2. NPM Packages कैसे इंस्टॉल करें? Modules (ESM vs CommonJS) 2026

┌─────────────────────────────────────────────┐
│                  NPM                        │
├─────────────────────────────────────────────┤
│  1. Website – packages search karne ke liye  │
│     (https://www.npmjs.com)                  │
│                                              │
│  2. CLI – command line tool                  │
│     (npm install, npm start)                 │
│                                              │
│  3. Registry – packages ka database           │
│     (2M+ packages stored here)               │
└─────────────────────────────────────────────┘

# Check if Node.js is installed
node --version
# Output: v24.0.0

# Check if NPM is installed
npm --version
# Output: 10.x.x

# Create project folder
mkdir my-npm-project
cd my-npm-project

# Initialize NPM project
npm init

# OR quick init (skip questions)
npm init -y

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

# Install express (production dependency)
npm install express

# Install nodemon (development dependency)
npm install -D nodemon

# Install globally
npm install -g npm-check-updates

# package.json now has dependencies
{
  "dependencies": {
    "express": "^4.18.2"
  },
  "devDependencies": {
    "nodemon": "^3.0.0"
  }
}

// index.js
const express = require('express');

const app = express();
const PORT = 3000;

app.get('/', (req, res) => {
  res.json({ message: 'Hello from Express!' });
});

app.listen(PORT, () => {
  console.log(`Server running on port ${PORT}`);
});

{
  "name": "ecommerce-api",
  "version": "1.0.0",
  "description": "E-commerce API built with Node.js and Express",
  "main": "server.js",
  "type": "module",
  "scripts": {
    "start": "node server.js",
    "dev": "nodemon server.js",
    "test": "jest",
    "build": "npm run lint && npm run test",
    "lint": "eslint ."
  },
  "dependencies": {
    "express": "^4.18.2",
    "mongoose": "^8.0.0",
    "dotenv": "^16.3.1",
    "bcrypt": "^5.1.1",
    "jsonwebtoken": "^9.0.2"
  },
  "devDependencies": {
    "nodemon": "^3.0.0",
    "jest": "^29.7.0",
    "eslint": "^8.56.0"
  },
  "keywords": ["api", "ecommerce", "nodejs", "express"],
  "author": "Rahul Sharma",
  "license": "MIT",
  "engines": {
    "node": ">=18.0.0",
    "npm": ">=9.0.0"
  }
}

my-project/
├── node_modules/          # 📦 Sab packages yahan install hote hain
│   ├── express/           # Express package
│   │   ├── index.js
│   │   ├── package.json
│   │   └── lib/
│   ├── mongoose/          # Mongoose package
│   ├── dotenv/            # Dotenv package
│   └── ... (hundreds of files)
├── package.json
├── package-lock.json
└── index.js

// package-lock.json (auto-generated)
{
  "name": "my-project",
  "lockfileVersion": 3,
  "requires": true,
  "packages": {
    "node_modules/express": {
      "version": "4.18.2",
      "resolved": "https://registry.npmjs.org/express/-/express-4.18.2.tgz",
      "integrity": "sha512-...",
      "dependencies": {
        "accepts": "~1.3.8",
        "body-parser": "1.20.1"
      }
    }
  }
}

# .gitignore
node_modules/
.env
.DS_Store

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

function subtract(a, b) {
  return a - b;
}

const PI = 3.14159;

// Export multiple things
module.exports = {
  add,
  subtract,
  PI
};

// OR export one by one
// exports.add = add;
// exports.subtract = subtract;

// app.js (CommonJS)
const math = require('./math.js');

console.log(math.add(5, 3));     // 8
console.log(math.subtract(5, 3)); // 2
console.log(math.PI);             // 3.14159

// Destructuring
const { add, subtract, PI } = require('./math.js');
console.log(add(10, 5)); // 15

// Node.js built-in modules – no install needed!
const fs = require('fs');        // File system
const path = require('path');    // Path utilities
const http = require('http');    // HTTP server
const crypto = require('crypto'); // Cryptography
const os = require('os');        // Operating system

console.log(os.platform());      // 'win32' or 'linux'
console.log(path.join('folder', 'file.txt')); // 'folder/file.txt'

// Third-party modules – need npm install first!
const express = require('express');
const mongoose = require('mongoose');
const jwt = require('jsonwebtoken');

// Use them
const app = express();

// package.json
{
  "name": "my-project",
  "type": "module",  // ✅ Enable ESM
  "dependencies": {
    "express": "^4.18.2"
  }
}

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

export const PI = 3.14159;

