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All Javascript in one file compressed or multiple files

I am compressing all of my files on the go, so it's easier to update rather than having to decompress. I have around 10 JS files, in total around 2,000 lines maybe more, would it be better to put them all in one file and compress it, would it speed up my website, or should I just leave it in individual files, and compress each?

I'm assuming this is for web development.
If all of your scripts were created by you, and you suspect each script will be needed for every page on your site, you should concat / compress them. The first load will take longer, but the scripts will be cached.
If all of your scripts were authored by you, but each page does not necessarily need all 10 of your scripts, you should consider lazy loading them on demand.
If any of those scripts were not authored by you and can be found on a CDN (like jQuery), then link the scripts to a CDN, as there is a chance users will already have them cached. For the remaining scripts, decide if you should lazy load or concat / compress them all.
What you shouldn't do, however, is load all 10 of your scripts individually on each page. That would just have the user's browser send more requests than needed.
It's all about trade-offs, and there isn't a 100% correct answer. Good luck :)
--edit--
You said "on the go". If the content of your scripts change, then you wouldn't want them to be cached. In that case, lazy-loading would probably be the answer.
--edit 2--
If you haven't already, you should really take a look at using Grunt to concat and minify your js files during development. If you decide to go that route, take a look at grunt-contrib-watch.

It is often a good idea to keep the number of files the browser needs to request to a minimum.
The browser may have to open one connection per HTTP request, but if you put all your JavaScript code in one big file it will only have to do one request and thus only one connection needs to be opened to fetch your js code.

It depends on how much of that 2000 lines you are using for different pages. If the file is like a library of which most is used in your distinct pages, then it might actually make little to no difference in terms of loading speed. But if only a part of that file is being used in each page, I would assume separating would be smart as less will be needed to load per page.

Is it possible to cache an HTML page in a way that it can be requested from multiple URIs?

I'm in the process of building a single-page web application with friendly URLs. As an example, for a chat you could request /chat/roomname or /chat/roomname2 to connect to a different chatroom. Since this is an SPA however, both of those would lead to the same HTML contents.
Is it possible to tell the browser to cache both pages as a single page (as in, going to /chat/roomname would also cache /chat/roomname2 in the browser), or something that would give a similar effect? This way the HTML contents could be large and only have to be loaded once.
Alternatively I could do /#/chat/roomname or similar, though I'd prefer not to if the above is possible.

Separate paths are separate things for the browser. Even the same pages with different query parameters are treated different (hence, the "cache busting" technique). However...
You can:
Off-load all scripts to external scripts so the browser caches them separate from the page.
Do the same with CSS as well.
Keep the page to a bare minimum.
If the resources (scripts, styles, images) won't be modified for a long period of time, you can set a longer expiry for them.
Instead of loading the entire layout using HTML, you can use AJAX to fly in templates. That way, those layout requests get cached as well. Have JS assemble them when they're needed.
In the end, your pages will be devoid of all JS, CSS and markup that can be flown in and assembled using JS. This will make the page lighter than usual. You can take it a step further by minifying the scripts and styles, compressing images, compress the HTML and stuff.

Is it better to load many small JavaScript files or one large JavaScript file?

I have noticed in chrome that if I load an image as a base64 string and then scroll through that part of the page it will slow down.
I have also noticed that when I navigate out of a tab with my Javascript in it and then move back to that tab it will be slow for a few seconds as though V8 is recompiling the js.
There are three options I can think of but I don't know which is best:
load a tiny loading page first and handle subsequent loading eloquently
load one huge js or css file with everything (jquery + my code + etc)
clump certain codes together (use jquery cdn but group my code together)
What is the best way to get your js loaded as quickly and eloquently as possible?

Generally, loading more files incurs more overhead in HTTP than combining them into fewer files. There are ways to combine files for all kinds of content:
For images, use CSS sprites.
For javascript, compile your client-side code and libraries into one file, and minify to reduce size.
For css, you can do something similar to the above. hem compiles stylus into one css file, for example, and this can help organizationally as well.
Additionally, when you concatenate Javascript and CSS, your webserver or reverse proxy can send them in compressed form for faster page loads. This will be more efficient for larger files as there is more to gain from compression.

