Node.js Event Loop

🧠 Node.js Event Loop: Microtasks vs Macrotasks

📦 Example

Let's take a look at this short Node.js script:

console.log("1");

setTimeout(() => console.log("macrotask"), 0);

Promise.resolve().then(() => console.log("microtask"));

console.log("2");

which outputs:

1
2
microtask
macrotask

🔍 Sequence

This example demonstrates how Node.js handles asynchronous code within a synchronous workflow using the event loop, specifically:

• The Call Stack executes synchronous code line by line.
• The Microtask Queue (e.g. Promises, await) runs after the stack is clear but before any macrotasks.
• The Macrotask Queue (e.g. setTimeout, I/O) runs after all microtasks are complete.

⚙️ Step-by-Step Breakdown:

1. console.log("1") → runs immediately.
2. setTimeout(..., 0) → schedules a macrotask.
3. Promise.resolve().then(...) → queues a microtask.
4. console.log("2") → runs next (still in call stack).
5. Microtask (console.log("microtask")) runs.
6. Macrotask (console.log("macrotask")) finally runs.

✅ Key Takeaways

• Node.js is single-threaded but asynchronous.
• Microtasks always run before macrotasks once the call stack is empty.