Node.js Event Loop

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

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