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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.

html5: a good loading approach?

I'm writing my first HTML5 + jquery.mobile web app.
The app is basically a menu which redirects to internal pages (data-role="page") defined in the same index.html. I do not write pages as external files to avoid reloading and rewriting the - substantially - same <head>: I suppose it's faster to jump to an internal tag than loading a new page...
Now, I have a page which needs some specific jquery plugins and some specific css. No other page needs these plugins or css.
Of course I could load these js/css in the main <head> section, but this approach would slow the first page loading, uselessly.
I could solve the problem with CSS, with:
$('head:first').append('<link rel="stylesheet" type="text/css" href="' + file + '" />');
I could even solve the problem with JS, but only for 'standard' JavaScript, with something like:
<script>
$(document).ready(function() {
$('#page-availability').live('pageinit', function () {
$.getScript("js/jqm-datebox.core.js");
$.getScript("js/jqm-datebox.mode.calbox.js");
$.getScript("js/jquery.mobile.datebox.i18n.en.utf8.js");
$('#datepicker').data({
"mode": "calbox",
...
});
...
});
...
});
Unfortunately this approach seems not to work (firebug croaks: "TypeError: a.mobile.datebox is undefined"...) with jquery plugins: it looks like they are not evaluated... (even if they are there, before the end of the <head> section, viewing at the "Generated Source"...).
I'm using Firefox (15) to debug, but I suppose this isn't the point...
Any hint?

The one page approach can be good for mobile if:
You don't have to load too much extra content in order to support all the content the user might show from that one page.
You don't have to load too much code to support all the behaviors.
The typical user actually does go to several different virtual pages so the scheme saves them load time and makes things quicker on subsequent virtual page loads.
Done well, the user gets OK performance on loading the first page and very quick performance when going to the other "embedded" pages that don't have to load new content over the network.
The one page approach is not so good if:
The initial load time is just more than it's worth because of the volume of stuff that must be loaded.
You have to dynamically load content for the sub-pages anyway.
You have SEO issues because the search engine can't really find/properly index all your virtual pages.
So, in the end, it's a real tradeoff and depends very much on how big things are, how many things you're loading and what the actual performance comes out to be. A compact mobile site can serve server-loaded page views from one page to the next pretty quickly if the pages are kept very lightweight and there are very few requests that must be satisfied for each page.
In general, you want to pursue these types of optimizations:
Compress/minify all javascript.
Reduce the number of separate items that must be loaded (stylesheets, javascript files, images).
Reduce the number of sequential things that must be loaded (load one, wait for it to load, load another). Mobile is bad at round-trips and loading lots of resources. It's OK at loading a few things.
Make it easy for the browser to cache javascript files. Use a few common javascript files that each serve the needs of many pages. Loading a little more at the start and then allowing the javascript file to be loaded from cache on all future pages loads is way, way better if the user will be visiting many successive pages on your site. The same is true for external CSS files.
Very very careful of lots of images, even small images. Lots of http requests in order to load a page is bad for load time on mobile and every image you request is an http request (unless it comes from the browser cache).
Make sure your server is configured to maximize browser caching for things that can be effectively cached.
Other things to be aware of:
By default dynamic loading of script files is asynchronous and unordered. If your script files must execute in a specific order, then you will have to either not load them dynamically or you will have to write code (or use a library) that serializes their execution in the desired order.

$.getscript is a shorthand AJAX function, it takes a callback as the second parameter.
Check out the docs:
http://dochub.io/#jquery/jquery.getscript
You could concatenate those scripts and then do your stuff in the callback.

This is not so dissimilar to old Flash asset loading issues.
My strategy for that? load only whats necessary for the initial page view. When its loaded and the page / app is viewable by the user, progressively load all other assets.
If the assets were particularly heavy, then I would disable the link to that specific page until its required assets were loaded.
In this case, you might disable the link to the particular page at the outset, initiate the load of its assets, and when they are ready, enable the link.
Not sure if you're having any syntax issues, but you can certainly just inject a new script element into the head with the correct source, and it will instigate a download (like you are doing with css. But you probably know that ;D )
Cheers

I would just combine/minify and compress all the JS in one file and always load that. This is something (with correct caching) which is only downloaded once so you don't have to worry about performance much.
Of course I could load these js/css in the main section
I often just add it just before the </body> and tag. Also note that besides the fact that .live() is deprecated it is also slow as hell. So don't use it, but use .on().