There are way too many maybes for this to have any guaranteed solutions, but here you go:
1) load CSS at the top -- load it all there, if you're doing a site with multiple pages.
If you're building a one-page application (where you're running galleries and twitter feeds and articles, etc on the same page, and you can open and close different sections), then you can consider loading widget-specific CSS, at the time you're loading your widget (if it's not needed at startup).
Do NOT use #import in your CSS, if you want it to load quickly (you do).
2) load the vast majority of your JS at the bottom of the page.
There is practically nothing that can't be lazy-loaded, or at least can't be initialized at the bottom of the page, after the DOM is ready, and if there really is, serve those as separate files at the top of the page, and consider how you might rewrite to depend on them less.
3) be careful with timers -- especially setInterval... ...you can get your page's performance into a lot of trouble with poorly-managed timers.
4) be even more careful with event-handlers on things like window-scroll, resize, mouse-move or key-down. These things fire many, many times a second, so if you've written fancy programs which depend on them, you need to rethink how you fire the program (ie: don't run it every time something the handler goes off).
5) serving JS files is a trade-off:
Compiling JS takes a while. So if you're loading 40,000 lines in one file, your browser is going to pause for a little while, as it compiles all of that.
If you serve 18 separate files, then you have to make 18 different server calls.
That's not cool, either.
So a good balance is to concatenate files together that you KNOW you're going to need for that page, and then lazy-load anything which is optional on the page (like a widget for adding a comment, or the lightbox widget, etc).
And either lazy-load them after all of the main products are up and running, OR load them at the last possible second (like when a user hits the "add comment" button).
If you need to have 40,000 lines loaded in your app, as soon as it starts, then take the hit, or decide what order you can load each one in, and provide "loading" indicators (which you should be doing on lazy-load always) for each widget until it's ready (loading the JS one at a time).
These are guidelines for getting around general performance issues.
Specifics are hard to answer even when you have the site directly in front of you.
Use the Chrome dev console for profiling information and network performance, and rendering performance, et cetera.

Well there is a very popular concept called concatenation. The idea is to have as few HTTP requests to your server as possible. Because each request means a new connection, for which DNS lookup happens, then handshake is negotiated and then after a few more protocol-based steps, the server sends the requested file as the response.
You can check http-archive for a list of performance best-practices.
So yeah, you should combine all JS files into one (there are certain exceptions, like js at head and js in footer)
This is the answer for your question-title and points 2 & 3.
As for the other part, I am not clear about the scenario you are talking of.

I recently had the same problem, and then I developed and released a JS library (MIT licence) to do this. Basically, you can put all your stuff (js, images, css ...) into a standard tar archive (which you can create server side), and the library reads it and allows you to easily use the files.
You'll find it here : https://github.com/sebcap26/FileLoader.js
It works with all recents browsers and IE >= 10.

The number of files to load has an impact on the load speed of the whole site. I would recommend to pack into a single javascript file all the required functionality for the website to display properly.

Include separate CSS and JS files into the header OR all code in one CSS and one JS file?

I'm trying to optimize my website. I have a few plugins to include (jquery plugins with CSS), and my own javascript code.
Now I have the CSS in separate files for different plugins, as I downloaded them. And if I need one on the actual page, I generate code to include that. The same with the JS files. But when it comes to render a complex page with a lot of stuff, 9 CSS files can be called and 7 or 8 JS files, each of which are HTTP requests.
Should I put the CSS into one big file to reduce the number of included CSS files? But then everything will be interpreted by the browser even if the current page doesn't need so much stuff.
I've thought of a third way: generate CSS and JS files with PHP. Then it'll be always one JS and one CSS file, and only with the things which are needed. Or is it an absurd way?
What do you say, which to use to reduce page load time?

It is better to include all CSS in a file and all JS in a file and the minify them using many online services that minify and compress CSS and Javascript. this will reduce the number of http requests as well as volume of data to be downloaded.
If you generate CSS with php then the CSS and JS should be downloaded with every page and generating them takes some time, but if you pack them in one file it downloads once and the browsers caches it.
if your site has many different sections and packing all css in a file makes a huge file then you can pack CSS in two or three file and in each section load the related one.
reducing number of http request is very important.

I think your last solution is the best one.
Generate one js file and one css file from php, and don't forget to minimized/gziped them :)
Here is a very good article about optimization : http://developer.yahoo.com/performance/rules.html

This depends largely on how your users use your page. If most of the users just view one page then it makes sense to only send them the stuff that they need to display that one page (combining everything into as few requests as possible). On the other hand if most of users view multiple pages then it makes sense to send them more than they need so they will already have the CSS&JS on the next page view. But in this case you have to make sure that you are always generating the same CSS&JS with the same URI, so that the browser will not re-download the same content under a different name. You also have to setup proper HTTP caching.
What I usually do is split JS/CSS in two parts. Every page has a "common.css" and "common.js", which has stuff that every page needs (header/footer/... styles for CSS, and then jquery/common js/... for JS). Then every subpage has it's own JS&CSS that has just the stuff you need for that page (if required).

