Comments in JavaScript

JavaScript comments explain code for people and are ignored by the JavaScript engine. Use // for a single-line comment and /* ... */ for a block comment that can span multiple lines.

Basic syntax:

// Single-line comment
const name = "plus2net";

/*
  Block comment
  across multiple lines
*/
console.log(name);

Single-Line JavaScript Comments Top ↑

Everything after // on that line is treated as a comment, unless those characters are inside a string or another token where they have a different meaning.

// Store the initial score
let score = 0;

// Increase it after a correct answer
score += 1;

Comments at the End of a Line Top ↑

A short comment can follow a statement. Use this sparingly; if the explanation is long, put it on its own line.

const timeout = 5000; // milliseconds
const retries = 3;         // maximum retry attempts

Multi-Line or Block Comments Top ↑

Block comments start with /* and end with */. They can span several lines.

/*
  Calculate the final amount after discount.
  The percentage is supplied as a whole number.
*/
function applyDiscount(price, percent) {
  return price * (1 - percent / 100);
}

Can JavaScript Block Comments Be Nested? Top ↑

No. Normal JavaScript block comments do not nest. The first */ closes the comment.

/* Outer comment
   /* Do not start another block comment here */
   // The outer comment has already been closed above.
*/

Using Comments to Temporarily Disable Code Top ↑

Comments can temporarily prevent a small piece of code from running while debugging, but version control is a better long-term record than leaving large amounts of dead code in production files.

const total = 25;
// console.log("Temporary diagnostic:", total);
console.log(total);

Documentation Comments Top ↑

JavaScript itself treats a documentation-style block such as /** ... */ as an ordinary comment. Tools such as JSDoc and editors can interpret structured tags inside it.

/**
 * Add two numbers.
 * @param {number} a First number.
 * @param {number} b Second number.
 * @returns {number} Sum of a and b.
 */
function add(a, b) {
  return a + b;
}

Use documentation comments where they help consumers understand a reusable API; do not document every obvious line.

JavaScript Comments and HTML Comments Are Different Top ↑

Use JavaScript comment syntax inside JavaScript. Do not wrap modern scripts in the old HTML-comment pattern <!-- ... //-->. That pattern belonged to very old browser compatibility techniques and should not be copied into current code.

What Makes a Useful Code Comment? Top ↑

• Explain why a non-obvious decision exists, not merely what readable code already says.
• Document assumptions, edge cases, units, compatibility constraints, or business rules.
• Keep comments synchronized with the code; an outdated comment can be worse than no comment.
• Prefer clearer names and smaller functions before adding comments to explain unnecessarily complicated code.
• Do not put secrets, passwords, tokens, or private information in comments committed to source control.

Common Comment Mistakes Top ↑

• Using HTML comments instead of JavaScript comments inside a script.
• Trying to nest /* ... */ comments.
• Commenting out large obsolete sections indefinitely instead of removing them through version control.
• Writing comments that repeat the statement without adding meaning.
• Allowing a comment to describe behavior the code no longer has.

Related Tutorials Top ↑

Continue with adding JavaScript to HTML, variables, and functions.

Subhendu Mohapatra

Author & Instructor at plus2net

I write and maintain practical tutorials on Python, PHP, SQL, JavaScript, HTML, jQuery, and web development at plus2net. The tutorials focus on clear explanations, working examples, and code that readers can test and adapt while learning.

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