Event Loop Phases

Definition

The specific sequence of queues the Event Loop cycles through in C++ to process different types of asynchronous callbacks.

Explain Like I'm New

The waiter doesn't just randomly grab food from the kitchen. They follow a strict circular path. First, they check Timers (`setTimeout`), then they check network connections (I/O callbacks), then they check `setImmediate` tasks, and finally they handle close events. The loop spins continuously, executing callbacks in their designated phase.

Real World Example

Understanding phases helps you predict exactly why a `setTimeout(cb, 0)` might run AFTER a `fs.readFile()` callback depending on what phase the loop is currently in.

Common Use Cases

•Advanced performance debugging
•Technical architectural interviews

Interactive Example

Console output will appear here...

Interview Questions

basic

What is the first phase of the Event Loop?

intermediate

Which phase handles network requests and file reads?

advanced

What is the difference between the Poll phase and the Check phase?

Flash Cards

Question

First phase?

Click to reveal answer

Answer

The Timers phase. It executes callbacks scheduled by `setTimeout()` and `setInterval()`.

Question

Poll vs Check phase?

Click to reveal answer

Answer

The **Poll** phase retrieves new I/O events (like database responses or incoming HTTP requests). The **Check** phase strictly runs callbacks registered by `setImmediate()`. The loop always goes from Poll -> Check.