For me, the best way is somewhere in the middle - for CSS files, you better grab them all, join and compress to one file. For JS code - make for example 3+ files: one with compressed and joined external libs, one with your common stuff, and maybe next files for each bigger section - but I dont think its needed. Maybe splitting your JS code on part needed before user login, and after user login.
Remember to minify and consider asynch loading (with LAB.js for example).
Oh, and this php script... I dont think it is good idea - better use/write some script which joins and minifies your statics on compile (or deploy, or even run by hand), so there is no need to generate everythin over and over again.

Where to place my JS code and where/how to load multiple jQuery plugins?

I have a couple of questions that are somewhat related so I'm posting them all on a single question on SO...
Question 1:
I'm currently doing this Facebook application where I'm using jQuery UI Tabs, there's only 4 where 2 of them are loaded through Ajax. The main page is index.html, this is where the tabs code is placed and for the 2 tabs loaded through Ajax, I have two different files, tab1.html and tab2.html.
Currently, the jQuery tabs initialization and Facebook JavaScript initialization is done on index.html. Both tab1.html and tab2.html have JavaScript code that belongs to those pages. For instance, tab2.html has a form and there's some JS (with jQuery) code to validate the form, this code is irrelevant to tab1.html as the JS code on tab1.html is irrelevant to tab2.html.
My question is, should I keep doing this or maybe aggregate all the JS/jQuery code in index.html, tab1.html and tab2.html in a single global.js file and then include it in index.html?
I though of doing this but there will be irrelevant code loaded if the user never opens tab1 or tab2. The benefit of using a single global.js file is that I could pack/minify the file, which I couldn't do if I included each code block in each respective tabX.html file.
Question 2:
As I'm using jQuery, I'm also using lots of plugins (actually only 3 for now, but that number can grow). Some of them provide a minified JS and I use those when available, when they are not, I use the normal versions of course.
There's also the requests problem. If I have lots of plugins, say 10, it will be 10 requests for those plugins. And there is also the fact that some plugins are used in tab1.html but not on tab2.html and vice-verse.
How should I load all the plugins in a minified/packed version on a single web request? Should I do that manually before publishing my app (packing and merging them into a single file) or could I use the PHP version of Dean Edwards's Packer and pack/merge all plugins on the fly? Would this be a good approach?
Question 3:
If the answer on Q1 was something like "merge all code in a single global.js file", should I include the global.js file in the packing/merging script I described above on Q2?
Doing this would simplify everything. I could have my development environment properly organized with all .js files, for the plugins and the global.js in the appropriate folders without bothering with anything else. The packing/merging should take care of the rest (pull the files from the respective folders, send the respective JS headers and output one single packed .js file).
The one thing that's confusing me the most is that not all plugins are used for every tab, not all code is for every tab too. Still, a chunk of the code is global to every tab and the index. This also simplifies everything as: a) I don't have to worry to add the needed code to each tabX.html file and can I simply look at them as HTML templates and nothing else; b) I don't have to be bothered in including the necessary plugins where I need them as I'm currently using $.getScript() from jQuery to load the plugins I need when and only when I need them, but I'm not sure this is a good approach and the code feels dirty and ugly like this.

Question 1:
Pack them all into a single .js file. This will make maintenance easier, and the tiny bit of overhead for the user loading a little js they they potentially may not use does not matter. I would also let Google load the jQuery library for you and then have all of your js code in a single separate file.
Question 2:
As these plugins don't really change I would manually combine them. Closure Compiler is good at this. When minifying use the highest setting that does not give any warnings.
Question 3:
Yes you will want to minify the global.js
When the browser downloads the global.js it's cached for an amount of time. Thus when you call the entire global.js again on a different page, its not re-downloaded it looks at your local copy first. So you do a little bit more work at first on the initial download, but from then on, it should be quicker.

Generally best practices related to javascript for speeding up website loads are:
Minify all javascript and put all of it into a single file (make as much of your javascript external as possible).
Put javascript at the bottom of the document.
Force web server to assign expiration date in the future and use a timestamped query string to invalidate old versions of javascript files, this will prevent unnecessary requests for your javascript if it has not changed. (ie: in httpd.conf ExpiresByType application/x-javascript "access plus 1 year", in your document: <script type="text/javascript" src="/allmy.js?v=1285877202"></script>)
Configure your web server to gzip all text files.

