Writing Comments in JavaScript: Single-Line and Multi-Line Syntax

What Is a Comment?

Acommentis text in your code that JavaScriptcompletely ignoreswhen running your program.

// This entire line is a comment — JavaScript skips it!let price = 500;   // Comments can also sit beside codeconsole.log(price); // Output: 500

When this runs, JavaScript behaves as if the comments don't exist. The output is exactly the same with or without them.

So why bother?Because code has two audiences:

1. 🖥️The computer— needs perfect syntax, zero explanation
2. 🧑Humans(teammates, future you) — need to understandwhythe code exists

Comments serve the second audience. And here's the industry secret:code is read 10x more often than it's written.Comments are a love letter to whoever reads your code next — which is usually you.

1️⃣ Single-Line Comments: //

Two forward slashes turneverything after them on that lineinto a comment:

// Ask the user for their namelet userName = "Priya";
// Calculate the total with 18% taxlet total = 500 + (500 * 0.18);
console.log(total);
// 590

Three Ways to Use Single-Line Comments

① Full-line comment— explains the next block of code:

// Convert the temperature from Celsius to Fahrenheitlet fahrenheit = (celsius * 9/5) + 32;

② End-of-line comment— a quick note beside the code:

const MAX_USERS = 100;
// server limit — do not increase!

③ Stacked comments— multiple//lines for longer explanations:

// Free delivery rules:
// - Order must be ₹500 or more
// - User must be logged in
// - Remote areas are excluded
if (cartTotal >= 500 && isLoggedIn && !isRemote) {
  console.log("Free delivery applied! 🚚");
}

2️⃣ Multi-Line Comments: /* */

For longer explanations, wrap text in/*and*/— everything between them becomes a comment, across as many lines as you want:

/*
  Discount Calculator
  -------------------
  Author:  Your Name
  Created: Lesson 8 practice
  Purpose: Applies member discounts to the cart total
           and logs the final price to the console.
*/let cartTotal = 1000;
let memberDiscount = 0.15;
let finalPrice = cartTotal -(cartTotal * memberDiscount);
console.log(finalPrice);
// 850

Multi-line comments can also sit mid-line:

let price = /* after tax */ 590;
let result = calculate( /* quantity */ 5, /* rate */ 120);

This works, but use it sparingly — it can make lines hard to read.

⚠️ The Classic Multi-Line Trap: No Nesting!

Youcannotput a/* */comment inside another/* */comment:

/*
  Outer comment starts here...
  /* inner comment */ ← ❌ The FIRST */ ends everything! This line is now live code → SyntaxError!*/

JavaScript ends the comment at thefirst*/it finds — everything after breaks. This bites every developer at least once.

3️⃣ JSDoc Comments — The Professional Upgrade

There's a third style you'll see in real projects:JSDoc. It uses/** */(note the double star) to document functions in a structured way:

/**
 * Calculates the final price after applying a discount.
 *
 * @param {number} price    - The original price
 * @param {number} discount - Discount as a decimal (e.g. 0.15 = 15%)
 * @returns {number} The discounted final price
 */function applyDiscount(price, discount) {
  return price -(price * discount);
}

Why this is magic:in VS Code, hover your mouse overapplyDiscountanywhere in your project — a tooltip pops up showing this documentation automatically! 🤯 JSDoc turns your comments into a live manual for your code. You'll use it heavily once we reach the functions lesson.

⌨️ Keyboard Shortcuts: Comment Like a Pro

Nobody types//on every line manually. Memorize these two VS Code shortcuts — they're among the most-used keys in all of programming:

The workflow:select 10 lines of code → pressCtrl + /→all 10 lines instantly commented. Press again → uncommented. This takes 0.2 seconds, and you'll do it hundreds of times a day.

🐛 Commenting Out Code — The Debugging Superpower

Comments aren't just for notes. One of the most practical uses istemporarily disabling codewithout deleting it — called"commenting out."

Testing two different approaches? Comment one out instead of deleting:

// let greeting = "Hello, " + userName + "!";   ← old way, saved for reference
let greeting = `Hello, ${userName}!`;
← new way, currently activeconsole.log(greeting);

Hunting a bug? Disable sections to isolate the problem:

let total = 500;
// total = total + tax;      ← temporarily disabled: is THIS line the bug?
// total = total - discount; ← and this one?
console.log(total);
// 500 — so the bug is in one of the disabled lines

This technique pairs perfectly with theconsole.log()debugging you learned inLesson 2: comment out code, log values, narrow down the problem, restore the code.

⚠️ One rule:"commenting out" is atemporarytool. If code stays commented-out for weeks, delete it — old dead code confuses everyone. (Your Git history remembers it forever anyway.)

When to Comment (And When NOT To)

Here's where beginners go wrong inbothdirections — writing zero comments, or writing a comment for every single line. The professional balance:

✅ DO Comment: The "Why"

script.js

Copy Code Run Online

// We add a 500ms delay because the animation// needs time to finish before the modal appearssetTimeout(openModal, 500);
// Round to 2 decimals — floating point math makes// prices like 19.900000000000002 otherwiseprice = price.toFixed(2);

The code showswhathappens; the comment explainswhy— that's the valuable part.

❌ DON'T Comment: The Obvious

script.js

Copy Code Run Online

let age = 25;
// ❌ BAD: "set age to 25" — the code already says this!
age = age + 1;
// ❌ BAD: "add 1 to age" — yes, we can see that
console.log(age);
// ❌ BAD: "log the age" — congratulations on the obvious

Comments that repeat the code add noise, not value. Worse, they becomelieswhen someone updates the code but forgets the comment.

🏆 The Golden Rule

Good code explains WHAT it does. Good comments explain WHY it does it.

❌ Useless Comment	✅ Useful Comment
// loop through users	// only process the first 100 users — API rate limit
// add tax	// GST is 18% for digital services as of 2024
// check if true	// admin accounts bypass the approval step

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Common Beginner Mistakes (And Fixes)

❌ Mistake	🔍 What Happens	✅ Fix
Using#for comments (Python habit)	SyntaxError	JavaScript uses//—#is Python/shell
Nesting/* */inside/* */	Comment ends early, code breaks	Never nest block comments
Commenting the obvious	Noisy, unmaintainable code	Comment thewhy, not thewhat
Leaving commented-out code forever	Confusing graveyard code	Delete it — Git remembers
Updating code but not the comment	Comment becomes a lie 😱	Update comments with the code, always

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Quick Self-Check: Lesson 8 Quiz 🧠

1. What are the two comment syntaxes in JavaScript?
2. Does JavaScript run commented code?
3. What's the VS Code shortcut to comment/uncomment a line?
4. Why can't you nest/* */comments inside each other?
5. Rewrite this bad comment as a good one:let tax = 0.18; // set tax to 0.18

(Answers: 1.//for single-line and/* */for multi-line · 2. No — comments are completely ignored at runtime · 3. Ctrl + / (Windows) or Cmd + / (Mac) · 4. JavaScript ends the comment at the FIRST*/it finds, leaving the rest as broken live code · 5. Something like:// GST rate for digital services (18% as of 2024)— explains WHY, not WHAT)