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.

Speed up web site loading

I am looking for the best way to speed up the load time of my js.
The problem is that I am working with a very large site that uses the jquery framework, and what's happening is because the site is also loading, facebook connect, addthis sharing, google analytics and another tracking code, the jquery is delayed a few seconds, and certain elements like the calendar just appear, and my users are complaining that things take to long.
I did a test in google chrome and the avg load time is 4s. Which is too much.
I am already doing minification, and the jquery UI/ Jquery is being loaded from google. What's the best way to approach this?

Make fewer http calls by combining images and script and css, and also use a Content Delivery Network for you static images and css might help!

You are not likely to be able to do much more about the load time of the external scripts, so what you can do is to change the order that things happen in the page so that the external scripts are loaded after you have initialised the page.
Scripts are loaded and executed in a serial manner, so if you change their order in the source code, you also change the order they are loaded.
Instead of using the ready event in jQuery, you can put your initialising code inline in the page, after all the content but before the body closing tag. That way the elements that you want to access are loaded when the script runs, and you can put external scripts below the initialising code to make them load after.

Small technical changes (such as serving the JSs from Google, minifying, etc..) will only get you so far.
Seems you simply have lots of dynamic stuff going on in your page. Have you though of an asynchronous way of building your content? One option might be to place placeholders instead of the external content, and asynchronously load it, so when all the scripts are loaded and ready, all you need to do is throw the markup into the placeholder.
This will create a better user experience, instead of the user waiting 10 seconds for the entire page, it will start loading incrementally after 2 seconds (and still fully load after 10).

In addition to Yuval's answer some options that might or might not bring you a speed gain:
the load time of external libraries is something beyond your control. Try to include them as late as possible, and better still, dynamically after the page has loaded. This way your page won't stall, if Google analytics or Facebook have another hickup.
It is not necessarily faster to load jQuery from Google. Consider putting jQuery, jQuery UI and as many of your own JS as reasonable in a single file, minify and gzip it and let the server serve the gzipped version where possible. Note here, that the gain in speed depends largely on what your users cache and where they cache it. If they already have jQuery from Google in their cache, this technique might make page load slower.
The bottomline is, that after some optimization you're out for experimenting. You must find out, what your average user has in her cache, if the page is accessed directly via deep links or if you can smuggle some JS or CSS (or even images) into her cache via a previous "landing page".

Make sure you deliver your content in gzip/deflate encrypted format. Combine multiple javascript files into 1 file, which helps to reduce the number of http requests.
P.S. Here's a test tool to check if compression is configured:
http://www.gidnetwork.com/tools/gzip-test.php

Where do you put your javascript?

Do you localize your javascript to the page, or have a master "application.js" or similar?
If it's the latter, what is the best practice to make sure your .js isn't executing on the wrong pages?
EDIT: by javascript I mean custom javascript you write as a developer, not js libraries. I can't imagine anyone would copy/paste the jQuery source into their page but you never know.

Putting all your js in one file can help performance (only one request versus several). And if you're using a content distribution network like Akamai it improves your cache hit ratio. Also, always throw inline js at the very bottom of the page (just above the body tag) because that is executed synchronously and can delay your page from rendering.
And yes, if one of the js files you are using is also hosted at google, make sure to use that one.