The main reason why you should keep too much javascript away from tab pages is because it will kill user experience. When a user clicks on a tab for the first time it will grab all the components needed on the fly which makes it kinda sluggish.
You're question is only semi-specific as we don't know a lot of things about your site like exact file sizes, how the modules are really used.
The general idea would be to find balance between modularity and speed.
When you're combining modules together these are the general ideas you should consider:
how often does this module change?
how often is this module used?
how big is this module (filesize)?
Then put the most used, stable codebase and merge it into one. Then you should include the rest site specific functionality on the tab pages.
Also, make sure to load javascript asynchronously as it won't block rendering of the page (and tabs).

Another combined answer:
if adding all the JS together in packed/minifed version generates no more than 30k of file size you're better off combining it. A single extra connection for a file (assuming it's not cached) is worth 10-20k of extra JS download. This has to do with browsers opening and closing connections vs streaming extra 20k on an established connection. The threshold also depends on your user distribution. If you have a lot of dial-up or low bandwidth users your threshold will be smaller.
I typically recommend combining and loading as 1 file unless the library is very obscure and requires a very edge case for it to be triggered on a page. Ex: Hover triggers functionality Y but it's on a feedback widget that gets less than 1% of traffic- don't bother combining.
Minifying and Packing is a little overrated these days. With the vast majority of browsers supporting gZip the amount of data consolidation gZip provides of the file over the wire during browser transmission has virtually the same effect as min/pack. However, there is a small cost on the browser to unpack it. Having said that, it's still good practice to min/pack the code since not all browsers support it, you may not want the file to be gZip enabled, etc.
I've used online packers against 3rd party module and it works fairly well. However, there are times when it can cause an issue so make sure to test your manually packed version before deploying.
Alternate:
If you feel that your users will rest on your index page for longer than 10 seconds you could pre-load the additional libraries separately using Js Loader Prototype pattern.

Steve Souder's Even Faster Websites is a book you should look into.
Firstly one experience slowdowns because whenever an external script is linked the browser waits for the script to download, parse and then execute. After this only it regains processing rest of the request. So to avoid such slow downs one can look at parallely downloading the scripts. Few techniques are Ajax the scripts if the scripts are in the same domain or use Script Dom element or Script in iframe if the scripts are on external domains
Q1 : For me modularising all the content is a better option with respect to further development if the page content has to be changed constantly. Responsiveness is very important for the end user. A small global.js will help in getting the app up and running.Parallely one can download the tabX.html.
Q2: As the jquery plugins rarely change. The plugins for the tabX.html pages can be downloaded parallely and locally cached so when the tabX.html is loaded the required plugins need not be fetched. SO all the plugins required by the main page should be in one single file and the ones used by the tabX.html's should be in different files.
Q3 : its a personal choice here. Do you want it to be developer friendly or user friendly. I bank on user friendliness. Making responsive and efficient apps is our job !!!. All the advantages of packing everything into a singe files is you will have ease in development. Well ugly code begets beautiful apps :). Users are speed-aholics. For eg. when google changed its 10 results per page to 20 they saw a considerable drop in search queries. So my opinion is not to pack all of them into one and load each parallely
some of the techniques and relevant links on testing each:
XHR eval /ajax : http://stevesouders.com/cuzillion/?ex=10009
XHR Injection : http://stevesouders.com/cuzillion/?ex=10015
Script in Iframe : http://stevesouders.com/cuzillion/?ex=10012
Script DOM element : http://stevesouders.com/cuzillion/?ex=10010

Question 1:
The best practice would be to place all js files in a single "global" file. This minimizes your HTTP Requests. Let's say you have 5 plug-ins, this would me you need to do 5 request, wherein if you combine them as one, you only need to request it once. This might be a little bit heavy on the first load, but the next time around this file will be cached by the browser, so..no worries about the size. HOWEVER, be careful about the sequence of the scripts when combining it. (I.E. : JQuery script should be placed first on the js file before JQuery UI's)
http://articles.sitepoint.com/article/web-site-optimization-steps/4
http://code.google.com/speed/page-speed/docs/rtt.html
Question 2:
You can do it manually or automatically.Dean Edward's Packer is a good choice. If you're using ASP.NET, you can check MB Compression Handler, if you're using APACHE with PHP perhaps you can change the configuration of your htaccess to gzip it
Question 3:
It'd be better if you pack the "global" javascript file as well. This could save up bandwidth and save more time to load. You got the point, combining all the js files you need for the site will save you time from including individual scripts.