Develop Reference

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.

Are JavaScript files downloaded with the HTML-body

I have a Java Web Application, and I'm wondering if the javascript files are downloaded with the HTML-body, or if the html body is loaded first, then the browser request all the JavaScript files.
The reason for this question is that I want to know if importing files with jQuery.getScript() would result in poorer performance. I want to import all files using that JQuery function to avoid duplication of JavaScript-imports.

The body of the html document is retrieved first. After it's been downloaded, the browser checks what resources need to be retrieved and gets those.
You can actually see this happen if you open Chrome Dev Console, go to network tab (make sure caching is disabled and logs preserved) and just refresh a page.
That first green bar is the page loading and the second chunk are the scripts, a stylesheet, and some image resources

The HTML document is downloaded first, and only when the browser has finished downloading the HTML document can it find out which scripts to fetch
That said, heavy scripts that don't influence the appearance of the HTML body directly should be loaded at the end of the body and not in the head, so that they do not block the rendering unless necessary

I'm wondering if the javascript are downloaded with the html body during a request
If it's part of that body then yes. If it's in a separate resource then no.
For example, suppose your HTML file has this:
<script type="text/javascript">
$(function () {
// some code here
});
</script>
That code, being part of the HTML file, is included in the HTML resource. The web server doesn't differentiate between what kind of code is in the file, it just serves the response regardless of what's there.
On the other hand, if you have this:
<script type="text/javascript" src="someFile.js"></script>
In that case the code isn't in the same file. The HTML is just referencing a separate resource (someFile.js) which contains the code. In that case the browser would make a separate request for that resource. Resulting in two requests total.

The HTML document is downloaded first, or at least it starts to download first. While it is parsed, any script includes that the browser finds are downloaded. That means that some scripts may finish loading before the document is completely loaded.
While the document is being downloaded, the browser parses it and displays as much as it can. When the parsing comes to a script include, the parsing stops and the browser will suspend it until the script has been loaded and executed, then the parsing continues. That means that
If you put a call to getScript instead of a script include, the behaviour will change. The method makes an asynchronous request, so the browser will continue parsing the rest of the page while the script loads.
This has some important effects:
The parsing of the page will be completed earlier.
Scripts will no longer run in a specific order, they run in the order that the loading completes.
If one script is depending on another, you have to check yourself that the first script has actually loaded before using it in the other script.
You can use a combination of script includes and getScript calls to get the best effect. You can use regular scripts includes for scripts that other scripts depend on, and getScript for scripts that are not affected by the effects of that method.

Browser cache - versioning files - but what if browser uses old version?

Say I have a JavaScript SPA that loads one file X_version1.js into the browser, and that file will be run. What is preventing X_version1.js from accidentally calling a function in X_version0.js that was cached in the browser at an earlier time? Are cached .js scripts not invoked unless they are explicitly invoked and under what circumstances could that happen?
This doesn't seem that far-fetched, because sometimes we use cached versions of jQuery, or whatever, which might not be inside the .js file that is loaded in the most recent server request...

The caching doesn't happen at the method level it happens at the file level.
So if you have
<script src="X_version0.js"></script>
Then the browser goes "ah I've downloaded that before! I'll just return the cached version." But when you change your source to:
<script src="X_version1.js"></script>
The browser hasn't seen that file before and goes and fetches it. At this point nothing in the page says load version0 so the browser doesnt

Caching Javascript inlined in HTML

Instead of having an external .js file, we can inline Javascript directly in HTML, i.e.
Externalized version
<html>
<body>
<script type="text/javascript" src="/app.js"></script>
</body>
</html>
Inlined version
<html>
<body>
<script type="text/javascript">
// app.js inlined
</script>
</body>
</html>
However, it's not recommended:
https://developer.yahoo.com/performance/rules.html#external
Put javascript and css inline in a single minified html file to improve performance?
The main reason is caching and pre-compiling - in the externalized version, the browser can download, pre-compile and store the file once for multiple pages, while it cannot do the same for inlined version.
However, is it possible to do something along these lines:
Inlined keyed version
<html>
<body>
<script type="text/javascript" hash="abc">
// app.js inlined
</script>
</body>
</html>
That is, do this:
In the first invocation, send the whole script and somehow tell the browser that the script hash is abc
Later, when the browser loads that or other pages containing the same script, it will send this key as a cookie. The server will only render the contents of the script if the key has been received.
That is, if the browser already knows about the script, the server will render just this:
Inlined keyed version, subsequent fetches (of the same or other pages)
<html>
<body>
<script type="text/javascript" hash="abc">
</script>
</body>
</html>
where notably the script contents are empty.
This would allow for shorter script fetching with a natural fallback.
Is the above possible? If not, is some other alternative to the above possible?

I don't know of a way to do what you asked, so I'll provide an alternative that might still suit your needs.
If you're really after a low latency first page load, you could inline the script, and then after the page loads, load the script via url so that it's in the browser cache for future requests. Set a cookie once you've loaded the script by direct url, so that your server can determine whether to inline the script or provide the external script url.
first page load
<script>
// inlined my-script.js goes here.
</script>
<script>
$(function(){
// load it again, so it's in the browser cache.
// notice I'm not executing the script, just loading it.
$.ajax("my-script.js").then(function(){
// set a cookie marking this script as cached
});
});
</script>
second page load
<script src="my-script.js"></script>
Obviously, this has the drawback that it loads the script twice. It also adds additional complexity for you to take care of when you update your script with new code - you need to make sure you address the cookie being for a old version.
I wouldn't bother with all this unless you really feel the need to optimize the first page. It might be worth it in your case.