Here's my "guidelines". Note that none of these are formal, they just seem like the right thing to do.
All shared JS code lives in the SITE/javascripts directory, but it's loaded in 'tiers'
For site-wide stuff (like jquery, or my site wide application.js), the site wide layout (this would be a master page in ASP.net) includes the file. The script tags go at the top of the page.
There's also 'region-wide' stuff (eg: js code which is only needed in the admin section of the site). These regions either have a common layout (which can then include the script tags) or will render a common partial, and that partial can include the script tags)
For less-shared stuff (say my library that's only needed in a few places) then I put a script tag in those HTML pages individually. The script tags go at the top of the page.
For stuff that's only relevant to the single page, I just write inline javascript. I try to keep it as close to it's "target" as possible. For example, if I have some onclick js for a button, the script tag will go below the button.
For inline JS that doesn't have a target (eg: onload events) it goes at the bottom of the page.
So, how does something get into a localised library, or a site-wide library?.
The first time you need it, write it inline
The next time you need it, pull the inline code up to a localised library
If you're referencing some code in a localized library from (approximately) 3 or more places, pull the code up to a region-wide library
If it's needed from more than one region, pull it up to a site-wide library.
A common complaint about a system such as this, is that you wind up with 10 or 20 small JS files, where 2 or 3 large JS files will perform better from a networking point of view.
However, both rails and ASP.NET have features which handle combining and caching multiple JS files into one or more 'super' js files for production situations.
I'd recommend using features like this rather than compromising the quality/readability of the actual source code.

Yahoo!'s Exceptional Performance Team has some great performance suggestions for JavaScript. Steve Souders used to be on that team (he's now at Google) and he's written some interesting tools that can help you decide where to put JavaScript.

I try to avoid putting javascript functions on the rendered page. In general, I have an application.js (or root.js) that has generic functionality like menu manipulation. If a given page has specific javascript functionality, I'll create a .js file to handle that code and mimic the dir structure on how to get to that file (also using the same name as the rendered file).
In other words, if the rendered page is in public/dir1/dir2/mypage.html, the js file would be in public/js/dir1/dir2/mypage.js. I've found this style works well for me, especially when doing templating on a site. I build the template engine to "autoload" my resources (css and js) by taking the request path and doing some checking for the css and js equivalents in the css and js directories on the root.

Personally, I try to include several Javascript files, sorted by module (like YUI does). But once in a while, when I'm writing essentially a one-liner, I'll put it on the page.

Best practice is probably to put it on Google's servers.
(Depends what you mean by "your" javascript though I suppose :)

This is something I've been wrestling with, too. I've ended up by using my back-end PHP script to intelligently build a list of required JS files based on the content requested by the user.
By organizing my JS files into a repository that contains multiple files organized by purpose (be they general use, focused for a single page, single section, etc) I can use the chain of events that builds the page on the back-end to selectively choose which JS files get included based on need (see example below).
This is after implementing my web app without giving this aspect of the code enough thought. Now, I should also add that the javascript I use enhances but does not form the foundation of my site. If you're using something like SproutCore or Ext I imagine the solution would be somewhat different.
Here's an example for a PHP-driven website:
If your site is divided into sections and one of those sections is calendar. The user navigates to "index.phhp?module=calendar&action=view". If the PHP code is class-based the routing algorithm instantiates the CalendarModule class which is based on 'Module' and has a virtual method 'getJavascript'. This will return those javascript classes that are required to perform the action 'view' on the 'calendar' module. It can also take into account any other special requirements and return js files for those as well. The rendering code can verify that there are no duplicates of js files when the javascript include list is built for the final page. So the getJavascript method returns an array like this
return array('prototype.js','mycalendar.js');
Note that this, or some form of this, is not a new idea. But it took me some time to think it important enough to go to the trouble.

If it's only a few hundred bytes or less, and doesn't need to be used anywhere else, I would probably inline it. The network overhead for another http request will likely outweigh any performance gains that you get by pulling it out of the page.
If it needs to be used in a few places, I would put the function(s) into a common external file, and call it from an inline script as needed.
If you are targeting an iphone, try to keep anything that you want cached under 25k.
No hard and fast rules really, every approach has pros and cons, would strongly recommend you check out the articles that can be found on yahoo's developer section, so you can make informed decisions on a case by case basis.