The Node.js Event Loop
📌 Covered in this chapter:
Call Stack · Event Loop Phases · Microtasks vs Macrotasks · Non-Blocking I/O
A deep look at how the event loop schedules callbacks, microtasks, and macrotasks to make single-threaded Node.js feel concurrent.
1The Node.js Event Loop — What You'll Learn
A deep look at how the event loop schedules callbacks, microtasks, and macrotasks to make single-threaded Node.js feel concurrent.
Here's everything this chapter covers, in the order you'll learn it:
- Synchronous vs asynchronous code
- The call stack
- The event loop and its phases
- The callback queue (macrotasks)
- The microtask queue (Promises)
- Non-blocking I/O
- Blocking code and why to avoid it
- CPU-intensive work and the event loop
- Common event loop mistakes
- Debugging asynchronous code
2Working Example
💻 Example: The Node.js Event Loop
JavaScript
▶ Run in Compiler
console.log("1: Synchronous");
Promise.resolve().then(() => console.log("3: Microtask (Promise)"));
setTimeout(() => console.log("4: Macrotask (setTimeout)"), 0);
console.log("2: Synchronous");3Best Practices & Common Pitfalls
💡 Key things to remember:
- Microtasks (Promise callbacks, queueMicrotask) always run before the next macrotask (setTimeout, setInterval, I/O callbacks), even with a 0ms delay.
- Output order above: 1, 2, 3, 4 — synchronous code first, then all pending microtasks, then macrotasks.
❓ Frequently Asked Questions (FAQ)
Q What's the most important thing to understand about the node.js event loop?
Focus on: Call Stack · Event Loop Phases · Microtasks vs Macrotasks · Non-Blocking I/O. These are the core building blocks this chapter's examples are built around, and they show up repeatedly in later chapters of this course.
Q Do I need external npm packages for the node.js event loop?
Only where explicitly shown in the code examples above (like Express, Zod, or Socket.IO) — otherwise, this chapter relies entirely on Node.js's own built-in capabilities.