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Same Language, Different Environment

JavaScript in the browser and Node.js share the same core language (ECMAScript), but their environments are completely different:

Browser Environment          Node.js Environment
┌──────────────────┐         ┌──────────────────┐
│ ECMAScript       │         │ ECMAScript       │
│ DOM, BOM, fetch  │         │ fs, http, path   │
│ window, document │         │ global, process  │
│ user interaction │         │ server/CLI ops   │
└──────────────────┘         └──────────────────┘

Global Objects

What Node.js Has That the Browser Doesn’t

1. File System Access

const fs = require('fs');

// Read a file
const data = fs.readFileSync('/etc/hostname', 'utf8');
console.log('Hostname:', data.trim());

// Write a file
fs.writeFileSync('log.txt', 'Server started at ' + new Date());

Security: Browsers can never access your local file system — that would be a massive security risk.

2. Process & System Info

console.log('PID:', process.pid);
console.log('Platform:', process.platform);
console.log('Memory:', process.memoryUsage());
console.log('Args:', process.argv);
console.log('CWD:', process.cwd());

3. Environment Variables

// Set: PORT=3000 node app.js
const port = process.env.PORT || 3000;
console.log('Listening on port', port);

4. HTTP Server (No Browser Required)

const http = require('http');

const server = http.createServer((req, res) => {
  res.end('Hello from Node.js!');
});

server.listen(3000);
// Visit http://localhost:3000

What the Browser Has That Node.js Doesn’t

1. The DOM

// ❌ This does NOT work in Node.js
document.getElementById('app')   // ReferenceError
document.querySelector('div')   // ReferenceError

2. Window & BOM APIs

// ❌ Node.js errors
alert('Hi')       // ReferenceError
confirm('Sure?')  // ReferenceError
localStorage      // ReferenceError

Module Systems

// Node.js — CommonJS
const fs = require('fs');
const path = require('path');

// Node.js — ES Modules (with "type": "module")
import fs from 'fs';
import path from 'path';

Event Loop Differences

Both use the same event loop concept, but Node.js adds extra phases:

Browser Event Loop:        Node.js Event Loop:
┌──────────────┐           ┌──────────────────┐
│ Microtasks   │           │ timers           │
│ (Promises)   │           │ pending callbacks│
│ Render       │           │ idle, prepare    │
│ Macro tasks  │           │ poll             │
└──────────────┘           │ check (setImmediate)│
                           │ close callbacks  │
                           └──────────────────┘

Node.js phases include timers (setTimeout), I/O callbacks, poll (retrieve I/O events), and setImmediate.

Practical Test

Run this file in both environments to see the difference:

// env-test.js
console.log('Global:', typeof globalThis);
console.log('process:', typeof process);
console.log('window:', typeof window);
console.log('document:', typeof document);
console.log('fs:', typeof require);

# Node.js output:
Global: object
process: object
window: undefined
document: undefined
fs: function

In the browser console, window exists but process and require don’t.

Key Takeaways

• Node.js and browser share ECMAScript but have totally different APIs
• Node.js adds fs, process, http, path, crypto for server-side work
• Browsers add DOM, BOM, localStorage for client-side interaction
• Node.js uses CommonJS by default (.require); browsers use ES Modules
• The V8 engine is the same — only the environment APIs differ