Introduction [UPDATE: This post has new 2015 followup] This is about managing dependencies in a well-tested client-side JavaScript codebase, how I got overwhelmed, and how I automated the pieces to make it Just work ™. The JavaScript world has grown a lot of tools for dependency management. I've dipped my toes into it into the past, but didn't really know much about it, especially on the client-side. Now I've done so, and I'm somewhat overwhelmed. I also found some solutions. If you don't feel overwhelmed by JavaScript dependency management yet, this document can help you to become overwhelmed. And then perhaps it can help a little to become less so. [update: I've created a followup to this post further analyzing the various options available]

A Client-side Codebase The JavaScript client-side codebase I work on is fairly large; it's a JavaScript web framework called Obviel. Obviel is large immaterial to this document however, so don't worry about it. I'm going to talk about tools like Grunt and RequirejS and Bower and Buster instead. So far I've managed dependencies for Obviel manually by adding the right <script> tags on web pages. If something needs foo.js and bar.js I need to manually make sure it's being included. Even if bar.js is only needed indirectly by foo.js and not by the code I'm writing myself. Managing <script> tags by hand is doable but annoying. Obviel is extensively unit tested. For the tests the dependency situation is more involved. Obviel uses this nice JavaScript test runner called Buster.JS. It runs on the server and is based on Node.js. It features a command-line test runner that can be hooked up to actual web browsers so you can run a whole set of different tests automatically without having to go clicking about in web browsers. Buster needs to be configured properly so that it knows what dependencies there are to run tests in the browser. This is done in a file called buster.js that you put in your project. You still need to explicitly manage dependencies by hand, though there are a few shortcuts such as including all .js files in a whole directory as dependencies at once.

The Goal I've always been dissatisfied with all this explicit dependency management in Obviel's development. I want to use more fine-grained modules in Obviel. Even it is decently modular already, I want to break up Obviel into smaller modules still so I can more easily manage the code during development. A hash table implementation that can have object keys, for instance, should be in its own module, and not in, say, obviel-session.js . I want smaller modules that do one thing well. Deployment of client-side JavaScript code favors large .js files, but during development I want smaller ones. Moreover, I want everything to Just Work ™. If use a new internal module in my project, or if I start depending on a new external library in my project, I should be able to use them right away without the hassle of config file editing. I should be able to hack on my code and write unit tests and depend on stuff in them without worrying.

My Python Background Over the years, I've worked a lot with Python dependency management; PyPI, distutils, setuptools, pip etc. And build tools like buildout. And of course, the Python import statement. I learned a lot about JavaScript tools, and the Python tools I'm already familiar with helped me put them in the right context. For JavaScript dependency management so far I've piggy-backed on Python's systems through Fanstatic, which I helped create. Fanstatic is a Python HTTP middleware that I helped create. It can automatically insert needed dependencies ( .js , .css , etc) into a web page on the server-side. Using Fanstatic-wrapped Obviel (the js.obviel Python library) works pretty well in a Python project, but it doesn't work for developing Obviel and it won't work for people who don't use Fanstatic or even Python.

Dependency management What do I mean by dependency management anyway? Let's write down a few concepts: a project is the codebase I'm hacking on. It could be an application or a framework or a library. It's in hackable form, checked out from a version control repository such as git. You can check out jQuery or the Linux Kernel or Obviel as a project. A project can typically be used to generate one or more packages.

a module is a source code unit (normally a file) that provides some functionality. It contains things like functions and classes. Usually it exposes an API. A project typically contains multiple modules. So foo.js is a module, and so is foo.py , foo.c , etc.

a module may depend on another module. A module dependency is expressed in the module itself, in source code. In Python and many languages this is done by importing the module by name somehow: import foo The C module system is bolted on through textual inclusion during the compilation phase: #include <stdio.h> In JavaScript there is no native way to express module dependencies, but people have created frameworks for it such as Node's module loading system and RequireJS, which I'll go into later.

