Synchronous vs asynchronous code in Node.js

Sync vs Async in Node.js

Node.js is single-threaded. Understanding synchronous vs asynchronous execution is fundamental to writing performant, non-blocking Node.js code.

Synchronous (Blocking)

Synchronous code blocks the event loop — nothing else runs until the operation completes.

const fs = require('fs');

// BLOCKING — waits until file is fully read
const data = fs.readFileSync('./file.txt', 'utf8');
console.log(data);
console.log('This runs after file is read');

Problem: If reading a 1GB file, all other requests are blocked!

Asynchronous (Non-blocking)

Async code hands off the operation and continues executing. The callback/Promise runs when the operation completes.

Callbacks (old style)

const fs = require('fs');

fs.readFile('./file.txt', 'utf8', (err, data) => {
  if (err) throw err;
  console.log(data);
});

console.log('This runs BEFORE file is read!');

Promises

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

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

console.log('This runs BEFORE file is read!');

async/await (modern)

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

async function readFile() {
  try {
    const data = await fs.readFile('./file.txt', 'utf8');
    console.log(data);
  } catch (err) {
    console.error(err);
  }
}

readFile();
console.log('This runs BEFORE file is read!');

What Happens Under the Hood

Async I/O request
       ↓
  libuv thread pool (OS handles it)
       ↓
  Event Loop polls for completion
       ↓
  Callback/Promise resolves on main thread

The main thread is never blocked — it keeps processing other events.

Common Async Patterns

Pattern	Pros	Cons
Callbacks	Simple, universal	Callback hell, hard to read
Promises	Chainable, better error handling	Verbose for complex flows
async/await	Readable, linear flow	Needs try/catch

Rule of Thumb

Never use sync I/O in production server code (readFileSync, writeFileSync, etc.)
Use async/await for most use cases
Use Promise.all() to run multiple async operations in parallel

// Parallel execution — much faster!
const [users, posts] = await Promise.all([
  fetchUsers(),
  fetchPosts()
]);

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