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Eric Hu

Core Node.js Modules

Module	Purpose
`fs`	Read, write, and manage files.
`path`	Build and parse file paths cross-platform.
`http`	Create HTTP servers and clients.
`events`	Implement the observer pattern with EventEmitter.
`stream`	Process data in chunks.
`crypto`	Hashing, encryption, and random values.
`os`	Operating system information.
`process`	Current process info and environment variables.
`url`	Parse and format URLs.
`util`	Utilities like promisify and inspect.

`fs`

Work with the file system. Promise-based API lives in fs/promises.

import { readFile, writeFile, mkdir, readdir, stat } from "fs/promises";

Read the contents of a file as a string.

const data = await readFile("config.json", "utf-8");

Write data to a file, creating or overwriting it.

await writeFile("output.txt", "Hello");

Create a new directory, optionally creating parent directories.

// Creates just "uploads"
await mkdir("uploads");
 
// Creates "images", then "2024", then "photos" inside it
await mkdir("images/2024/photos", { recursive: true });

Get a list of files and folders in a directory.

const files = await readdir("./src");

Get metadata about a file or directory.

const stats = await stat("file.txt");
stats.isFile(); // true
stats.size; // bytes

`path`

Build file paths that work on every operating system.

import path from "path";

Join path segments using the correct separator for the current OS.

path.join("/users", "ada", "file.txt");
// "/users/ada/file.txt" on Linux and macOS
// "\users\ada\file.txt" on Windows

Resolve a sequence of paths to an absolute path from the current working directory.

path.resolve("src", "index.js");
// "/home/user/project/src/index.js"

Extract the filename portion from a file path.

path.basename("/users/ada/file.txt");
// "file.txt"

Extract the directory portion from a file path.

path.dirname("/users/ada/file.txt");
// "/users/ada"

Get the file extension from a file path.

path.extname("file.txt");
// ".txt"

Parse a file path into an object with root, dir, base, ext, and name properties.

path.parse("/users/ada/file.txt");
 
// {
//   root: "/",
//   dir: "/users/ada",
//   base: "file.txt",
//   ext: ".txt",
//   name: "file",
// }

`http`

Create a web server without a framework.

import { createServer } from "http";
 
const server = createServer((req, res) => {
  if (req.url === "/api/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("Listening on :3000"));

In practice, use Express or other web frameworks. The http module powers them underneath.

`events`

Implement publish-subscribe with EventEmitter.

import { EventEmitter } from "events";
 
const emitter = new EventEmitter();

emitter.on("order", (order) => console.log("New order:", order.id));

emitter.once("init", () => console.log("Runs once"));

Emit an event to trigger all registered listeners with optional data.

emitter.emit("order", { id: 42 });

Remove a specific listener from an event.

emitter.off("order", handler);

Many core modules (stream, server) extend EventEmitter.

`stream`

Process data in chunks instead of loading everything into memory.

Ideal for large files, network requests, or any scenario where data arrives incrementally.

import { createReadStream } from "fs";
import { Transform } from "stream";

There are four types of streams:

Readable - for reading data
Writable - for writing data
Duplex - for both reading and writing (e.g., network sockets)
Transform - a Duplex that modifies data between reading and writing

Create a readable stream to read data from a source in chunks.

const readable = createReadStream("input.txt");

Create a writable stream to write data to a destination.

import { createWriteStream } from "fs";
const writable = createWriteStream("output.txt");

Create a transform stream that modifies data passing through it.

const upperCase = new Transform({
  transform(chunk, encoding, callback) {
    callback(null, chunk.toString().toUpperCase());
  },
});

Piping is when you connect a readable stream to a writable or transform stream, allowing data to flow automatically.

readable.pipe(upperCase).pipe(writable);

Pipe a readable stream directly to the console output.

readable.pipe(process.stdout);

Use the 'data' event to process chunks individually without piping.

readable.on("data", (chunk) => {
  console.log(`Received ${chunk.length} bytes`);
});
 
readable.on("end", () => {
  console.log("No more data");
});

Handle stream errors to prevent crashes.

readable.on("error", (err) => {
  console.error("Stream error:", err.message);
});

`crypto`

This module provides cryptographic functionality for securing data, hash generation, random value creation, and password hashing.

