Node ships with a standard library. fs, path, os and events cover most of what a backend needs before you install a single package.

Learning objectives

• Read, write and inspect files with fs.
• Build safe paths with path instead of string concatenation.
• Read system information with os.
• Emit and listen for custom events with EventEmitter.

1. fs — the file system

Synchronous calls block until the operation finishes. They are fine in scripts and at startup:

const fs = require("node:fs");

fs.writeFileSync("notes.txt", "first line\n", "utf8");
const text = fs.readFileSync("notes.txt", "utf8");
console.log(text);

fs.appendFileSync("notes.txt", "second line\n");
console.log(fs.existsSync("notes.txt")); // true

Asynchronous calls keep the process free while the disk works:

const fs = require("node:fs/promises");

async function main() {
  await fs.writeFile("notes.txt", "hello", "utf8");
  const text = await fs.readFile("notes.txt", "utf8");
  console.log(text);
}

main();

Inside an HTTP server, always use the async version — a synchronous read blocks every other request.

Useful operations:

fs.mkdirSync("data", { recursive: true }); // create folders, ignore if present
fs.readdirSync(".");                        // names in a directory
fs.statSync("notes.txt").size;              // bytes
fs.unlinkSync("notes.txt");                 // delete a file

2. path — never join with "+"

const path = require("node:path");

path.join("content", "posts", "one.md"); // content\posts\one.md on Windows
path.resolve("./data");                  // absolute path from cwd
path.extname("app.js");                  // ".js"
path.basename("/tmp/report.pdf");        // "report.pdf"
path.dirname("/tmp/report.pdf");         // "/tmp"

path.join normalises separators for the current operating system. Manual string concatenation breaks on Windows and on paths that already end with a slash.

__dirname (CommonJS) and import.meta.dirname (ESM) give the folder of the current file — the reliable anchor for reading project files regardless of where the process was started.

3. os — information about the machine

const os = require("node:os");

os.platform();     // "win32", "linux", "darwin"
os.cpus().length;  // logical cores
os.totalmem();     // bytes
os.homedir();      // C:\Users\...

Useful for diagnostics and for choosing sensible defaults (for example, a worker count).

4. events — EventEmitter

Node's core is event-driven. EventEmitter exposes that pattern to your own code:

const { EventEmitter } = require("node:events");

const bus = new EventEmitter();

bus.on("order:created", (order) => {
  console.log(`Send receipt for ${order.id}`);
});

bus.once("ready", () => console.log("Ready once"));

bus.emit("order:created", { id: 42 });
bus.emit("order:created", { id: 43 });

on stays subscribed; once removes itself after the first call. HTTP servers, streams and sockets are all EventEmitters underneath.

Common mistakes

• Forgetting the "utf8" argument and getting a Buffer back.
• Using __dirname inside an ES module ("type": "module") — use import.meta.dirname.
• Blocking the event loop with readFileSync inside a request handler.

Practice

Write the directory report tool in the assignment. It combines fs, path and process.argv into a script you will actually reuse.