How to force browsers to reload cached CSS and JS files?

Question

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.

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 have found John Millikin's 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.)

Answer 1

This solution is written in PHP, but it should be easily adapted to other languages.

The original .htaccess regex can cause problems with files like json-1.3.js. The 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 $file  The file to be loaded. works on all type of paths.
 */
function auto_version($file) {
  if($file[0] !== '/') {
    $file = rtrim(str_replace(DIRECTORY_SEPARATOR, '/', dirname($_SERVER['PHP_SELF'])), '/') . '/' . $file;
  }
  
  if (!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 2

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 information 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 3

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, Ruby on Rails, Python, static HTML -- anything) and rewrites links to CSS, JavaScript, 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 4

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 5

I am not sure why you guys/gals 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 6

You can just put ?foo=1234 at the end of your CSS / JavaScript import, changing 1234 to be whatever you like. Have a look at the Stack Overflow 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 7

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

See also:

• Automatic asset versioning in Django
• Automatically Version Your CSS and JavaScript Files

Answer 8

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 rethink 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 XMLHttpRequest (XHR) to the server to get /some.template

• (Your browser hasn't refreshed the page, so you're still running version M)

1. 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 applications run into this issue when two conditions are met:

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

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

1. Install all site files into versioned directories: /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)

1. Update all <script> and <link> tags, etc. to point to that file in one of the versioned directories

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 9

In Laravel (PHP) we can do it in the 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 10

Don’t 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 Internet Explorer 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 11

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 12

The RewriteRule needs a small update for JavaScript 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 13

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 14

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 15

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

https://superuser.com/a/512833

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 lightweight way to get what you want!

Answer 16

I have not found the client-side 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 17

Thanks to 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.

/**
 * 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;
            }
        }
    }
}

Answer 18

I recently solved this using Python. Here is the code (it 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 %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 content again, touching the CSS file will automatically trigger a cache invalidation. It works very well and the overhead is not noticeable.

Answer 19

Say you have a file available at:

/styles/screen.css

You 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 20

You could simply add some random number with the CSS and JavaScript URL like

example.css?randomNo = Math.random()

Answer 21

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

Answer 22

You can force a "session-wide caching" if you add the session-id as a spurious parameter of the JavaScript/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 23

For ASP.NET I propose the following solution with advanced options (debug/release mode, versions):

Include JavaScript or CSS files this 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 are 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 24

If you're using Git and PHP, you can reload the script from the cache each time there is a change in the Git repository, 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 25

Simply add this code where you want to do a hard reload (force the browser to reload cached CSS and JavaScript files):

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

Do this inside the .load, so it does not refresh like a loop.

Answer 26

For development: use a browser setting: for example, Chrome network tab has a disable cache option.

For production: append a unique query parameter to the request (for example, q?Date.now()) with a server-side rendering framework or pure JavaScript code.

// Pure JavaScript unique query parameter generation
//
//=== myfile.js

function hello() { console.log('hello') };

//=== end of file

<script type="text/javascript">
    document.write('<script type="text/javascript" src="myfile.js?q=' + Date.now() + '">
    // document.write is considered bad practice!
    // We can't use hello() yet
</script>')

<script type="text/javascript">
    hello();
</script>

Answer 27

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 web server's response, in particular the HTTP headers - for how long is this resource valid? Was this resource updated since I last retrieved it? etc.

If things are configured 'correctly', just updating the files of your application should (at some point) refresh the browser's 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 in How Web Caches Work.

Answer 28

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/custom.css?d=@(System.Text.RegularExpressions.Regex.Replace(File.GetLastWriteTime(Server.MapPath("~/css/custom.css")).ToString(),"[^0-9]", ""))" />

<script type="text/javascript" src="~/js/custom.js?d=@(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 29

For developers with this problem while developing and testing:

Remove caching briefly.

