Your First Program · Lesson 7 of 15

Comments

A free lesson from Swipe Coding · JavaScript, level 2 · updated October 2026

By the Swipe team, founded by Colin · how we make lessons

🤖

Notes to your future self are one of the most underrated tools in coding. 📝

A comment is a note written directly in your code that the JavaScript engine completely ignores when it runs. Comments exist purely for people — to explain what a piece of code does, why it's written a certain way, or to leave yourself a reminder.

// This line calculates the total price
let total = price * quantity;

Everything after // on a line is a single-line comment — the engine skips right over it.

That's a single-line comment, made with two forward slashes (//). Everything from // to the end of that line is ignored. For longer notes, or for text that spans several lines, JavaScript also has multi-line comments, which start with /* and end with */.

/*
  This function calculates a discount.
  It takes the original price and a percentage off.
*/

Multi-line comments start with /* and end with */ — everything between them is ignored, no matter how many lines it spans.

🤖

Commenting out code is basically a pause button, not a delete button. ⏸️

Multi-line comments are also handy for temporarily disabling a block of code without deleting it — a trick often called "commenting out" code — which is useful while you're testing or debugging.

Fill in the blank

A single-line comment in JavaScript starts with ____.

Quick check

Which symbols mark the start and end of a multi-line comment in JavaScript?

True or false

The JavaScript engine executes the text inside comments as part of the program.

Good comments explain why code exists or why a choice was made, not just what it does — code that simply restates the obvious ("// add one to x") tends to clutter a file rather than help anyone reading it.

Chapter quiz