Modules and Packages¶

What Are Modules?¶

Modules are independent blocks of reusable code. In Node.js, every file is treated as a separate module.

There are three types of modules:

Core (built-in)
Custom (user-defined)
Third-party (npm packages)

CommonJS (require/module.exports)¶

Exporting in one file¶

// math.js
function add(a, b) {
  return a + b;
}
module.exports = add;

Importing in another¶

// app.js
const add = require("./math");
console.log(add(2, 3)); // 5

You can export multiple values:

module.exports = {
  add,
  subtract
};

ES Modules (import/export)¶

Supported with .mjs extension or "type": "module" in package.json.

// math.mjs
export function add(a, b) {
  return a + b;
}

// app.mjs
import { add } from './math.mjs';
console.log(add(5, 6));

Built-in Modules¶

Node provides many core modules out of the box. Common ones:

fs – file system
path – file paths
http – create web servers
os – system info
events – event emitter

Example:

const path = require("path");

console.log(__filename);                  // full file path
console.log(path.basename(__filename));  // file name

Creating Custom Modules¶

Every file is its own module. Use module.exports to export, and require() to import.

// logger.js
function log(msg) {
  console.log(`[LOG]: ${msg}`);
}
module.exports = log;

// main.js
const log = require("./logger");
log("App started");

Intro to npm (Node Package Manager)¶

npm lets you install and manage third-party packages.

Check version:

npm -v

Initialize a project:

npm init -y

This creates a package.json file that stores metadata and dependencies.

Installing Packages¶

Install locally:

npm install chalk

Use:

const chalk = require("chalk");
console.log(chalk.green("Success!"));

Install globally:

npm install -g nodemon

Dev Dependencies¶

Install a package for development only:

npm install nodemon --save-dev

package.json Basics¶

{
  "name": "project-name",
  "version": "1.0.0",
  "main": "index.js",
  "scripts": {
    "start": "node index.js",
    "dev": "nodemon index.js"
  },
  "dependencies": {
    "chalk": "^5.0.0"
  },
  "devDependencies": {
    "nodemon": "^2.0.0"
  }
}

package-lock.json¶

Tracks exact versions of all installed packages to ensure consistency across environments.

node_modules¶

Stores all installed packages. Do not push this folder to GitHub — use .gitignore.