"keep caching consistent with the file" .. it's 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 application - 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 DevTools, you can follow this disable-caching approach like in the image below:

And if you have Firefox caching issues:

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 application frequently and you don't provide a dedicated cache synchronisation mechanism like the ones described in the answers above.

Yes, this information is already in previous answers, but I still needed to do a Google search to find it.

Answer 30

You can use SRI to break the browser cache. You only have to update your index.html file with the new SRI hash every time. When the browser loads the HTML and finds out the SRI hash on the HTML page didn't match that of the cached version of the resource, it will reload your resource from your servers. It also comes with a good side effect of bypassing cross-origin read blocking.

<script src="https://jessietessie.github.io/google-translate-token-generator/google_translate_token_generator.js" integrity="sha384-muTMBCWlaLhgTXLmflAEQVaaGwxYe1DYIf2fGdRkaAQeb4Usma/kqRWFWErr2BSi" crossorigin="anonymous"></script>

Answer 31

TomA's answer is right.

Using the "querystring" method will not be cached as quoted by Steve Souders below:

...that Squid, a popular proxy, doesn’t cache resources with a querystring.

TomA's suggestion of using style.TIMESTAMP.css is good, but MD5 would be much better as only when the contents were genuinely changed, the MD5 changes as well.

Answer 32

I suggest implementing the following process:

• version your CSS and JavaScript 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 33

I see a problem with the approach of using a timestamp- or hash-based differentiator in the resource URL which gets stripped out on request at the server. The page that contains the link to e.g. the style sheet might get cached as well. So the cached page might request an older version of the style sheet, but it will be served the latest version, which might or might not work with the requesting page.

To fix this, you either have to guard the requesting page with a no-cache header or meta, to make sure it gets refreshed on every load. Or you have to maintain all versions of the style file that you ever deployed on the server, each as an individual file and with their differentiator intact so that the requesting page can get at the version of the style file it was designed for. In the latter case, you basically tie the versions of the HTML page and the style sheet together, which can be done statically and doesn't require any server logic.

Answer 34

For a Java Servlet environment, you can look at the Jawr library. The features page explains how it handles caching:

Jawr will try its best to force your clients to cache the resources. If a browser asks if a file changed, a 304 (not modified) header is sent back with no content. On the other hand, with Jawr you will be 100% sure that new versions of your bundles are downloaded by all clients. Every URL to your resources will include an automatically generated, content-based prefix that changes automatically whenever a resource is updated. Once you deploy a new version, the URL to the bundle will change as well so it will be impossible that a client uses an older, cached version.

The library also does JavaScript and CSS minification, but you can turn that off if you don't want it.

Answer 35

A SilverStripe-specific answer worked out from reading: http://api.silverstripe.org/3.0/source-class-SS_Datetime.html#98-110:

Hopefully this will help someone using a SilverStripe template and trying to force reload a cached image on each page visit / refresh. In my case it is a GIF animation which only plays once and therefore did not replay after it was cached. In my template I simply added:

?$Now.Format(dmYHis)

to the end of the file path to create a unique time stamp and to force the browser to treat it as a new file.

Answer 36

Disable caching of script.js only for local development in pure JavaScript.

It injects a random script.js?wizardry=1231234 and blocks regular script.js:

<script type="text/javascript">
  if(document.location.href.indexOf('localhost') !== -1) {
    const scr = document.createElement('script');
    document.setAttribute('type', 'text/javascript');
    document.setAttribute('src', 'scripts.js' + '?wizardry=' + Math.random());
    document.head.appendChild(scr);
    document.write('<script type="application/x-suppress">'); // prevent next script(from other SO answer)
  }
</script>

<script type="text/javascript" src="scripts.js">

Answer 37

One of the best and quickest approaches I know is to change the name of the folder where you have CSS or JavaScript files.

Or for developers: Change the name of your CSS and JavaScript files something like versions.

<link rel="stylesheet" href="cssfolder/somecssfile-ver-1.css"/>

Do the same for your JavaScript files.