// math.js (ESM)
// Named exports
export function add(a, b) {
  return a + b;
}

export function subtract(a, b) {
  return a - b;
}

export const PI = 3.14159;

// Default export (only one per module)
export default function multiply(a, b) {
  return a * b;
}

// app.js (ESM)
// Named imports
import { add, subtract, PI } from './math.js';

// Default import
import multiply from './math.js';

// Import everything as object
import * as math from './math.js';

console.log(add(5, 3));        // 8
console.log(multiply(5, 3));   // 15
console.log(math.PI);           // 3.14159

// ESM – dynamic import (lazy loading)
const module = await import('./math.js');
console.log(module.add(5, 3));

// Conditional import
if (condition) {
  const { encrypt } = await import('./crypto.js');
  encrypt(data);
}

// ESM – need import assertion
import data from './config.json' assert { type: 'json' };

console.log(data.apiUrl);

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

// app.cjs
const { add } = require('./math.cjs');
console.log(add(2, 3));

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

// app.mjs
import { add } from './math.mjs';
console.log(add(2, 3));

// CommonJS – __dirname available
console.log(__dirname); // /home/user/project

// ESM – need to create __dirname
import { fileURLToPath } from 'url';
import { dirname } from 'path';

const __filename = fileURLToPath(import.meta.url);
const __dirname = dirname(__filename);

console.log(__dirname); // /home/user/project

npm install -D typescript @types/node
npx tsc --init

{
  "compilerOptions": {
    "target": "ES2020",
    "module": "CommonJS",      // Output CommonJS
    "moduleResolution": "node",
    "outDir": "./dist",
    "rootDir": "./src",
    "strict": true,
    "esModuleInterop": true
  }
}

{
  "compilerOptions": {
    "target": "ES2020",
    "module": "ESNext",         // Output ESM
    "moduleResolution": "bundler",
    "outDir": "./dist",
    "rootDir": "./src",
    "strict": true,
    "esModuleInterop": true,
    "resolveJsonModule": true
  }
}

// src/math.ts (TypeScript)
export function add(a: number, b: number): number {
  return a + b;
}

export const PI = 3.14159;

// src/app.ts
import { add, PI } from './math.js'; // Note: .js extension in import!

console.log(add(5, 3));
console.log(PI);

{
  "scripts": {
    "build": "tsc",
    "start": "node dist/app.js",
    "dev": "tsx watch src/app.ts"
  },
  "devDependencies": {
    "typescript": "^5.3.0",
    "tsx": "^4.6.0",
    "@types/node": "^20.10.0"
  }
}

# Create project
mkdir my-api
cd my-api
npm init -y

# Install all dependencies
npm install express mongoose dotenv bcrypt jsonwebtoken cors helmet morgan joi

# Install dev dependencies
npm install -D nodemon jest eslint prettier

// server.js (ESM style)
import express from 'express';
import mongoose from 'mongoose';
import dotenv from 'dotenv';
import cors from 'cors';
import helmet from 'helmet';
import morgan from 'morgan';
import bcrypt from 'bcrypt';
import jwt from 'jsonwebtoken';

dotenv.config();

const app = express();

// Middleware
app.use(express.json());
app.use(cors());
app.use(helmet());
app.use(morgan('dev'));

const PORT = process.env.PORT || 3000;

app.get('/', (req, res) => {
  res.json({ message: 'API is running!' });
});

app.listen(PORT, () => {
  console.log(`Server running on port ${PORT}`);
});

// ❌ Error: Cannot use import statement outside a module
import express from 'express';

// ✅ Solution – use require
const express = require('express');

// OR set "type": "module" in package.json

// ❌ Error – Node.js thinks it's a core module
const math = require('math.js');

// ✅ Correct – local file needs ./
const math = require('./math.js');

// ❌ Error – ESM needs file extension
import { add } from './math';

// ✅ Correct
import { add } from './math.js';

// ❌ ESM code but package.json has no "type": "module"
import express from 'express'; // Error!

// ✅ Add to package.json
{
  "type": "module"
}

# ❌ Wrong – no such package
npm install bcryptjs-instead

# ✅ Check exact name on npmjs.com first
npm install bcrypt

# ❌ Package installs but not saved
npm install express

# ✅ Save to dependencies
npm install express --save

# ✅ Save to devDependencies
npm install nodemon --save-dev

{
  "scripts": {
    "start": "node server.js",
    "dev": "nodemon server.js",
    "test": "jest",
    "build": "tsc",
    "lint": "eslint .",
    "format": "prettier --write ."
  }
}

import { fileURLToPath } from 'url';
import { dirname } from 'path';
const __dirname = dirname(fileURLToPath(import.meta.url));