Node.js Basics

JavaScript Recap for Node.js

Key Differences from Browser JS:

• No DOM - No document, window, or browser APIs

• Different Global Object - global instead of window

• Module System - CommonJS (require()) by default

• File System Access - Can read/write files

• Built-in Modules - fs, http, path, etc.

ES6+ Features You'll Use Often:

javascript

// Arrow functions
const add = (a, b) => a + b;

// Template literals
console.log(`Server running on port ${PORT}`);

// Destructuring
const { readFile, writeFile } = require('fs');

// Spread/Rest operators
const config = { port: 3000, ...defaultConfig };

// Promises & Async/Await
const fetchData = async () => {
  try {
    const data = await someAsyncOperation();
    return data;
  } catch (error) {
    console.error(error);
  }
};

// ES Modules (alternative to CommonJS)
import express from 'express'; // Instead of: const express = require('express')

Global Objects in Node.js

global Object:

• Similar to window in browsers

• Properties attached to global are available everywhere

• Don't pollute the global namespace!

Important Global Objects & Variables:

1. __dirname - Absolute path to the directory containing the current file

javascript

console.log(__dirname);
// Output: /Users/username/projects/myapp/src

2. __filename - Absolute path to the current file

javascript

console.log(__filename);
// Output: /Users/username/projects/myapp/src/app.js

3. process - Information and control over the current Node.js process

javascript

// Get environment variables
console.log(process.env.NODE_ENV);

// Command line arguments
console.log(process.argv); // ['node', 'app.js', 'arg1', 'arg2']

// Current working directory
console.log(process.cwd()); // Different from __dirname!

// Exit the process
process.exit(0); // 0 = success, non-zero = error

// Standard I/O streams
process.stdin;   // Standard input
process.stdout;  // Standard output
process.stderr;  // Standard error

// Platform information
console.log(process.platform); // 'darwin', 'win32', 'linux'
console.log(process.version);  // Node.js version
console.log(process.pid);      // Process ID

// Memory usage
console.log(process.memoryUsage());

4. console - Similar to browser but with extra methods

javascript

console.log('Standard log');
console.error('Error message');
console.warn('Warning');
console.table([{name: 'John', age: 30}, {name: 'Jane', age: 25}]);
console.time('timer'); console.timeEnd('timer'); // Performance timing

5. module, exports, require - CommonJS module system

javascript

// module.exports and exports reference the same object
exports.myFunction = () => {};
// Equivalent to: module.exports.myFunction = () => {};

// require() caches modules - second call returns cached version
const fs = require('fs');

globalThis (ES2020):

javascript

// Works in both Node.js and browsers
globalThis.setTimeout === setTimeout; // true

The Node.js REPL

REPL = Read-Eval-Print-Loop - Interactive Node.js shell

Starting REPL:

bash

# Just type 'node' without arguments
node

REPL Commands & Features:

javascript

> 2 + 2            // Simple calculations
4

> const name = 'Node.js'  // Variable declaration

> .help             // Show all commands
> .break            // Exit multi-line input
> .clear            // Clear context
> .exit             // Exit REPL (or Ctrl+D twice)
> .save filename.js // Save session to file
> .load filename.js // Load file into REPL
> .editor           // Enter editor mode (Ctrl+D to save, Ctrl+C to cancel)

// Special variable '_' contains last result
> 5 + 5
10
> _ * 2
20

// Multi-line input (automatic on brackets)
> function hello() {
... console.log('Hello!');
... }

Useful REPL Features:

1. Tab Completion - Press Tab for autocomplete

2. Up/Down Arrows - Command history

3. Run JavaScript files from REPL:

javascript

> .load app.js      // Load and execute file

Understanding package.json

The package.json is the manifest file for Node.js projects.

Creating package.json:

bash

npm init              # Interactive setup
npm init -y           # Accept all defaults
npm init --scope=@username  # Create scoped package

Key Fields in package.json:

json

{
  "name": "my-project",
  "version": "1.0.0",
  "description": "A sample Node.js project",
  "main": "index.js",           // Entry point
  
  "scripts": {                  // Custom npm commands
    "start": "node index.js",
    "dev": "nodemon index.js",
    "test": "jest",
    "build": "tsc"
  },
  
  "keywords": ["node", "example"],
  "author": "Your Name",
  "license": "MIT",
  
  "dependencies": {             // Production dependencies
    "express": "^4.18.0",
    "mongoose": "^6.0.0"
  },
  
  "devDependencies": {          // Development dependencies
    "nodemon": "^2.0.0",
    "jest": "^28.0.0"
  },
  
  "engines": {                  // Required Node.js version
    "node": ">=14.0.0",
    "npm": ">=6.0.0"
  },
  
  "repository": {
    "type": "git",
    "url": "https://github.com/username/repo.git"
  },
  
  "bugs": {
    "url": "https://github.com/username/repo/issues"
  },
  
  "homepage": "https://github.com/username/repo#readme"
}

Versioning Syntax (Semantic Versioning):

json

"dependencies": {
  "express": "^4.18.0",    // Accept 4.x.x (major version locked)
  "lodash": "~4.17.21",    // Accept 4.17.x (minor version locked)
  "react": "16.13.1",      // Exact version only
  "moment": "*",           // Any version (dangerous!)
  "axios": "latest"        // Latest version
}

Useful Commands:

bash

npm list              # Show installed dependencies
npm outdated          # Check for outdated packages
npm audit             # Security audit
npm audit fix         # Fix security vulnerabilities
npm version patch     # Bump patch version (1.0.0 → 1.0.1)
npm version minor     # Bump minor version (1.0.0 → 1.1.0)
npm version major     # Bump major version (1.0.0 → 2.0.0)

npm vs npx

npm (Node Package Manager):

• Installs packages globally or locally

• Manages dependencies in node_modules

• Runs scripts defined in package.json

bash

# Package management
npm install express          # Install locally (adds to package.json)
npm install -g nodemon       # Install globally
npm uninstall express        # Remove package
npm update express           # Update package

# Run scripts from package.json
npm start
npm run dev
npm test

npx (Node Package Execute):

• Executes packages without permanent installation

• Runs binaries from locally installed packages

• Downloads and runs packages temporarily

bash

# Run a package without installing it
npx create-react-app my-app          # One-time use
npx jest                             # Run locally installed Jest
npx cowsay "Hello World"             # Fun temporary packages

# Execute specific version
npx express@4.16.0 --version

# Clear npx cache
npx clear-npx-cache

Key Differences:

Practical Examples:

bash

# Before npx: Had to install globally first
npm install -g create-react-app
create-react-app my-app

# With npx: No global installation needed
npx create-react-app my-app

# Run different Node.js versions
npx node@14 --version
npx node@16 script.js

# Execute TypeScript compiler without installing
npx tsc --init

When to Use Which:

• Use npm when you need a package for your project (dependencies)

• Use npx when you need to execute a tool/CLI temporarily

• Use npx for project scaffolding (create-react-app, vite, etc.)

• Use npm for managing project dependencies and scripts

Bonus: Package Lock Files

bash

# package-lock.json or yarn.lock or pnpm-lock.yaml
# These files lock exact dependency versions
# Commit them to version control for reproducible builds

This foundation helps understand how Node.js projects are structured and managed!