Understanding Node.js Single-Threaded Runtime: A Guide for JavaScript Developers

Published on Dec 2025 · 10 min read

In many organizations, software development is often treated as an isolated process:

The goal of this article is to encourage developers to think like server architects, understanding how, where, and when each piece of code executes on the server.

What is Node.js?

Node.js is a runtime environment that executes JavaScript code using the V8 engine (provided by Google). A runtime environment exists while a program is running, sitting between your code and the operating system, providing all the resources needed to execute programs.

What Node.js Provides

Single-Threaded Architecture in Node.js

The V8 engine is single-threaded, meaning it executes one task at a time.

How Requests Flow

When a user visits a website, the request reaches the VPS (server) and is stored in the Kernel TCP buffers. Node.js is loaded into RAM, providing the runtime environment and V8 engine. The request reaches your JavaScript program. Node.js stores events in a queue, and the event loop picks one event at a time in FIFO order.

Event Loop and Asynchronous Execution

“For many asynchronous I/O operations (especially network requests), Node.js uses the OS’s non‑blocking I/O mechanisms, and Libuv monitors their completion. Only certain operations (like filesystem or CPU‑heavy crypto tasks) are offloaded to Libuv’s internal thread pool.” 

console.log("Start");

setTimeout(() => {
    console.log("Async Task Done");
}, 1000);

console.log("End");

// Output:
// Start
// End
// Async Task Done

Here, setTimeout is asynchronous. Node.js hands it to Libuv, which schedules it outside the main thread, then puts the result back in the queue for the event loop to process.

Node.js Queue in RAM

All tasks—whether handled by V8 directly or by workers/Libuv—are added to a Node.js queue in RAM.

This is the core of Node.js’s single-threaded execution model.

Concurrency in Node.js

Even though V8 executes tasks one at a time, Node.js can handle thousands of concurrent operations. Libuv is multi-threaded and manages tasks like:

By default, Libuv uses 4 threads, but this can be configured. No matter how many users connect, CPU cores, or RAM you have, V8 always executes JavaScript code one task at a time. Concurrency comes from Libuv handling I/O in parallel.

Why This Matters

What if Blocking

worker_threads is a built-in module in Node.js

Key points

console.log("Start");

const { Worker } = require('worker_threads');

const worker = new Worker(`
  const { parentPort } = require('worker_threads');
  let sum = 0;
  for (let i = 0; i < 1e8; i++) { sum += i; }
  parentPort.postMessage(sum);
`, { eval: true });

worker.on('message', (result) => console.log('Result from worker:', result));

console.log('Main thread continues...');

In situation like JSON.parse(hugeString), the program will be blocked

Summary