Node.js: JavaScript on the Server

// This is valid Node.js — no browser needed
const fs = require("fs");
const path = require("path");

const content = fs.readFileSync("data.txt", "utf8");
console.log(content);

# Run a file
node app.js
node --watch app.js  # auto-restart on changes (Node 18+)

# REPL (interactive shell)
node
> 2 + 2
4
> .exit

# Check version
node --version

// File System
const fs = require("fs");
const { readFileSync, writeFileSync, existsSync } = require("fs");
const fsPromises = require("fs/promises");  // async versions

// Path utilities
const path = require("path");
path.join(__dirname, "data", "users.json");  // cross-platform paths
path.resolve("./config.json");  // absolute path
path.extname("file.jpg");       // ".jpg"
path.basename("/dir/file.txt"); // "file.txt"

// OS information
const os = require("os");
os.platform();    // "win32", "linux", "darwin"
os.cpus().length; // number of CPU cores
os.homedir();     // "/Users/alice"

// Child processes
const { execSync, exec, spawn } = require("child_process");
execSync("git status");  // synchronous

// HTTP server (low-level)
const http = require("http");

// Events
const { EventEmitter } = require("events");

const fs = require("fs/promises");
const path = require("path");

async function readConfig() {
    try {
        const content = await fs.readFile("config.json", "utf8");
        return JSON.parse(content);
    } catch (err) {
        if (err.code === "ENOENT") return {};  // file not found
        throw err;
    }
}

async function writeData(filename, data) {
    await fs.mkdir(path.dirname(filename), { recursive: true });
    await fs.writeFile(filename, JSON.stringify(data, null, 2));
}

// List directory contents
const files = await fs.readdir("./src");
const stats = await fs.stat("./package.json");
stats.isFile();      // true
stats.isDirectory(); // false
stats.size;          // bytes

// Watch for file changes
const watcher = fs.watch("./src", { recursive: true }, (eventType, filename) => {
    console.log(`${eventType}: ${filename}`);
});

// Read environment variables
const PORT = process.env.PORT ?? 3000;
const DB_URL = process.env.DATABASE_URL;
const NODE_ENV = process.env.NODE_ENV ?? "development";
const IS_PROD = NODE_ENV === "production";

// Set in terminal
// PORT=8080 node app.js

// .env file (with dotenv package)
require("dotenv").config();
// or in modern Node.js (--env-file flag):
// node --env-file=.env app.js

// process.env is always a string
const port = parseInt(process.env.PORT ?? "3000", 10);

// Check for required environment variables
function requireEnv(name) {
    const value = process.env[name];
    if (!value) throw new Error(`Environment variable ${name} is required`);
    return value;
}

const apiKey = requireEnv("API_KEY");

// cli.js
const args = process.argv.slice(2);  // remove 'node' and script path

// node cli.js --name Alice --verbose
const flags = {};
for (let i = 0; i < args.length; i++) {
    if (args[i].startsWith("--")) {
        const key = args[i].slice(2);
        flags[key] = args[i + 1] && !args[i + 1].startsWith("--") ? args[++i] : true;
    }
}

console.log(flags.name);     // "Alice"
console.log(flags.verbose);  // true

// Exit codes
process.exit(0);   // success
process.exit(1);   // error

// Clean up on exit
process.on("exit", (code) => {
    console.log(`Exiting with code ${code}`);
});

process.on("SIGINT", () => {
    console.log("\nCtrl+C pressed — shutting down");
    cleanup();
    process.exit(0);
});

const http = require("http");

const server = http.createServer((req, res) => {
    console.log(`${req.method} ${req.url}`);
    
    if (req.url === "/health") {
        res.writeHead(200, { "Content-Type": "application/json" });
        res.end(JSON.stringify({ status: "ok" }));
        return;
    }
    
    res.writeHead(404);
    res.end("Not found");
});

server.listen(3000, () => {
    console.log("Server running at http://localhost:3000");
});

// CommonJS (require/module.exports)
__dirname;   // absolute path of the current directory
__filename;  // absolute path of the current file

// ESM (import/export) — __dirname doesn't exist, use this instead:
import { fileURLToPath } from "url";
import { dirname } from "path";

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