Create a hash digest of data using a specified algorithm (SHA-256, SHA-512, MD5, etc.).

import { createHash } from "crypto";
 
const hash = createHash("sha256").update("data").digest("hex");
// "3a6eb0790f39ac87c94f3856b2dd2c5d110e6811602261a9a923d3bb23adc8b7"

Generate cryptographically secure random bytes for API keys, tokens, or salts.

import { randomBytes } from "crypto";
 
const token = randomBytes(32).toString("hex");
// "f4a8d7c2b0e9f6a1b3c8d4e5f6a7b8c9d0e1f2a3b4c5d6e7f8a9b0c1d2e3f4a5"

Generate a secure random UUID v4 for unique identifiers.

import { randomUUID } from "crypto";
 
const id = randomUUID();
// "1b9d6bcd-bbfd-4b2d-9b5d-ab8dfbbd4bed"

Hash passwords asynchronously with scrypt.

import { randomBytes, scrypt } from "crypto";
 
const salt = randomBytes(16).toString("hex");
scrypt(password, salt, 64, (err, key) => {
  const hashedPassword = key.toString("hex");
});

Hash passwords synchronously with scrypt.

import { scryptSync } from "crypto";
 
const key = scryptSync(password, salt, 64).toString("hex");

Create an HMAC (Hash-based Message Authentication Code) for verifying message integrity and authenticity.

import { createHmac } from "crypto";
const hmac = createHmac("sha256", "secret-key").update("message").digest("hex");

`os`

Get system-level information like CPU, memory, and operating system details.

import os from "os";

Get the number of CPU cores available on the machine.

os.cpus().length; // 8

Get the total system memory in bytes.

os.totalmem(); // 17179869184 (16GB)

Get the amount of free system memory in bytes.

os.freemem(); // 4294967296 (4GB)

Get the machine's hostname.

os.hostname(); // "macbook-pro.local"

Get the operating system platform.

os.platform(); // "linux", "darwin", or "win32"

Get the operating system release version.

os.release(); // "22.6.0"

Get system uptime in seconds.

os.uptime(); // 86400 (1 day)

Get network interface information.

os.networkInterfaces();
// { en0: [ { address: '192.168.1.100', ... } ] }

Get the user's home directory.

os.homedir(); // "/Users/username"

`process`

Access and interact with the current Node.js process, including environment, arguments, and lifecycle events. process is globally available, no import needed.

Access environment variables from the shell.

process.env.NODE_ENV; // "production" or "development"

Read command-line arguments passed to the script.

process.argv; // ["node", "script.js", "--flag", "value"]

Get the current process ID.

process.pid; // 12345

Get the current working directory where the script is executed.

process.cwd(); // "/home/user/project"

Exit the process immediately with an optional status code.

process.exit(1); // 0 = success, non-zero = error

Listen for process events like termination signals.

process.on("SIGTERM", () => {
  console.log("Received SIGTERM, cleaning up...");
  gracefulShutdown();
});

Listen for uncaught exceptions to prevent crashes.

process.on("uncaughtException", (err) => {
  console.error("Unhandled error:", err.message);
});

Get memory usage of the Node.js process.

process.memoryUsage();
// { rss: 50.5 MB, heapTotal: 20 MB, heapUsed: 15 MB, external: 2 MB }

Get the Node.js version.

process.version; // "v22.0.0"

Get the current platform the process is running on.

process.platform; // "linux", "darwin", or "win32"

Set an environment variable for the current process only.

process.env.MY_VAR = "some-value";

Get the time spent in user and system CPU time.

process.cpuUsage();
// { user: 150000, system: 30000 } (microseconds)

Measure the elapsed time between two points.

process.hrtime.bigint(); // Returns high-resolution timestamp in nanoseconds

Handle before-exit events to perform cleanup.

process.on("beforeExit", (code) => {
  console.log("Process about to exit with code:", code);
});

`url`

Parse, construct, and manipulate URLs easily using the built-in URL class.

import { URL } from "url";

Create a URL object from a string to access its components.

const url = new URL("https://example.com/users?page=2");

Get the hostname portion of the URL.

url.hostname; // "example.com"

Get the path portion of the URL.

url.pathname; // "/users"

Access query parameters using the searchParams API.

url.searchParams.get("page"); // "2"

Set or modify query parameters on a URL.

url.searchParams.set("sort", "asc");
url.toString(); // "https://example.com/users?page=2&sort=asc"

Get the full URL as a string.

url.href; // "https://example.com/users?page=2"

Parse legacy URLs using the deprecated url.parse() (prefer the URL class).

import { parse } from "url";
const parsed = parse("https://example.com/path?query=1");
// { protocol: 'https:', hostname: 'example.com', pathname: '/path', ... }

Build a URL from component parts.

const myUrl = new URL("/path", "https://example.com");
// "https://example.com/path"

Get the search parameters as a key-value object.

