npm_logoAs any one who has created a software system can tell you, after some point you get to a point in time where putting all your code in one big location becomes unmanageable and leads to maintenance issues.  The logical first step is to try to separate code that is related into smaller distinct entities and "include" them in the project in question.  But, what happens if two included file "A" and file "B" both define a "doSomething()" method?  Global Scope is sometimes convenient, but more often a cause for troubles when development and enhancing a product.

Different languages handle these "namespace" collisions in different ways.  Some make them optional, some a bit more strict.  Node avoids potential "global scope" issues by not offering an easy way to accidentally over-stuff the global scope.  This is done in the form of "Modules".

At a high level, modules are just an easy way to group similar code together.  Every file in Node.js is considered a module, although modules can be a bit more complex than a single file.  Node modules allow you to select which functions and variables that are in the file are exposed to the calling application and, consequently, which ones are private.

Modules can be packaged and published to an online repository (most likely the Node Package Manager or npm) without those using the modules having to worry about one module conflicting with another in the project.

CREATING MODULES

For the sake of simplicity, we'll focus the following examples on a single file module solution.  To create a new module, just create a new file with the name of your module as it's file name.  Inside that file,  there is a special object calls "exports".  It is with this object, that you specify what you want exposed to the caller.

/* indexGenerator.js */
var counter = 0;
exports.nextIndex = function() {
  return counter++;
}
exports.reset = function() {
  counter = 0;
}

In this example module titled "indexGenerator", I've exposed the following functions:

  • "nextIndex" that will return the next 0-based index in the set.
  • "reset" that will reset the index to 0

That's it, you've created your first module!  The next step is to figure out how to load it and get access to it's functionality.  That is done with the "require" command.  I went into this in detail in my "L is for Loading" article.  I'll point you to that article on the ins and outs of loading a module.

/* test.js */
var generator = require("./indexGenerator.js");
console.log("INDEX: " + generator.nextIndex() + "\n");
console.log("INDEX: " + generator.nextIndex() + "\n");
console.log("resetting index\n");
generator.reset();
console.log("INDEX: " + generator.nextIndex() + "\n");

will produce the following output

INDEX: 0
INDEX: 1
resetting index
INDEX: 0

MODULE PATTERNS

The above example illustrates how to return a set of independent functions from a module.  More often then not, you will want to package your functions into an object and return that to the caller. There are several methods for returning objects from modules: The Factory and The Constructor models

The Factory Model

With the factory module, a creation function is returned in the "exports" object that, when called, will create a new instance of the object in question.  The following code illustrates this:

/* hello.js */
function helloObject() {
  this.sayHi = function() { return "Hi"; }
  this.sayHowdy = function() { return "Howdy"; }
}
exports.createHello = function() {
  return new helloObject();
}
/* test.js */
var helloFactory = require("./hello.js");
var helloObj = helloFactory.createHello();
console.log("HI: " + helloObj.sayHi());

The benefit of this approach is that you can expose many numbers of factories by adding each of them to the exports object along with all other variables and exported standalone functions.

The Constructor Model

Another way to return an object from a module is to override the exports object with the new object in question.  With this approach, you can only return a single item (in this case an object) from your module.

/* hello.js */
function helloObject() {
  this.sayHi = function() { return "Hi"; }
  this.sayHowdy = function() { return "Howdy"; }
}
module.exports = helloObject;
/* test.js */
var helloClass = require("./hello.js");
var helloObject = new helloClass();
console.log("HI: " + helloObj.sayHi());

MODULE FOLDER FORMAT

As I mentioned above, modules can be more than just a file.  You can create a folder structure which contains meta-data, version information, and the module implementation and test files.

/my-module
  /package.json
  /lib
    /module_impl.js
    /file2.js
    ...

package.json
{
  "name": "my-module",
  "version": "1.0.0",
  "main": "./lib/module_impl.js"
}

More information on the Folder Module and the loader rules can be found in my article on Node.js Loading.

PUBLISHING

So you've written a pretty awesome index generator module and you want to share it with the rest of the world.  The Node Package Manager makes it really simple to do so:
- The first step is to thoroughly test out your module.
- Ensure your package.json file has all the info you want to include (help can be found with the "npm help json" command).
- Create an account in the npm registry servers with the "npm adduser" command.
- run "npm publish" from within the module directory.

Pretty simple isn't it!  If you need to manage your package within the repository, you can do so with the "npm unpublish" command.

For more information on Node.js modules, check out the Modules section of the Node.js API reference.