Core Concepts

Async Programming

Master callbacks, Promises, and async/await for handling asynchronous operations in Node.js.

Asynchronous Node.js

Almost everything in Node.js is asynchronous. Reading files, querying databases, making HTTP requests — they all happen asynchronously.

Evolution of Async Patterns

Callbacks (original): Functions called when operation completes. Can lead to "callback hell"
Promises (ES6): Chainable, more readable
async/await (ES2017): Looks synchronous, is asynchronous — the modern standard

Error Handling

Callbacks: First argument is always an error (err-first pattern)
Promises: Use .catch()
async/await: Use try/catch

Example

nodejs

const fs = require('fs').promises;

// ---- Callbacks (old style) ----
const fsOld = require('fs');
fsOld.readFile('./data.txt', 'utf8', (err, data) => {
  if (err) {
    console.error('Error:', err);
    return;
  }
  console.log(data);
});

// ---- Promises ----
fs.readFile('./data.txt', 'utf8')
  .then(data => console.log(data))
  .catch(err => console.error(err));

// ---- async/await (recommended) ----
async function readData() {
  try {
    const data = await fs.readFile('./data.txt', 'utf8');
    console.log(data);
  } catch (err) {
    console.error('Failed to read file:', err.message);
  }
}

// Parallel async operations
async function fetchMultiple() {
  const [users, products] = await Promise.all([
    fetch('/api/users').then(r => r.json()),
    fetch('/api/products').then(r => r.json()),
  ]);
  return { users, products };
}

readData();

Try it yourself — NODEJS