Answer 38

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 39

"Another idea which 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."

<link rel="stylesheet" href="file.css?<?=hash_hmac('sha1', session_id(), md5_file("file.css")); ?>" />

Hashing the file means when it has changed, the query string will have changed. If it hasn't, it will remain the same. Each session forces a reload too.

Optionally, you can also use rewrites to cause the browser to think it's a new URI.

Answer 40

Another suggestion for ASP.NET websites,

1. Set different cache-control:max-age values, for different static files.

2. For CSS and JavaScript files, the chances of modifying these files on server is high, so set a minimal cache-control:max-age value of 1 or 2 minutes or something that meets your need.

3. For images, set a far date as the cache-control:max-age value, say 360 days.

4. By doing so, when we make the first request, all static contents are downloaded to client machine with a 200-OK response.

5. On subsequent requests and after two minutes, we see 304-Not Modified requests on CSS and JavaScript files which avoids us from CSS and JavaScript versioning.

6. Image files will not be requested as they will be used from cached memory till the cache expires.

7. By using the below web.config configurations, we can achieve the above described behavior,

   <system.webServer>
       <modules runAllManagedModulesForAllRequests="true"/>
       <staticContent>
           <clientCache cacheControlMode="UseMaxAge" cacheControlMaxAge="00.00:01:00"/>
       </staticContent>
       <httpProtocol>
           <customHeaders>
               <add name="ETAG" value=""/>
           </customHeaders>
       </httpProtocol>
   </system.webServer>
   
   <location path="Images">
       <system.webServer>
           <staticContent>
               <clientCache cacheControlMode="UseMaxAge" cacheControlMaxAge="180.00:00:00" />
           </staticContent>
       </system.webServer>
   </location>
   

Answer 41

If you are using a modern browser, you could use a manifest file to inform the browsers which files need to be updated. This requires no headers, no versions in URLs, etc.

For more details, see: Using the application cache

Answer 42

Many answers here advocate adding a timestamp to the URL. Unless you are modifying your production files directly, the file's timestamp is not likely to reflect the time when a file was changed. In most cases this will cause the URL to change more frequently than the file itself. This is why you should use a fast hash of the file's contents such as MD5 as levik and others have suggested.

Keep in mind that the value should be calculated once at build or run, rather than each time the file is requested.

As an example, here's a simple bash script that reads a list of filenames from standard input and writes a JSON file containing hashes to standard output:

#!/bin/bash
# Create a JSON map from filenames to MD5 hashes
# Run as hashes.sh < inputfile.list > outputfile.json

echo "{"
delim=""
while read l; do
    echo "$delim\"$l\": \"`md5 -q $l`\""
    delim=","
done
echo "}"

This file could then be loaded at server startup and referenced instead of reading the file system.

Answer 43

I came to this question when looking for a solution for my SPA, which only has a single index.html file listing all the necessary files. While I got some leads that helped me, I could not find a quick-and-easy solution.

In the end, I wrote a quick page (including all of the code) necessary to autoversion an HTML/JavaScript index.html file as part of the publishing process. It works perfectly and only updates new files based on date last modified.

You can see my post at Autoversion your SPA index.html. There is a stand-alone Windows application there too.

The guts of the code is:

private void ParseIndex(string inFile, string addPath, string outFile)
{
    string path = Path.GetDirectoryName(inFile);
    HtmlAgilityPack.HtmlDocument document = new HtmlAgilityPack.HtmlDocument();
    document.Load(inFile);

    foreach (HtmlNode link in document.DocumentNode.Descendants("script"))
    {
        if (link.Attributes["src"]!=null)
        {
            resetQueryString(path, addPath, link, "src");
        }
    }

    foreach (HtmlNode link in document.DocumentNode.Descendants("link"))
    {
        if (link.Attributes["href"] != null && link.Attributes["type"] != null)
        {
            if (link.Attributes["type"].Value == "text/css" || link.Attributes["type"].Value == "text/html")
            {
                resetQueryString(path, addPath, link, "href");
            }
        }
    }

