Modules

One major feature Node introduced to JavaScript was the ability to encapsulate code into separate files using modules. The approach adopted by Node is the CommonJS module pattern.

Warning

Node’s use of modules predates ECMA6’s adoption of modules, and the CommonJS approach Node adopted is fundamentally different than the ECMA6 version. For Node 16 (installed on your Codio Box), ECMA6 modules are an optional feature that has to be enabled with a flag when invoking the node command, i.e.:

$ node --input-type=module <file>

We can also enable ECMA6 module support by using the .mjs file extension, or by setting an option in the package.json file present in our project. See the NodeJS Documentation for more information.

Writing a Module

A module is simply a JavaScript file (typically saved with a js extension). Code within the module is locally scoped to the module; code intended to be accessed outside of the module must be explicitly exported by assigning it to the module.exports parameter. This example exports a function to print “Hello World”:

/* hello.js */

function helloWorld() {
  console.log("Hello World");
}

module.exports = helloWorld;

You can export a value, function, or object from a module. If you don’t specify an export, a module exports undefined.

Loading a Module

CommonJS Modules are loaded using the require() function, which loads the JavaScript in the module and returns any exports from the module. Objects, functions, or even values can be exported, i.e. this example loads our earlier module, assigns the result to the variable greet, and invokes it:

var greet = require('./hello');
greet();

This example will print “Hello World” to the console.

There are three kinds of modules you can load in Node: 1) core libraries, 2) npm packages, and 3) user-authored modules.

Core libraries are part of Node. Examples are the fs (file system) library, the http library, and the crypto library. A full list of libraries can be found in the Node documentation. Node libraries are required using their name as as the argument, i.e. require('fs') will require the filesystem library.

Npm packages are typically open-source libraries created by the community and are downloaded using the Node Package Manager (npm). These are installed either globally, or within a node_modules folder created by npm within your project. Npm modules are required by their package name, i.e. require('webpack') will load the webpack package.

User-written modules are loaded from the filesystem, and are typically located in your project directory or subdirectories within it. They are required using the filepath to their file, i.e. require('./hello.js). It is important to use the period-slash (./) indicating current directory to distinguish files from npm packages.