JavaScript Comments

A comment is a note you write inside your code for humans. The browser ignores it completely — it never runs, never shows on the page, and never affects the result. Comments exist for one reason: so that a person reading the code later — often you, three months later — can understand what the code is doing and why.

Diagram — see how it fits together

There are two kinds of comments in JavaScript. A single-line comment starts with two slashes // and runs to the end of the line. A multi-line comment starts with /* and ends with */, and can cover as many lines as you like. Both are ignored by the browser.

Single-line comments

Use // for a short note on one line — what a variable holds, why a number is what it is, or what the next line does. You can put it on its own line, or at the end of a line of code.

// Price of one cup of Masala Chai
let price = 10;
let total = price * 3; // 3 cups = ₹30

The first comment explains a value. The second comment, at the end of a line, shows the maths. Both are invisible to the visitor.

Multi-line comments

Use /* ... */ when the note needs more than one line — for example, a short description at the top of a file, or an explanation of a tricky piece of logic. Everything between /* and */ is ignored, even if it spans ten lines.

/*
  This function calculates the bill for a chai order.
  It adds a 5% tax and gives a 10% discount on orders above ₹100.
*/
function calculateBill(cups, price) {
  let total = cups * price;
  return total;
}

What makes a good comment

A good comment explains why, not what. The code already shows what is happening — let total = cups * price; is obvious. What is not obvious is why the price is 10, or why the discount only applies above ₹100. Write the reason, not the repetition. And keep comments short: if a comment is longer than the code it explains, the code probably needs rewriting, not more comments.

Common mistakes

• Forgetting the closing */ on a multi-line comment — everything after it becomes a comment, and the code stops working.
• Writing comments that repeat the code — // add 1 to count above count = count + 1; adds nothing. Say why instead.
• Leaving old, wrong comments behind after changing the code — a comment that lies is worse than no comment at all.
• Commenting out code and forgetting about it — delete dead code instead of leaving it commented in the file.

Remember

• // comments out one line; /* ... */ comments out many lines.
• The browser ignores comments completely — they are notes for humans only.
• Good comments explain why, not what.
• Always close a multi-line comment with */.

Example explained

• // Price of one cup of Kulhad Chai — a single-line comment above the variable it describes. It tells the reader what the number 20 means.
• let price = 20; // ₹20 per cup — a comment at the end of a line, adding a small fact about the value on that line.
• /* ... */ around the discount rule — a multi-line comment explaining the shop's rule: orders above ₹100 get 10% off. The rule is business logic, not code logic, so it needs a note.
• let total = cups * price; — no comment needed here. The line is clear on its own, and a comment would only repeat it.
• // Show the final total on the page — a comment before the last line, marking the step where the answer reaches the visitor.