Callbacks

This lesson covers Callbacks. If this feels new, that is normal. We will build up slowly.

What you will learn

• What callbacks 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: Event Loop — Complete Guide

Explain it simply

A callback is a function you pass to another function to run later — when a file is read, a DB query finishes, etc.

Why developers use this

• Original Node style for async code
• Still used in many APIs
• Understanding callbacks helps read older code

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

const fs = require('fs');
fs.readFile('package.json', 'utf8', (err, data) => {
  if (err) return console.error(err);
  console.log(data.slice(0, 80) + '...');
});

Always check err first. The callback runs when the file read completes.

What each part does

• const fs = require('fs'); — Loads a built-in module or package you installed with npm.
• fs.readFile('package.json', 'utf8', (err, data) => { — Line 2: runs as written.
• if (err) return console.error(err); — Line 3: runs as written.
• console.log(data.slice(0, 80) + '...'); — Prints to the terminal — great for learning; use proper logging in production.
• }); — Line 5: runs as written.

Real life: where Callbacks shows up

An online store uses Callbacks 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. Point readFile at a real file in your project
2. Log err when the file is missing
3. Wrap the logic in a named function

Common mistakes (avoid these)

• Ignoring the err parameter — silent failures are hard to debug.

Interview note

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

Summary

• Callbacks run after async work finishes
• First argument is often an error
• Nested callbacks led to "callback hell"