Put javascript in one .js file or break it out into multiple .js files?

My web application uses jQuery and some jQuery plugins (e.g. validation, autocomplete). I was wondering if I should stick them into one .js file so that it could be cached more easily, or break them out into separate files and only include the ones I need for a given page.

I should also mention that my concern is not only the time it takes to download the .js files but also how much the page slows down based on the contents of the .js file loaded. For example, adding the autocomplete plugin tends to slow down the response time by 100ms or so from my basic testing even when cached. My guess is that it has to scan through the elements in the DOM which causes this delay.

I think it depends how often they change. Let's take this example:

• JQuery: change once a year
• 3rd party plugins: change every 6 months
• your custom code: change every week

If your custom code represents only 10% of the total code, you don't want the users to download the other 90% every week. You would split in at least 2 js: the JQuery + plugins, and your custom code. Now, if your custom code represents 90% of the full size, it makes more sense to put everything in one file.

When choosing how to combine JS files (and same for CSS), I balance:

• relative size of the file
• number of updates expected

Common but relevant answer:

> It depends on the project.

If you have a fairly limited website where most of the functionality is re-used across multiple sections of the site, it makes sense to put all your script into one file.

In several large web projects I've worked on, however, it has made more sense to put the common site-wide functionality into a single file and put the more section-specific functionality into their own files. (We're talking large script files here, for the behavior of several distinct web apps, all served under the same domain.)

The benefit to splitting up the script into separate files, is that you don't have to serve users unnecessary content and bandwidth that they aren't using. (For example, if they never visit "App A" on the website, they will never need the 100K of script for the "App A" section. But they would need the common site-wide functionality.)

The benefit to keeping the script under one file is simplicity. Fewer hits on the server. Fewer downloads for the user.

As usual, though, YMMV. There's no hard-and-fast rule. Do what makes most sense for your users based on their usage, and based on your project's structure.

If people are going to visit more than one page in your site, it's probably best to put them all in one file so they can be cached. They'll take one hit up front, but that'll be it for the whole time they spend on your site.

At the end of the day it's up to you.

However, the less information that each web page contains, the quicker it will be downloaded by the end-viewer.

If you only include the js files required for each page, it seems more likely that your web site will be more efficient and streamlined

If the files are needed in every page, put them in a single file. This will reduce the number of HTTP request and will improve the response time (for lots of visits).

See Yahoo best practice for other tips

I would pretty much concur with what bigmattyh said, it does depend.

As a general rule, I try to aggregate the script files as much as possible, but if you have some scripts that are only used on a few areas of the site, especially ones that perform large DOM traversals on load, it would make sense to leave those in separate file(s).
e.g. if you only use validation on your contact page, why load it on your home page?

As an aside, you can sometimes sneak these files into interstitial pages, where not much else is going on, so when a user lands on an otherwise quite heavy page that needs it, it should already be cached - use with caution - but can be a handy trick when you have someone benchmarking you.

So, as few script files as possible, within reason.

If you are sending a 100K monolith, but only using 20K of it for 80% of the pages, consider splitting it up.

It depends pretty heavily on the way that users interact with your site.

Some questions for you to consider:

• How important is it that your first page load be very fast?
• Do users typically spend most of their time in distinct sections of the site with subsets of functionality?
• Do you need all of the scripts ready the moment that the page is ready, or can you load some in after the page is loaded by inserting <script> elements into the page?

Having a good idea of how users use your site, and what you want to optimize for is a good idea if you're really looking to push for performance.

However, my default method is to just concatenate and minify all of my javascript into one file. jQuery and jQuery.ui are small and have very low overhead. If the plugins you're using are having a 100ms effect on page load time, then something might be wrong.

A few things to check:

• Is gzipping enabled on your HTTP server?
• Are you generating static files with unique names as part of your deployment?
• Are you serving static files with never ending cache expirations?
• Are you including your CSS at the top of your page, and your scripts at the bottom?
• Is there a better (smaller, faster) jQuery plugin that does the same thing?

I've basically gotten to the point where I reduce an entire web application to 3 files.

• vendor.js
• app.js
• app.css

Vendor is neat, because it has all the styles in it too. I.e. I convert all my vendor CSS into minified css then I convert that to javascript and I include it in the vendor.js file. That's after it's been sass transformed too.

Because my vendor stuff does not update often, once in production it's pretty rare. When it does update I just rename it to something like vendor_1.0.0.js.

Also there are minified versions of those files. In dev I load the unminified versions and in production I load the minified versions.

I use gulp to handle doing all of this. The main plugins that make this possible are....

• gulp-include
• gulp-css2js
• gulp-concat
• gulp-csso
• gulp-html-to-js
• gulp-mode
• gulp-rename
• gulp-uglify
• node-sass-tilde-importer

Now this also includes my images because I use sass and I have a sass function that will compile images into data-urls in my css sheet.

function sassFunctions(options) {
options = options || {};
options.base = options.base || process.cwd();

var fs = require('fs');
var path = require('path');
var types = require('node-sass').types;

var funcs = {};

funcs['inline-image($file)'] = function (file, done) {
    var file = path.resolve(options.base, file.getValue());
    var ext = file.split('.').pop();
    fs.readFile(file, function (err, data) {
        if (err) return done(err);
        data = new Buffer(data);
        data = data.toString('base64');
        data = 'url(data:image/' + ext + ';base64,' + data + ')';
        data = types.String(data);
        done(data);
    });
};

return funcs;
}

So my app.css will have all of my applications images in the css and I can add the image's to any chunk of styles I want. Typically i create classes for the images that are unique and I'll just take stuff with that class if I want it to have that image. I avoid using Image tags completely.