    document.Save(outFile);
    MessageBox.Show("Your file has been processed.", "Autoversion complete");
}

private void resetQueryString(string path, string addPath, HtmlNode link, string attrType)
{
    string currFileName = link.Attributes[attrType].Value;

    string uripath = currFileName;
    if (currFileName.Contains('?'))
        uripath = currFileName.Substring(0, currFileName.IndexOf('?'));
    string baseFile = Path.Combine(path, uripath);
    if (!File.Exists(baseFile))
        baseFile = Path.Combine(addPath, uripath);
    if (!File.Exists(baseFile))
        return;
    DateTime lastModified = System.IO.File.GetLastWriteTime(baseFile);
    link.Attributes[attrType].Value = uripath + "?v=" + lastModified.ToString("yyyyMMddhhmm");
}

Answer 44

Small improvement from existing answers...

Using a random number or session id would cause it to reload on each request. Ideally, we may need to change only if some code changes were done in any JavaScript or CSS file.

When using a common JSP file as a template to many other JSP and JavaScript files, add the below in a common JSP file

<%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core"%>
<c:set var = "version" scope = "application" value = "1.0.0" />

Now use the above variable in all locations as below in your JavaScript file inclusions.

<script src='<spring:url value="/js/myChangedFile.js?version=${version}"/>'></script>

Advantages:

1. This approach will help you in changing version number at one location only.

2. Maintaining a proper version number (usually build/release number) will help you to check/verify your code changes being deployed properly (from developer console of the browser).

Another useful tip:

If you are using the Chrome browser, you can disable caching when Dev Tools is open. In Chrome, hit F12 → F1 and scroll to Settings → Preferences → Network → *Disable caching (while DevTools is open)

Answer 45

A simple solution for static files (just for development purposes) that adds a random version number to the script URI, using script tag injections

<script>
    var script = document.createElement('script');
    script.src = "js/app.js?v=" + Math.random();
    document.getElementsByTagName('head')[0].appendChild(script);
</script>

Answer 46

In ASP.NET Core you could achieve this by adding 'asp-append-version':

<link rel="stylesheet" href="~/css/xxx.css" asp-append-version="true" />

 <script src="~/js/xxx.js" asp-append-version="true"></script>

It will generate HTML:

<link rel="stylesheet" href="/css/xxx.css?v=rwgRWCjxemznsx7wgNx5PbMO1EictA4Dd0SjiW0S90g" />

The framework will generate a new version number every time you update the file.

Answer 47

If you don't want the client to cache the file ever, this solution seems to be quickest to implement. Adjust the part with time() if you e.g. load the file in footer.php:

<script src="<?php echo get_template_directory_uri(); ?>/js/main.js?v=<?= time() ?>"></script>

Answer 48

My method to do this is simply to have the link element into a server-side include:

<!--#include virtual="/includes/css-element.txt"-->

where the contents of css-element.txt is

<link rel="stylesheet" href="mycss.css"/>

so the day you want to link to my-new-css.css or whatever, you just change the include.

Answer 49

Well, I have made it work my way by changing the JavaScript file version each time the page loads by adding a random number to JavaScript file version as follows:

// Add it to the top of the page
<?php
    srand();
    $random_number = rand();
?>

Then apply the random number to the JavaScript version as follow:

<script src="file.js?version=<?php echo $random_number;?>"></script>

Answer 50

We have one solution with some different way for implementation. We use the above solution for it.

datatables?v=1

We can handle the version of the file. It means that every time that we change our file, change the version of it too. But it's not a suitable way.

