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what is the filename.js?m=1497883755 in script attachment src? [duplicate]

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.)

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 $file The file to be loaded. Must be an absolute path (i.e.
* starting with slash).
*/
function auto_version($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.

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.

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.

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.

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.

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.

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

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:
Your CD pipeline deploys the new version N of the application onto the server
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)
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:
Install all site files into versioned directories: /v<release_tag_1>/…files…, /v<release_tag_2>/…files…
Set HTTP headers to let browsers cache files forever
(Or better yet, put everything in a CDN)
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.

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">

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.

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.

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]

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?

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.

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!

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;
}
}
}
}

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>

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".

You could simply add some random number with the CSS and JavaScript URL like
example.css?randomNo = Math.random()

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.

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.

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>

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();
}
...
}

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;

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.

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>

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.

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.

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;
}
}

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>

How does "?v=" in HTML linking work?

I have been developing a website for testing new stuff, and I need to figure out the "?v=" thing. But I have no clue how it works, so can someone explain this to me please haha? Like how to, and how it works.
So what would this look like and how would the file names on the server vary for this:
<script src="assets/js/moticulous.js"></script>
<link rel="stylesheet" href="assets/js/platforms.css"/>
as opposed to this:
<script src="assets/js/moticulous.js?v=1"></script>
<link rel="stylesheet" href="assets/js/platforms.css?v=1"/>

This can be added to prevent Caching of js/css/image files. By adding ?anything=123 You force browser/client to download the updated version of js/css/image file from the server.
Read more on: https://css-tricks.com/can-we-prevent-css-caching/

That is a technique used to control caching of script, css and image files.
The browser will download the script file with the ?v=1 parameter (example"http://example.com/path/to/script.js?v=1") and cache it to the visitors disk. The next time the browser visits the page, if the URL is still "http://example.com/path/to/script.js?v=1" then the cached version will be loaded.
If you change the ?v=1 to ?v=2 then the cached version is no longer valid as the full URL is no longer the same as what the browser has cached. This results in a new file being downloaded, and cached. This forces recent changes to every visitor regardless of cache settings set at the server config or browser.
This technique is often used with a version number (likely why its a v=) to force a new download of the js when the software version gets updated.
In your backend code, you would replace the =1 part with whatever the current software version is to make this cache control dynamic. Alternately, you could increment the version number whenever the asset changes but that's less dynamic or more work to make it so.

The dummy HTTP GET string is passed to prevent caching as some browsers cache the .js and .css files. It is usually done to prevent the older version of the file from loading by the browsers via browser cache when a change is made to the .css or .js file. Adding the timestamp value to the name (as <filename>?<timestamp>) is more popular than adding the version as it forces the browser to download the files every time the page is viewed as no two request times have the same timestamp.

importing javascript library once

I'm building a website which imports a javascript library (located inside <head>):
<script type="text/javascript" src="#routes.Assets.at("nicedit/nicEdit.js")"></script>
which means,
every page loading (no matter which page in my website), this line exists.
I wanted to know if modern browsers download the library once, and then cache it,
or every page load - the javascript is library re-downloaded.
thanks

Under normal circumstances most browsers will cache a javascript file if it is repeatedly requested from the same url; however, my definition of normal circumstances is wrapped up in how servers are usually set up. The real result depends on what cache headers the server is sending, as well as whether or not the URL changes (which is not clear from your question).
There are lots of questions on StackOverflow about caching JavaScript. This one includes something of a compendium.

Please try configuring http.cacheControl
http.cacheControl
HTTP Response headers control for static files: sets the default
max-age in seconds, telling the user’s browser how long it should
cache the page. This is only read in prod mode, in dev mode the cache
is disabled. For example, to send no-cache:
http.cacheControl=0 Default: 3600 - set cache expiry to one hour.
Source of the information.

Force refresh of client browser

I have added a pretty substantial change to a javascript file. However, in order for the new changes to register, I need to refresh my browser. I imagine this is how it will be for all the other people using this site as well.
Is there a way to invalidate the client's browser cache and force it to download the new js file?