Additionally, use html to js plugin I compile all of my html to the js file into a template object hashed by the path to the html files, i.e. 'html\templates\header.html' and then using something like knockout I can data-bind that html to an element, or multiple elements.

The end result is I can end up with an entire web application that spins up off one "index.html" that doesn't have anything in it but this:

<html>
<head>
    <script src="dst\vendor.js"></script>
    <script src="dst\app.css"></script>
    <script src="dst\app.js"></script>
</head>
<body id="body">
    <xyz-app params="//xyz.com/api/v1"></xyz-app>
    <script>
        ko.applyBindings(document.getTagById("body"));
    </script>
</body>
</html>

This will kick off my component "xyz-app" which is the entire application, and it doesn't have any server side events. It's not running on PHP, DotNet Core MVC, MVC in general or any of that stuff. It's just basic html managed with a build system like Gulp and everything it needs data wise is all rest apis.

• Authentication -> Rest Api
• Products -> Rest Api
• Search -> Google Compute Engine (python apis built to index content coming back from rest apis).

So I never have any html coming back from a server (just static files, which are crazy fast). And there are only 3 files to cache other than index.html itself. Webservers support default documents (index.html) so you'll just see "blah.com" in the url and any query strings or hash fragments used to maintain state (routing etc for bookmarking urls).

Crazy quick, all pending on the JS engine running it.

Search optimization is trickier. It's just a different way of thinking about things. I.e. you have google crawl your apis, not your physical website and you tell google how to get to your website on each result.

So say you have a product page for ABC Thing with a product ID of 129. Google will crawl your products api to walk through all of your products and index them. In there you're api returns a url in the result that tells google how to get to that product on a website. I.e. "http://blah#products/129";.

So when users search for "ABC thing" they see the listing and clicking on it takes them to "http://blah#products/129";.

I think search engines need to start getting smart like this, it's the future imo.

I love building websites like this because it get's rid of all the back end complexity. You don't need RAZOR, or PHP, or Java, or ASPX web forms, or w/e you get rid of those entire stacks.... All you need is a way to write rest apis (WebApi2, Java Spring, or w/e etc etc).

This separates web design into UI Engineering, Backend Engineering, and Design and creates a clean separation between them. You can have a UX team building the entire application and an Architecture team doing all the rest api work, no need for full stack devs this way.

Security isn't a concern either, because you can pass credentials on ajax requests and if your stuff is all on the same domain you can just make your authentication cookie on the root domain and presto (automatic, seamless SSO with all your rest apis).

Not to mention how much simpler server farm setup is. Load balance needs are a lot less. Traffic capabilities a lot higher. It's way easier to cluster rest api servers on a load balancer than entire websites.

Just setup 1 nginx reverse proxy server to serve up your index .html and also direct api requests to one of 4 rest api servers.

• Api Server 1
• Api Server 2
• Api Server 3
• Api Server 4

And your sql boxes (replicated) just get load balanced from the 4 rest api servers (all using SSD's if possible)

• Sql Box 1
• Sql Box 2

All of your servers can be on internal network with no public ips and just make the reverse proxy server public with all requests coming in to it.

You can load balance reverse proxy servers on round robin DNS.

This means you only need 1 SSL cert to since it's one public domain.

If you're using Google Compute Engine for search and seo, that's out in the cloud so nothing to worry about there, just $.

If you like the code in separate files for development you can always write a quick script to concatenate them into a single file before minification.

One big file is better for reducing HTTP requests as other posters have indicated.

I also think you should go the one-file route, as the others have suggested. However, to your point on plugins eating up cycles by merely being included in your large js file:

Before you execute an expensive operation, use some checks to make sure you're even on a page that needs the operations. Perhaps you can detect the presence (or absence) of a dom node before you run the autocomplete plugin, and only initialize the plugin when necessary. There's no need to waste the overhead of dom traversal on pages or sections that will never need certain functionality.

A simple conditional before an expensive code chunk will give you the benefits of both the approaches you are deciding on.

I tried breaking my JS in multiple files and ran into a problem. I had a login form, the code for which (AJAX submission, etc) I put in its own file. When the login was successful, the AJAX callback then called functions to display other page elements. Since these elements were not part of the login process I put their JS code in a separate file. The problem is that JS in one file can't call functions in a second file unless the second file is loaded first (see Stack Overflow Q. 25962958) and so, in my case, the called functions couldn't display the other page elements. There are ways around this loading sequence problem (see Stack Overflow Q. 8996852) but I found it simpler put all the code in one larger file and clearly separate and comment sections of code that would fall into the same functional group e.g. keep the login code separate and clearly commented as the login code.