Another way used a GUID. It wasn't suitable either, because each time it fetches the file and doesn't use from the browser cache.

datatables?v=Guid.NewGuid()

The last way that is the best way is:

When a file change occurs, change the version too. Check the follow code:

<script src="~/scripts/main.js?v=@File.GetLastWriteTime(Server.MapPath("/scripts/main.js")).ToString("yyyyMMddHHmmss")"></script>

By this way, when you change the file, LastWriteTime change too, so the version of the file will change and in the next when you open the browser, it detects a new file and fetch it.

Answer 51

Here is my Bash script-based cache busting solution:

1. I assume you have CSS and JavaScript files referenced in your index.html file
2. Add a timestamp as a parameter for .js and .css in index.html as below (one time only)
3. Create a timestamp.txt file with the above timestamp.
4. After any update to .css or .js file, just run the below .sh script

Sample index.html entries for .js and .css with a timestamp:

<link rel="stylesheet" href="bla_bla.css?v=my_timestamp">
<script src="scripts/bla_bla.js?v=my_timestamp"></script>

File timestamp.txt should only contain same timestamp 'my_timestamp' (will be searched for and replaced by script later on)

Finally here is the script (let's call it cache_buster.sh :D)

old_timestamp=$(cat timestamp.txt)
current_timestamp=$(date +%s)
sed -i -e "s/$old_timestamp/$current_timestamp/g" index.html
echo "$current_timestamp" >timestamp.txt

(Visual Studio Code users) you can put this script in a hook, so it gets called each time a file is saved in your workspace.

Answer 52

I've solved this issue by using ETag:

ETag or entity tag is part of HTTP, the protocol for the World Wide Web. It is one of several mechanisms that HTTP provides for Web cache validation, which allows a client to make conditional requests. This allows caches to be more efficient and saves bandwidth, as a Web server does not need to send a full response if the content has not changed. ETags can also be used for optimistic concurrency control,1 as a way to help prevent simultaneous updates of a resource from overwriting each other.

• I am running a Single-Page Application (written in Vue.JS).
• The output of the application is built by npm, and is stored as dist folder (the important file is: dist/static/js/app.my_rand.js)
• Nginx is responsible of serving the content in this dist folder, and it generates a new Etag value, which is some kind of a fingerprint, based on the modification time and the content of the dist folder. Thus when the resource changes, a new Etag value is generated.
• When the browser requests the resource, a comparison between the request headers and the stored Etag, can determine if the two representations of the resource are the same, and could be served from cache or a new response with a new Etag needs to be served.

Answer 53

location.reload(true)

Or use "Network" from the inspector ([CTRL] + [I]), click "disable cache", click trash icon, click "load"/"get"

Answer 54

If you are using jQuery, there is an option called cache that will append a random number.

This is not a complete answer I know, but it might save you some time.

Answer 55

Another way for JavaScript files would be to use the jQuery $.getScript in conjunction with $.ajaxSetup option cache: false.

Instead of:

<script src="scripts/app.js"></script>

You can use:

$.ajaxSetup({
  cache: false
});

$.getScript('scripts/app.js'); // GET scripts/app.js?_1391722802668

Answer 56

Changing the filename will work. But that's not usually the simplest solution.

An HTTP cache-control header of 'no-cache' doesn't always work, as you've noticed. The HTTP 1.1 spec allows wiggle-room for user-agents to decide whether or not to request a new copy. (It's non-intuitive if you just look at the names of the directives. Go read the actual HTTP 1.1 spec for cache... it makes a little more sense in context.)

In a nutshell, if you want iron-tight cache-control use

Cache-Control: no-cache, no-store, must-revalidate

in your response headers.

Answer 57

The simplest method is to take advantage of the PHP file read functionality. Just have the PHP echo the contents of the file into tags.

<?php
//Replace the 'style.css' with the link to the stylesheet.
echo "<style type='text/css'>".file_get_contents('style.css')."</style>";
?>

If you're using something besides PHP, there are some variations depending on the language, but almost all languages have a way to print the contents of a file. Put it in the right location (in the section), and that way, you don't have to rely on the browser.