always check the freshness of the resource with the server (this typically results in a 304 if you delegate to Tomcat's DefaultServlet and the .css, .js, etc. file hasn't changed) while in deployment mode I set headers that say "cache forever".

Answer

You could simply add some random number with the CSS/JS url like

example.css?randomNo=Math.random()
Answer

For ASP.NET I suppose next solution with advanced options (debug/release mode, versions):

Js or Css files included by such way:

<script type="text/javascript" src="Scripts/exampleScript<%=Global.JsPostfix%>" />
<link rel="stylesheet" type="text/css" href="Css/exampleCss<%=Global.CssPostfix%>" />

Global.JsPostfix and Global.CssPostfix is calculated by the following way in Global.asax:

protected void Application_Start(object sender, EventArgs e)
{
    ...
    string jsVersion = ConfigurationManager.AppSettings["JsVersion"];
    bool updateEveryAppStart = Convert.ToBoolean(ConfigurationManager.AppSettings["UpdateJsEveryAppStart"]);
    int buildNumber = System.Reflection.Assembly.GetExecutingAssembly().GetName().Version.Revision;
    JsPostfix = "";
#if !DEBUG
    JsPostfix += ".min";
#endif      
    JsPostfix += ".js?" + jsVersion + "_" + buildNumber;
    if (updateEveryAppStart)
    {
        Random rand = new Random();
        JsPosfix += "_" + rand.Next();
    }
    ...
}
Answer

I recently solved this using Python. Here the code (should be easy to adopt to other languages):

def import_tag(pattern, name, **kw):
    if name[0] == "/":
        name = name[1:]
    # Additional HTML attributes
    attrs = ' '.join(['%s="%s"' % item for item in kw.items()])
    try:
        # Get the files modification time
        mtime = os.stat(os.path.join('/documentroot', name)).st_mtime
        include = "%s?%d" % (name, mtime)
        # this is the same as sprintf(pattern, attrs, include) in other
        # languages
        return pattern % (attrs, include)
    except:
        # In case of error return the include without the added query
        # parameter.
        return pattern % (attrs, name)

def script(name, **kw):
    return import_tag("""<script type="text/javascript" """ +\
        """ %s src="/%s"></script>""", name, **kw)

def stylesheet(name, **kw):
    return import_tag('<link rel="stylesheet" type="text/css" ' +\
        """%s href="/%s">', name, **kw) 

This code basically appends the files time-stamp as a query parameter to the URL. The call of the following function

script("/main.css")

will result in

<link rel="stylesheet" type="text/css"  href="/main.css?1221842734">

The advantage of course is that you do never have to change your html again, touching the CSS file will automatically trigger a cache invalidation. Works very good and the overhead is not noticeable.

Answer

If you're using git + PHP, you can reload the script from cache each time there is a change in the git repo, using the following code:

exec('git rev-parse --verify HEAD 2> /dev/null', $gitLog);
echo '  <script src="/path/to/script.js"?v='.$gitLog[0].'></script>'.PHP_EOL;
Answer

It seems all answers here suggest some sort of versioning in the naming scheme, which has its downsides.

Browsers should be well aware of what to cache and what not to cache by reading the webservers response, in particular the http headers - for how long is this resource valid ? was this resource updated since I last retrieved it ? etcetera.

If things are configured 'correctly', just updating the files of your application should (at some point) refresh the browsers caches. You can for example configure your web server to tell the browser to never cache files (which is a bad idea).

A more in-depth explanation of how that works is here https://www.mnot.net/cache_docs/#WORK

Answer

Simply add this code, where you want to do hard reload (force browser to reload cached CSS/JS files ) Do this inside the .load so it does not refreshes like a loop

 $( window ).load(function() {
   location.reload(true);
});
Answer

Just use server side code to add the date of the file... that way it WILL be cached and only reloaded when the file changes

In ASP.NET

<link rel="stylesheet" href="~/css/[email protected](System.Text.RegularExpressions.Regex.Replace(File.GetLastWriteTime(Server.MapPath("~/css/custom.css")).ToString(),"[^0-9]", ""))" />

<script type="text/javascript" src="~/js/[email protected](System.Text.RegularExpressions.Regex.Replace(File.GetLastWriteTime(Server.MapPath("~/js/custom.js")).ToString(),"[^0-9]", ""))"></script>    

This can be simplified to:

<script src="<%= Page.ResolveClientUrlUnique("~/js/custom.js") %>" type="text/javascript"></script>

By adding an extension method to your project to extend Page:

public static class Extension_Methods
{
    public static string ResolveClientUrlUnique(this System.Web.UI.Page oPg, string sRelPath)
    {
        string sFilePath = oPg.Server.MapPath(sRelPath);
        string sLastDate = System.IO.File.GetLastWriteTime(sFilePath).ToString();
        string sDateHashed = System.Text.RegularExpressions.Regex.Replace(sLastDate, "[^0-9]", "");

        return oPg.ResolveClientUrl(sRelPath) + "?d=" + sDateHashed;
    }
}
Answer

If you are a developer looking to avoid caching, chrome network tab has disable cache option. Otherwise you can do it without a server rendering framework using two script tags.

<script type="text/javascript">
    document.write('<script type="text/javascript" src="myfile.js?q=' + Date.now() + '">
    // can't use myfile.js stuff yet
</script>')
<script type="text/javascript">
    // do something with myfile.js
</script>
Answer

I suggest implementing the following process:

  • version your css/js files whenever you deploy, something like: screen.1233.css (the number can be your SVN revision if you use a versioning system)

  • minify them to optimize loading times

Answer

I put an MD5 hash of the file's contents in its URL. That way I can set a very long expiration date, and don't have to worry about users having old JS or CSS.

I also calculate this once per file at runtime (or on file system changes) so there's nothing funny to do at design time or during the build process.

If you're using ASP.NET MVC then you can check out the code in my other answer here.

Answer

This question is super old, and appears first thing when anybody googles this issue. This is not an answer to the question the way op wants it, instead is an answer to devs with this problem while developing and testing. And i can't post a new question on this topic since will be marked as duplicate.

Like many others, i just wanted to remove caching briefly.

"keep caching consistent with the file" .. its way too much hassle ..

Generally speaking, I don't mind loading more - even loading again files which did not change - on most projects - is practically irrelevant. While developing an app - we are mostly loading from disk, on localhost:port - so this increase in network traffic issue is not a deal breaking issue.

Most small projects are just playing around - they never end-up in production. so for them you don't need anything more..

As such if you use Chrome Dev Tools, you can follow this disable caching approach like in the image below: how to force chrome to reload cached files

And if you have firefox caching issues: how to force asset reload on firefox

how to disable caching in Firefox while in development Do this only in development, you also need a mechanism to force reload for production, since your users will use old cache invalidated modules if you update your app frequently and you don't provide a dedicated cache sync mechanism like the ones described in the answers above.

Yes, this info is already in previous answers but i still needed to do a google search to find it.

Hopefully this answer is very clear and now you don't need to.

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