"Modules" in Node.js are similar to libs in C or JAR files in Java. A module can be a single file (or a collection of files) that you include in your code for additional methods and functionality. Node.js is partially compatible with the CommonJS securable module system, so there's an entire set of specifications on how to create new modules. We won't get into that here.

Instead, here's how you can include a module into your project:

var fooModule = require('foo');

fooModule.someNewFunction();

That's it! You call the global require() function, and assign it to a variable. That variable then contains the exported members of the foo module. This paradigm is essential to developing in Node.js, since all the core libraries are considered modules, and must be assigned in this way.

You can also specify a certain directory to look for a module:

var farAway = require(../../../bar);

Technically, you don't even need to specify the .. directory change. Node.js always looks in the curent directory for a module. If it doesn't find it, it walks up the directory, checking each parent, looking for either a file matching your module's name (bar.js), or, a folder called node_modules that contains your file. If it can't find a file matching your module name by the time it hits the root directory ('/'), Node.js throws an exception.

Now, the fun part: there's an entire ecosystem of passionate third-party and open source developers creating their own modules for Node.js. The Node Package Manager, or npm, is designed to be both a source of finding modules to install, and a way to manage module dependencies within your own projects.

Right after installing Node.js, you'll almost certainly want to install the npm command-line tool for managing packages. Once you've done that, you can start browsing the npm registry to find and install new packages.

Local and global installs

There's a distinction worth pointing out when you first begin installing packages. By default, npm installs any modules you need in the current directory it's called at. Thus, if you're working on a project call theLatestAndGreatest, and you want to install express, the package is only relative to your project. If you decide to work on a new project in a different directory called thisIsIt, you'll need to reinstall express again via npm.

To work around this, you could try installing Node.js packages globally. You can do this by specifying the -g switch on the command prompt. This installs the module in a special location that varies depending on your operating system. Any project you start can then safely require() modules.