a package is a collection of one or more modules that is published somewhere so others may use it (this may be published on the internet, or internal to a project). It has metadata that describes the package, its version number, who wrote it, and what other published packages it depends on. In Python this information is in a file called setup.py , in JavaScript it's... well, it depends. I'll note that Linux distributions also feature packages that can be installed. I'll call these deployment packages. Deployment packages for various reasons are not very convenient to develop against. This is why many languages such as Python or JavaScript or Ruby or Perl have language-specific package systems. I'm focusing on such development-oriented packaging systems here.

a module in package A may depend on another module in package B. In this case package A describes in its metadata that it depends on package B. This is an external dependency on a module in another package.

A package manager is a tool that installs a package into your system so you can start using it. It's distinct from a version control system which installs a project into your system so you can start hacking on it, though package managers can be built on top of version control system software.

A package registry is a system where packages can be registered so that others may find and download them. CPAN is the package registry for Perl code, for instance. Some of these systems allow manual download of packages through a web interface as well as automated downloads; Python's PyPI is the example I'm most familiar with. JavaScript has several package registries. When I develop a project I want to be able to express in my metadata somewhere that it depends on some packages, and in my project's modules I want to express what other modules they depend on. I don't want to have to worry about other config files for this, as that's only more code to maintain and more mistakes to make. And if I check out a project, I want to start hacking on it as soon as I can. To get the project's external dependencies I want to be able to run a command that does that for me. Again, it should Just Work ™.

server-side js packaging I mentioned Buster.JS before. Using Buster.JS and its various plugins for linting (jshint) and code coverage and so on introduced me to the world of npm, the package manager for Node.js. npm is built around the CommonJS specs for JavaScript package management. There's a file called package.json that describes what dependencies packages have on others. Like what setup.py does for Python packages. There's also a registry of published packages for npm; this is like PyPI for Python, CPAN for Perl, etc. npm lets you to download and install packages from this registry, either by hand or by reading a config file. This is much like easy_install or pip for Python. So npm provides the equivalent of what pypi + distutils + distribute + pip is for Python. I haven't really studied npm in detail yet, but it seems nice. npm appears to be more coherent than the Python equivalents, which grew over the years layering on top of each other, sometimes glued together in hacky ways. npm looks cleaner.

server-side imports Unlike Python and many other languages, JavaScript doesn't have a standard way to import modules. So people had to invent some! In Node.JS, import works like this: var otherModule = require('otherModule'); You can then use otherModule like a module, so call functions on it for instance: otherModule.someFunction(); require() doesn't just take names, but paths, such as relative paths, so you can see stuff like this: ``require('../otherModule');`` to get a module from one directory higher up. npm installs modules in a directory called node_modules which is often in the project's home directory. There are some lookup rules I don't quite fully grasp yet for getting modules with require() from other places.

client-side imports But Obviel isn't a server-side JavaScript codebase; it's a client-side one that runs in the browser. So I need dependency management for browser JS. The big fish here is RequireJS. It is a client-side library that implements a spec called Asynchronous Module Definition, or AMD. AMD modules look like this: define(['jquery', 'obviel'], function($, obviel) { ... obviel.view({...}); ... $('.foo').render(blah); ... }); So, a module is wrapped in a function that's passed to define() . AMD also allows this: define(function(require) { var otherModule = require('otherModule'); }); which starts to look a lot like CommonJS, expect this wrapper function around it all. AMD was actually born originally as part of the CommonJS project, but there was some difference of opinion and the AMD folks went off on their own. My understanding is that the main point of contention is that the AMD folks figured an explicit function wrapper was the way to go for client-side code to ensure maximum portability of JavaScript code (no preprocessing of JS code necessary), and because it's good practice anyway on the browser to avoid global variables from leaking out of your code. The CommonJS folks wanted the client-side code to look more like server-side modules. See the Why AMD? document for more on this from the AMD perspective. How does RequireJS know where to find modules? You need to specify where it looks for modules in a config file. In that config file you can map one path to another, so you can tell it where to look for your project's modules, and where jQuery is and so on.

client-side packaging That brings us to client-side packaging. One popular package manager for doing this is called Bower. It introduces this config file called bower.json which is like package.json (or setup.py ) but then for declaring front-end package metadata and dependencies. Bower also introduces its own package registry and you have a command-line tool to install packages, which end up in a components directory, much like the way npm installs server-side packages into node_modules .

