npm & Packages

npm (Node Package Manager) gives you access to over 2 million packages. Need to send emails, parse dates, validate forms, or build APIs? There's a package for that.

Initializing a Project

Every Node.js project starts with package.json. Create it:

mkdir my-project
cd my-project
npm init -y

The -y flag accepts all defaults. You'll get:

{
  "name": "my-project",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "keywords": [],
  "author": "",
  "license": "ISC"
}

Installing Packages

Let's install lodash, a popular utility library:

npm install lodash

• lodash was added to node_modules/ folder
• package.json now has a "dependencies" section
• package-lock.json was created (locks exact versions)

{
  "dependencies": {
    "lodash": "^4.17.21"
  }
}

Using an Installed Package

// app.js
const _ = require('lodash');

const numbers = [1, 2, 3, 4, 5];

console.log(_.sum(numbers));           // 15
console.log(_.reverse([...numbers]));  // [5, 4, 3, 2, 1]
console.log(_.chunk(numbers, 2));      // [[1, 2], [3, 4], [5]]

const users = [
  { name: 'Alice', age: 30 },
  { name: 'Bob', age: 25 },
  { name: 'Charlie', age: 35 }
];

console.log(_.maxBy(users, 'age'));    // { name: 'Charlie', age: 35 }

node app.js

15
[ 5, 4, 3, 2, 1 ]
[ [ 1, 2 ], [ 3, 4 ], [ 5 ] ]
{ name: 'Charlie', age: 35 }

dependencies vs devDependencies

# Regular dependency (needed to run your app)
npm install express

# Dev dependency (only needed during development)
npm install -D nodemon

{
  "dependencies": {
    "express": "^4.18.2"
  },
  "devDependencies": {
    "nodemon": "^3.0.1"
  }
}

• Testing (jest, mocha)
• Linting (eslint)
• Build tools (webpack, typescript)
• Development utilities (nodemon)

Common npm Commands

# Install all dependencies from package.json
npm install

# Install a specific package
npm install axios

# Install as dev dependency
npm install -D jest

# Install globally (available everywhere)
npm install -g nodemon

# Uninstall a package
npm uninstall lodash

# Update packages
npm update

# See outdated packages
npm outdated

# Run a script from package.json
npm run start
npm test

npm Scripts

Add custom commands to package.json:

{
  "scripts": {
    "start": "node app.js",
    "dev": "nodemon app.js",
    "test": "jest",
    "lint": "eslint ."
  }
}

Run them:

npm start        # Runs: node app.js
npm run dev      # Runs: nodemon app.js
npm test         # Runs: jest (test is special, no 'run' needed)
npm run lint     # Runs: eslint .

The node_modules Folder

When you npm install, packages go into node_modules/. It gets big:

du -sh node_modules
# 50M    node_modules  (can easily be 200MB+ in real projects)

Create .gitignore:

node_modules/

Anyone who clones your project just runs npm install to recreate it.

package-lock.json

This file locks exact versions:

{
  "packages": {
    "node_modules/lodash": {
      "version": "4.17.21",
      "resolved": "https://registry.npmjs.org/lodash/-/lodash-4.17.21.tgz"
    }
  }
}

Finding Packages

• npmjs.com - Search the registry
• GitHub - Most packages are open source
• bundlephobia.com - Check package size before installing

Try It Yourself

Create a project that uses multiple packages:

mkdir date-project
cd date-project
npm init -y
npm install dayjs chalk

const dayjs = require('dayjs');
const chalk = require('chalk');

const now = dayjs();

console.log(chalk.blue('Current time:'), now.format('HH:mm:ss'));
console.log(chalk.green('Today:'), now.format('MMMM D, YYYY'));
console.log(chalk.yellow('This week:'), 'Week', now.week());
console.log(chalk.red('Days until 2025:'), dayjs('2025-01-01').diff(now, 'day'), 'days');

node app.js

Current time: 14:32:15
Today: January 22, 2026
This week: Week 4
Days until 2025: -386 days

Key Takeaways

• npm init creates package.json
• npm install pkg adds a dependency
• npm install -D pkg adds a dev dependency
• node_modules/ is auto-generated, never commit it
• package-lock.json locks versions, do commit it
• npm scripts let you define custom commands

Next, let's understand the magic behind Node.js - the Event Loop!