Object.fromEntries(url.searchParams);
// { page: '2', sort: 'asc' }

`util`

Access a collection of handy utility functions for common tasks like converting callbacks and inspecting objects.

Convert a callback-based function into a promise-based one.

import fs from "fs";
import { promisify } from "util";
 
const readFile = promisify(fs.readFile);
const data = await readFile("file.txt", "utf-8");

Inspect an object for deep, formatted logging (more readable than console.log).

import { inspect } from "util";
inspect(obj, { depth: null, colors: true }); // Full depth with colors

Inspect without formatting limitations to see hidden properties.

console.log(inspect(complexObject, { showHidden: true, depth: null }));

Format a string with placeholders (similar to printf).

import { format } from "util";
format("Hello %s, you have %d messages", "Alice", 5);
// "Hello Alice, you have 5 messages"

Get the type of a value as a normalized string.

import { types } from "util";
types.isDate(new Date()); // true
types.isMap(new Map()); // true
types.isRegExp(/regex/); // true

Parse a string to a JavaScript value (like JSON.parse but more flexible).

import { parseArgs } from "util";
const { values, positionals } = parseArgs({
  args: process.argv,
  options: { verbose: { type: "boolean" } },
});

Create a callback that ignores errors and passes through values.

import { callbackify } from "util";
const fn = callbackify(async () => "result");
// Converts async function to error-first callback style

❤ Like

🔖 Save

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Eric Hu

Core Node.js Modules

Module	Purpose
`fs`	Read, write, and manage files.
`path`	Build and parse file paths cross-platform.
`http`	Create HTTP servers and clients.
`events`	Implement the observer pattern with EventEmitter.
`stream`	Process data in chunks.
`crypto`	Hashing, encryption, and random values.
`os`	Operating system information.
`process`	Current process info and environment variables.
`url`	Parse and format URLs.
`util`	Utilities like promisify and inspect.

`fs`

Work with the file system. Promise-based API lives in fs/promises.

import { readFile, writeFile, mkdir, readdir, stat } from "fs/promises";

Read the contents of a file as a string.

const data = await readFile("config.json", "utf-8");

Write data to a file, creating or overwriting it.

await writeFile("output.txt", "Hello");

Create a new directory, optionally creating parent directories.

// Creates just "uploads"
await mkdir("uploads");
 
// Creates "images", then "2024", then "photos" inside it
await mkdir("images/2024/photos", { recursive: true });

Get a list of files and folders in a directory.

const files = await readdir("./src");

Get metadata about a file or directory.

const stats = await stat("file.txt");
stats.isFile(); // true
stats.size; // bytes

`path`

Build file paths that work on every operating system.

import path from "path";

Join path segments using the correct separator for the current OS.

path.join("/users", "ada", "file.txt");
// "/users/ada/file.txt" on Linux and macOS
// "\users\ada\file.txt" on Windows

Resolve a sequence of paths to an absolute path from the current working directory.

path.resolve("src", "index.js");
// "/home/user/project/src/index.js"

Extract the filename portion from a file path.

path.basename("/users/ada/file.txt");
// "file.txt"

Extract the directory portion from a file path.

path.dirname("/users/ada/file.txt");
// "/users/ada"

Get the file extension from a file path.

path.extname("file.txt");
// ".txt"

Parse a file path into an object with root, dir, base, ext, and name properties.

path.parse("/users/ada/file.txt");
 
// {
//   root: "/",
//   dir: "/users/ada",
//   base: "file.txt",
//   ext: ".txt",
//   name: "file",
// }

`http`

Create a web server without a framework.

import { createServer } from "http";
 
const server = createServer((req, res) => {
  if (req.url === "/api/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("Listening on :3000"));

In practice, use Express or other web frameworks. The http module powers them underneath.

