Core Node.js APIs

Work with essential built-in APIs including file systems, paths, operating-system information, URLs, events, and environment configuration.

File System Operations

The node:fs and node:fs/promises modules provide APIs for reading, writing, updating, deleting, and inspecting files and directories. Prefer asynchronous APIs in server applications when the operation may involve significant I/O.

Synchronous file operations block the JavaScript execution thread and should generally be reserved for startup tasks or scripts where blocking is acceptable.

Reading a File Asynchronously

import { readFile } from 'node:fs/promises';

const data = await readFile('config.json', 'utf8');
console.log(data);

💡 The Promise-based file system API allows the application to read a file without using a callback.

Key Takeaways

Prefer asynchronous file APIs in request-handling code.
Use node:fs/promises for Promise-based file operations.
Avoid blocking file operations inside high-throughput request handlers.

Knowledge Check

Why should synchronous file operations usually be avoided inside HTTP handlers?

Path, URL, and OS APIs

The path module provides platform-aware path manipulation. The URL API handles URL parsing and construction. The os module provides information about the operating system and hardware.

Use path.join() or path.resolve() rather than manually concatenating file paths so applications work correctly across operating systems.

Working with Paths and URLs

import path from 'node:path';

const filePath = path.join(process.cwd(), 'data', 'users.json');
const url = new URL('/users?id=10', '[https://example.com');

console.log(filePath);
console.log(url.searchParams.get('id')](https://example.com'%29;

console.log%28filePath%29;
console.log%28url.searchParams.get%28'id'%29));

💡 The example builds a platform-aware path and reads a query parameter from a URL.

Key Takeaways

Use path instead of manual path-string concatenation.
The URL API provides structured access to URL components.
Platform-aware APIs make applications more portable.

Knowledge Check

Which API is best suited for joining filesystem paths?

EventEmitter

Node.js uses events extensively. The EventEmitter class allows objects to emit named events and allows listeners to subscribe to them.

Event-driven design can decouple producers from consumers, but excessive event usage can make application flow difficult to understand. Use events when they represent meaningful domain or infrastructure events.

Custom EventEmitter

import { EventEmitter } from 'node:events';

const emitter = new EventEmitter();

emitter.on('user.created', (user) => {
  console.log('Created:', user.name);
});

emitter.emit('user.created', { name: 'Suriya' });

💡 A listener subscribes to a custom event, and the application emits that event with a user object.

Key Takeaways

EventEmitter provides publish-subscribe behavior inside a process.
Listeners should be removed when their lifecycle requires it.
Use events carefully to avoid hidden control flow.

Knowledge Check

Which method emits an EventEmitter event?

File System Operations

Path, URL, and OS APIs

EventEmitter

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