Event Loop Deep Dive

Definition

A highly granular look at the Node.js Event Loop phases: Timers, Pending Callbacks, Idle/Prepare, Poll, Check, and Close Callbacks.

Explain Like I'm New

The browser Event Loop is fairly simple. The Node.js Event Loop is a giant factory with 6 different conveyor belts (phases). The manager (Event Loop) walks to belt 1, processes everything, walks to belt 2, processes everything, and repeats the circle. `setTimeout` goes on belt 1. File reading goes on belt 4. `setImmediate` goes on belt 5.

Real World Example

Understanding why `setImmediate` might fire BEFORE a `setTimeout(cb, 0)` depending on whether they were called from the global script or inside an I/O callback.

Common Use Cases

•Node.js server optimization
•Predicting complex async execution order

Terminal Output

bash / terminal

const fs = require('fs'); // 1. Drains first, before the Event Loop even continues process.nextTick(() => console.log('nextTick 1')); // 2. Microtask, drains after nextTick Promise.resolve().then(() => console.log('Promise 1')); // 3. Timers phase setTimeout(() => console.log('setTimeout 1'), 0); // 4. Check phase setImmediate(() => console.log('setImmediate 1')); // Output order (Usually): // nextTick 1 // Promise 1 // setTimeout 1 (Non-deterministic vs immediate in main module) // setImmediate 1

Interview Questions

basic

What are the main phases of the Node.js Event Loop?

intermediate

What is the difference between `process.nextTick()` and `setImmediate()`?

advanced

If you call `setTimeout(cb, 0)` and `setImmediate(cb)` in the main module, which runs first?

Flash Cards

Question

process.nextTick vs setImmediate?

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Answer

`process.nextTick` is NOT part of the Event Loop phases. It is a special microtask queue that fires IMMEDIATELY after the current operation completes, regardless of the Event Loop phase. `setImmediate` is an actual Event Loop phase (the Check phase) that runs after I/O events.

Question

Which runs first in the main module?

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Answer

It is NON-DETERMINISTIC! The performance of the machine dictates it. `setTimeout` has a minimum delay of 1ms. If the Node startup takes more than 1ms to reach the timers phase, the timer fires first. If it reaches it in 0.5ms, it skips the timer phase, hits the Check phase, and `setImmediate` fires first. (If placed inside an I/O callback like `fs.readFile`, `setImmediate` ALWAYS fires first).