The Concept
Here's an interesting approach (after being bugged by notifications :P)
You could have the server render your script this way. Notice the weird type attribute. That's to prevent the script from executing. We'll get to that in a second.
<script type="text/cacheable" data-hash="9182n30912830192c83012983xm019283x">
//inline script
</script>
Then create a library that looks for these scripts with weird types, get the innerHTML of these scripts, and execute them in the global context as if they were normally executing (via eval or new Function). This makes them execute like normal scripts. Here's a demo:
<script type="text/cacheable" data-hash="9182n30912830192c83012983xm019283x">
alert(a);
</script>
<script type="text/cacheable" data-hash="9182n30912830192c83012983xm019283x">
alert(b);
</script>
<script>
// Let's say we have a global
var a = "foo";
var b = "bar"
// Getting the source
var scripts = Array.prototype.slice.call(
document.querySelectorAll('script[type="text/cacheable"]')
);
scripts.forEach(function(script){
// Grabbing
var source = script.innerHTML;
// Create a function (mind security on this one)
var fn = new Function(source);
// Execute in the global scope
fn.call(window);
});
</script>
However...
Since you have the script source (the innerHTML), you can cache them somewhere locally (like in localStorage) and use the hash as its identifier. Then you can store the same hash in the cookie, where future page-requests can tell the server "Hey, I have cached script with [hash]. Don't print the script on the page anymore". Then you'll get this in future requests:
<script type="text/cacheable" data-hash="9182n30912830192c83012983xm019283x"></script>
That covers up the first half. The second phase is when your library sees an empty script. The other thing your library should do is when it sees an empty script, it should look up for that script with that hash in your local storage, get the script's source and execute it like you just did in the first place.
The Catch
Now there's always a trade-off in everything, and I'll highlight what I can think of here:
Pros
You only need one request for everything. Initial pageload contains scripts, subsequent pages become lighter because of the missing code, which is already cached by then.
Instant cache busting. Assuming the hash and code are 1:1, then changing the content should change the hash.
Cons
This assumes that pages are dynamic and are never cached. That's because if you happen to create a new script, with new hash, but had the client cache the page, then it will still be using the old hashes thus old scripts.
Initial page load will be heavy due to inlined scripts. But this can be overcome by compressing the source using a minifier on the server. Overhead of minification can also be overcome by caching minified results on the server.
Security. You'll be using eval or new Function. This poses a big threat when unauthorized code manages to sneak in. In addition, the threat is persistent because of the caching.
Out of sync pages. What happens if you get an empty script, whose hash is not in the cache? Perhaps the user deleted local storage? You'll have to issue a request to the server for it. Since you want the source, you'll have to have AJAX.
Scripts are not "normal". Your script is best put at the end of the page so that all inline scripts will be parsed by then. This means your scripts execute late and never in the time they get parsed by the browser.
Storage limits. localStorage has a size limit of 5-10MB, depending on which browser we're talking about. Cookies are limited to 4KB generally.
Request size. Note that cookies are shipped up to the server on request and down to the browser on response. That additional load might be more of a hassle than it is for good.
Added server-side logic. Because you need to know what needs to be added, you need to program your server to do it. This makes the client-side implementation dependent on the server. Switching servers (say from PHP to Python) wouldn't be as easy, as you need to port over the implementation.

If your <script> is not introduced as type=text/javascript, it will simply not be executed.
So you could have many tags like theses:
<script type="text/hashedjavascript" hash="abc">...</script>
<script type="text/hashedjavascript" hash="efg">...</script>
Then when the DOM is loaded, pick one and evaluate it.
I made an example here: http://codepen.io/anon/pen/RNGQEM
But it smells, real bad. It's definitely better to fetch two different files.
Actually what you should do, is have a single file my-scripts.js that contains the code for each of your script, wrapped in a function
// file: my-scripts.js
function script_abc(){
// what script abc is supposed to do
}
function script_efg(){
// what script efg is supposed to do
}
Then execute whatever your cookie tells you to. This is how AMD builders concatenate multiples files in one.
Also look for an AMD library such as requirejs
Edit: I misunderstood your question, removed the irrelevant part.

Good or bad idea : load database as a separate .js file

I have a web page where you can customize your game character. In order to speed up browsing (gems) I load entire gems database (600 entries, 247KB) as a separate .js file, so it can be cached and I don't need to load it every time.
I don't notice a delay, is it still a bad idea?
Should I ajax-get necessary records on the fly instead?
FYI: I use ASP.NET MVC 2.0, here is loading the script:
<script type="text/javascript" src='./Data.aspx/Gems'></script>
And here is the action:
[OutputCache(Duration = 14400, VaryByParam = null)]
public ActionResult Gems() {...}
EDIT: My main concern is not load time, but memory usage. Is it going to have noticeable impact having excra 250KB of javascript loaded/parsed by browser?

I find it a pretty good idea. Plus, if you ever need to "upgrade" the GEMS database you can just load up the scripts with a version tag like
<script type="text/javascript" src='./Data.aspx/Gems?v=1232'></script>
Where v=123 will force the user to download the new version if required.

I assume the page won't function until the script is fully loaded anyway but to make the page feel faster you should load the javascript at the bottom of the page.

Embedding the data as a script will cause the browser to halt page loading until the script file has been downloaded and parsed.
If you fetch a static script or data file using ajax, the browser should cache it as if it was an inline script, so there isn't any downside to using ajax, and you don't have to worry about slowing the page load.