Event Loop

This lesson covers Event Loop. Let us learn this step by step — no rush, no jargon first.

What you will learn

• What event loop means — in normal words, not textbook words
• How it works step by step
• Code you can run today on your laptop
• Where teams use this in real projects

Before you start

• Software: Node.js LTS from nodejs.org, VS Code, and a terminal
• Knowledge: Earlier lessons in this Node.js course
• Previous lesson: Environment Variables — Complete Guide

Explain it simply

The event loop is how Node runs your code, handles async work, and runs callbacks — all on one main thread.

Why developers use this

• Explains async order in interviews
• Helps you avoid blocking the loop
• Core to Node's performance model

How it works (step by step)

1. Your code starts a task (read file, query DB, timer).
2. Node continues other work instead of waiting idle.
3. When the task finishes, your callback, Promise, or await runs.
4. Errors go in catch or .catch() — never ignore them.

Code example — type this yourself

console.log('1');
setTimeout(() => console.log('2'), 0);
Promise.resolve().then(() => console.log('3'));
console.log('4');

Order is 1, 4, 3, 2. Promises (microtasks) run before timers in this simple case.

What each part does

• console.log('1'); — Prints to the terminal — great for learning; use proper logging in production.
• setTimeout(() => console.log('2'), 0); — Prints to the terminal — great for learning; use proper logging in production.
• Promise.resolve().then(() => console.log('3')); — Prints to the terminal — great for learning; use proper logging in production.
• console.log('4'); — Prints to the terminal — great for learning; use proper logging in production.

Real life: where Event Loop shows up

An online store uses Event Loop so hundreds of users can check order status at once. While one request waits for the database, Node handles other users instead of freezing. Start small: one feature working beats a perfect architecture on paper.

Try it yourself — hands-on

1. Run the snippet and note the order
2. Add another Promise.then
3. Read the output and match it to the explanation

Common mistakes (avoid these)

• Assuming setTimeout(fn, 0) runs before Promise.then — usually the opposite.

Interview note

Interviewers often ask: “What is Event Loop?” Answer in one sentence, then give a tiny example you actually ran.

Summary

• Sync code runs first
• Microtasks (Promises) often run before timers
• Never run heavy CPU work on the main thread