Module System in Node.js

The module system in Node.js is fundamental to its design and allows developers to break down their code into manageable, reusable pieces called modules. Each module is a separate file with its own scope.

CommonJS Modules

CommonJS module system allows you to structure and organize your code into separate files, or modules, that can be required and reused across your application.

Defining a CommonJS Module

A module is defined in its own file and makes use of the exports or module.exports objects to expose functions, objects, or values to other files.

// myModule.js
function greet(name) {
  return `Hello, ${name}!`;
}

module.exports = greet;

In this example, greet is a function defined in myModule.js and is exported using module.exports.

tip

module.exports is a shorthand for module.exports a shorthand exports. Both refer to the same object initially. However, if you reassign exports directly, it will break the reference to module.exports.

exports = function () {
  console.log('This will not work as expected');
};

Using a CommonJS Module

To use a module in another file, you use the require function to import it.

// app.js
const greet = require('./myModule');

console.log(greet('Node.js'));

Globals

Node.js has several global objects available in all modules, making it unnecessary to require them explicitly.

global
The global namespace object.

process
Provides information and control over the current Node.js process.

__dirname
The directory name of the current module.

__filename
The file name of the current module.

require
Function to import modules.

module
Reference to the current module.

exports
Shortcut to module.exports.

ES6 Modules

Node.js also supports ES6 (ECMAScript 2015) modules, which use import and export syntax. This requires using the .mjs file extension or setting "type": "module" in the package.json file.

Defining an ES6 Module

// myModule.mjs
export function greet(name) {
  return `Hello, ${name}!`;
}

Using an ES6 Module

// app.mjs
import { greet } from './myModule.mjs';

console.log(greet('Node.js'));

Module Resolution

Core Modules
If the module is a core module, Node.js returns it.

File Modules
If the module is a relative path (e.g., ./module), Node.js resolves it to a file or directory.

Node Modules
If the module is not a core module or relative path, Node.js searches the node_modules directories.