`events`

Implement publish-subscribe with EventEmitter.

import { EventEmitter } from "events";
 
const emitter = new EventEmitter();

emitter.on("order", (order) => console.log("New order:", order.id));

emitter.once("init", () => console.log("Runs once"));

Emit an event to trigger all registered listeners with optional data.

emitter.emit("order", { id: 42 });

Remove a specific listener from an event.

emitter.off("order", handler);

Many core modules (stream, server) extend EventEmitter.

`stream`

Process data in chunks instead of loading everything into memory.

Ideal for large files, network requests, or any scenario where data arrives incrementally.

import { createReadStream } from "fs";
import { Transform } from "stream";

There are four types of streams:

Readable - for reading data
Writable - for writing data
Duplex - for both reading and writing (e.g., network sockets)
Transform - a Duplex that modifies data between reading and writing

Create a readable stream to read data from a source in chunks.

const readable = createReadStream("input.txt");

Create a writable stream to write data to a destination.

import { createWriteStream } from "fs";
const writable = createWriteStream("output.txt");

Create a transform stream that modifies data passing through it.

const upperCase = new Transform({
  transform(chunk, encoding, callback) {
    callback(null, chunk.toString().toUpperCase());
  },
});

Piping is when you connect a readable stream to a writable or transform stream, allowing data to flow automatically.

readable.pipe(upperCase).pipe(writable);

Pipe a readable stream directly to the console output.

readable.pipe(process.stdout);

Use the 'data' event to process chunks individually without piping.

readable.on("data", (chunk) => {
  console.log(`Received ${chunk.length} bytes`);
});
 
readable.on("end", () => {
  console.log("No more data");
});

Handle stream errors to prevent crashes.

readable.on("error", (err) => {
  console.error("Stream error:", err.message);
});

`crypto`

This module provides cryptographic functionality for securing data, hash generation, random value creation, and password hashing.

Create a hash digest of data using a specified algorithm (SHA-256, SHA-512, MD5, etc.).

import { createHash } from "crypto";
 
const hash = createHash("sha256").update("data").digest("hex");
// "3a6eb0790f39ac87c94f3856b2dd2c5d110e6811602261a9a923d3bb23adc8b7"

Generate cryptographically secure random bytes for API keys, tokens, or salts.

import { randomBytes } from "crypto";
 
const token = randomBytes(32).toString("hex");
// "f4a8d7c2b0e9f6a1b3c8d4e5f6a7b8c9d0e1f2a3b4c5d6e7f8a9b0c1d2e3f4a5"

Generate a secure random UUID v4 for unique identifiers.

import { randomUUID } from "crypto";
 
const id = randomUUID();
// "1b9d6bcd-bbfd-4b2d-9b5d-ab8dfbbd4bed"

Hash passwords asynchronously with scrypt.

import { randomBytes, scrypt } from "crypto";
 
const salt = randomBytes(16).toString("hex");
scrypt(password, salt, 64, (err, key) => {
  const hashedPassword = key.toString("hex");
});

Hash passwords synchronously with scrypt.

import { scryptSync } from "crypto";
 
const key = scryptSync(password, salt, 64).toString("hex");

Create an HMAC (Hash-based Message Authentication Code) for verifying message integrity and authenticity.

import { createHmac } from "crypto";
const hmac = createHmac("sha256", "secret-key").update("message").digest("hex");

`os`

Get system-level information like CPU, memory, and operating system details.

import os from "os";

Get the number of CPU cores available on the machine.

os.cpus().length; // 8

Get the total system memory in bytes.

os.totalmem(); // 17179869184 (16GB)

Get the amount of free system memory in bytes.

os.freemem(); // 4294967296 (4GB)

Get the machine's hostname.

os.hostname(); // "macbook-pro.local"

Get the operating system platform.

os.platform(); // "linux", "darwin", or "win32"

Get the operating system release version.

os.release(); // "22.6.0"

Get system uptime in seconds.

os.uptime(); // 86400 (1 day)

Get network interface information.

os.networkInterfaces();
// { en0: [ { address: '192.168.1.100', ... } ] }

Get the user's home directory.

os.homedir(); // "/Users/username"

`process`

Access and interact with the current Node.js process, including environment, arguments, and lifecycle events. process is globally available, no import needed.