Overwhelmed yet? So we have two different ways to define modules, and two different way to do packaging. I am simplifying matters here - CommonJS does offer definitions to transport modules to the client too, and there are ways to manage client-side packages using npm too, and I'm sure other package managers exist too. You may start to agree with me that this is all somewhat overwhelming, especially if you're new to this! But we're not there yet...

bower and RequireJS bower is agnostic as to what's in packages and how they're used; RequireJS is just one possibility. So when I install a package into a project using bower, RequireJS has no clue that it is even there, let alone where to import it from. As a result, I cannot import any modules from that package in my project's modules. I need to tell RequireJS first. So if I install jQuery using bower, I still need to manually tell RequireJS in its config file how to find it: look in components/jquery/jquery.js for it, please. Only after that I can depend on it in my module. This doesn't Just Work ™. I want to install something with bower and start using it right away. We need something to help glue this together.

Grunt To construct complex Python projects I use this system called buildout. It can be used to pull in dependencies, install scripts, and automate all sorts of other tasks too. Buildout is driven from a configuration file - it's kind of like Make and the Makefile configuration file. s So JavaScript has some build automation systems too. One popular one is called Grunt. It takes a config file called Gruntfile.js . It can be extended with plugins which you install with npm. Grunt is pretty useful to automate jobs such as gluing bower and RequireJS together.

grunt-bower-requirejs Using npm, you can install a grunt plugin called grunt-bower-requirejs. You configure it up in Gruntfile.js . Now if you run grunt it will automatically make any dependencies installed using bower available for RequireJS. It does this by manipulating the RequireJS config to tell it where bower-installed packages are. So now (at least after I run grunt ), I can require() whatever bower-installed packages I like from my own JS code. Awesome!

Gluing up Buster We're not all there yet. Remember the test runner I use, Buster? There is already a buster-amd plugin, which is needed to let Buster behave properly around RequireJS. Making this work did take somewhat tricky configuration featuring a pathMapper and a regex I don't quite understand, but okay. There is also already a grunt-buster plugin. This can automatically start a PhantomJS based web browser to run the tests, and then run them, if I type grunt test . Pretty neat! Is this enough to make things Just Work ™? After all I should be able to rely on RequireJS declare the dependencies for my test modules. But no... As mentioned before, Buster actually has a special requirement if you run tests against a web browser: it needs to know what JS resources to publish to the web browser it runs the tests in, so that they are even available for RequireJS at all. It is kind of what Fanstatic does, actually!

grunt-bower-busterjs So now Buster needs to know what client-side packages are installed through Bower too, just like RequireJS. Unfortunately there wasn't a grunt plugin for this yet that I could find. Balazs Ree, my friend from the Zope community who also is doing lots of stuff with JavaScript, suggested creating something like grunt-bower-requirejs to create bower integration for buster. Good idea! It turned out grunt-bower-requirejs was extremely close to what was needed already, so I forked it and hacked it up into grunt-bower-busterjs. When plugged into grunt, this generates a bowerbuster.json file. Following Balazs' advice I then tweaked Buster's buster.js configuration file to load up bowerbuster.json into the test sources list. And then, at last, everything started to Just Work ™!

jspak - a sample project There is a good chance you're now overwhelmed as I was. Hopefully I can help: I've pulled all this together into a sample project called jspak. It integrates bower and buster and grunt and RequireJS and seems to Just Work ™. I will consult it myself when I start the job of converting Obviel to use it but perhaps it's useful for others too.