You can rename the file, or just add a random parameter when loading the file:
<script src="/javascripts/application.js?version=4" type="text/javascript"></script>

You can add an argument to the file name, like
<script src="script.js?rnd=2131231"></script>

There is a very comprehensive discussion here, with several ways to do this.
The recommended solution (mod_pagespeed) might not be the best solution for you in case your project is not big enough or you don't use apache, in which case I would recommend a format such as:
<script src="/{sha_1_here}/script.js"></script>
instead of a GET parameter, because apparently some browsers do not cache scripts with parameters.
This should work everywhere, and you just need to rewrite the URL server side.

For development purposes, I disable the cache on the browser side so that it fetches always.
But for production, to reset so that all client user's browsers should download the new version you can look at apache's settings to expire cached files.

What is a "?" for in the src attribute of a html script tag?

If this has been asked before, I apologize but this is kinda of a hard question to search for. This is the first time I have come across this in all my years of web development, so I'm pretty curious.
I am editing some HTML files for a website, and I have noticed that in the src attribute of the script tags that the previous author appended a question mark followed by data.
Ex: <script src="./js/somefile.js?version=3.2"></script>
I know that this is used in some languages for value passing in GET request, such as PHP, but as I far as I ever knew, this wasn't done in javascript - at least in calling a javascript file. Does anyone know what this does, if anything?
EDIT: Wow, a lot of responses. Thanks one and all. And since a lot of people are saying similar things, I will post an global update instead of commenting everyone.
In this case the javascript files are static, hence my curiosity. I have also opened them up and did not see anything attempt to access variables on file load. I've never thought about caching or plain version control, both which seam more likely in this circumstance.

I believe what the author was doing was ensuring that if he creates version 3.3 of his script he can change the version= in the url of the script to ensure that users download the new file instead of running off of the old script cached in their browser.
So in this case it is part of the caching strategy.

My guess is it's so if he publishes a new version of the JavaScript file, he can bump the version in the HTML documents. This will not do anything server-side when requested, but it causes the browser to treat it as a different file, effectively forcing the browser to re-fetch the script and bypass the local cache of the file.
This way, you can set a really high cache time (like a week or a month!) but not sacrifice the ability to update scripts frequently if necessary.

What you have to remember is that this ./js/somefile.js?version=3.2 doesn't have to be a physical file. It can be a page which creates the file on the fly. So you could have it where the request says, "Hey give me version 3 of this js file," and the server side code creates it and writes it to the output stream.
The other option is to force the browser to not cache the file and pull down the new one when it makes the request. Since the URI changed, it will think the file is completely new.

A (well-configured) web server will send static files like JavaScript source code once and tell the web browser to cache that file locally for a certain period of time (could be a day, a week, a month, or longer). When the browser sees another request for that same file, it will just use that version instead of getting new code from the server.
If the URL changes -- for example by adding a query string -- then the browser suspects that its cached version is no good and gets a new one. As such, the ? helps developers say "Oops, I changed this file, make sure the browser gets a new copy."

In this case it's probably being used to ensure the source file isn't cached between versions.
Of course, it could also be used server side to generate the javascript file, without knowing what you have on the other end of the request, it's difficult to be definitive.
BTW, the ?... portion of the url is called the query string.

this is used to guarantee that the browser downloads a new version of the script when available. The version number in the url is incremented each time a new version is deployed so that the browser see it as a different file.

Just because the file extension is .js doesn't mean that the target is an actual .js file. They could set up their web server to pass the requested URL to a script (or literally have a script named somefile.js) and have that interpret the filename and version.

The query string has nothing to do with the javascript. Some server side code is hosting up a different version depending on that querystring it appears.
You should never assume anything about paths in a URL. The extension on a path in a URL doesn't really tell you anything. URLs can be completely dynamic and served by some server side code or can rewritten in web servers dynamically.
Now it is common to add a querystring to urls when loading javascript files to prevent client side caching. If the page updates and references a new version of the script then the page can bust through and cause the client to refresh it's script.