Access environment variables from the shell.

process.env.NODE_ENV; // "production" or "development"

Read command-line arguments passed to the script.

process.argv; // ["node", "script.js", "--flag", "value"]

Get the current process ID.

process.pid; // 12345

Get the current working directory where the script is executed.

process.cwd(); // "/home/user/project"

Exit the process immediately with an optional status code.

process.exit(1); // 0 = success, non-zero = error

Listen for process events like termination signals.

process.on("SIGTERM", () => {
  console.log("Received SIGTERM, cleaning up...");
  gracefulShutdown();
});

Listen for uncaught exceptions to prevent crashes.

process.on("uncaughtException", (err) => {
  console.error("Unhandled error:", err.message);
});

Get memory usage of the Node.js process.

process.memoryUsage();
// { rss: 50.5 MB, heapTotal: 20 MB, heapUsed: 15 MB, external: 2 MB }

Get the Node.js version.

process.version; // "v22.0.0"

Get the current platform the process is running on.

process.platform; // "linux", "darwin", or "win32"

Set an environment variable for the current process only.

process.env.MY_VAR = "some-value";

Get the time spent in user and system CPU time.

process.cpuUsage();
// { user: 150000, system: 30000 } (microseconds)

Measure the elapsed time between two points.

process.hrtime.bigint(); // Returns high-resolution timestamp in nanoseconds

Handle before-exit events to perform cleanup.

process.on("beforeExit", (code) => {
  console.log("Process about to exit with code:", code);
});

`url`

Parse, construct, and manipulate URLs easily using the built-in URL class.

import { URL } from "url";

Create a URL object from a string to access its components.

const url = new URL("https://example.com/users?page=2");

Get the hostname portion of the URL.

url.hostname; // "example.com"

Get the path portion of the URL.

url.pathname; // "/users"

Access query parameters using the searchParams API.

url.searchParams.get("page"); // "2"

Set or modify query parameters on a URL.

url.searchParams.set("sort", "asc");
url.toString(); // "https://example.com/users?page=2&sort=asc"

Get the full URL as a string.

url.href; // "https://example.com/users?page=2"

Parse legacy URLs using the deprecated url.parse() (prefer the URL class).

import { parse } from "url";
const parsed = parse("https://example.com/path?query=1");
// { protocol: 'https:', hostname: 'example.com', pathname: '/path', ... }

Build a URL from component parts.

const myUrl = new URL("/path", "https://example.com");
// "https://example.com/path"

Get the search parameters as a key-value object.

Object.fromEntries(url.searchParams);
// { page: '2', sort: 'asc' }

`util`

Access a collection of handy utility functions for common tasks like converting callbacks and inspecting objects.

Convert a callback-based function into a promise-based one.

import fs from "fs";
import { promisify } from "util";
 
const readFile = promisify(fs.readFile);
const data = await readFile("file.txt", "utf-8");

Inspect an object for deep, formatted logging (more readable than console.log).

import { inspect } from "util";
inspect(obj, { depth: null, colors: true }); // Full depth with colors

Inspect without formatting limitations to see hidden properties.

console.log(inspect(complexObject, { showHidden: true, depth: null }));

Format a string with placeholders (similar to printf).

import { format } from "util";
format("Hello %s, you have %d messages", "Alice", 5);
// "Hello Alice, you have 5 messages"

Get the type of a value as a normalized string.

import { types } from "util";
types.isDate(new Date()); // true
types.isMap(new Map()); // true
types.isRegExp(/regex/); // true

Parse a string to a JavaScript value (like JSON.parse but more flexible).

import { parseArgs } from "util";
const { values, positionals } = parseArgs({
  args: process.argv,
  options: { verbose: { type: "boolean" } },
});

Create a callback that ignores errors and passes through values.

import { callbackify } from "util";
const fn = callbackify(async () => "result");
// Converts async function to error-first callback style

Core Node.js Modules

`fs`

`path`

`http`

`events`

`stream`

`crypto`

`os`

`process`

`url`

`util`

Full-Stack AI Developer Roadmap

Core Node.js Modules

`fs`

`path`

`http`

`events`

`stream`

`crypto`

`os`

`process`

`url`

`util`

Full-Stack AI Developer Roadmap