JavaScript CommentsGet Certified

Comments are notes that you write inside your code. JavaScript completely ignores them when the program runs. They exist only to help humans understand the code.

Single-line Comments

A single-line comment starts with two forward slashes //. Everything after the // on that line is ignored.

// This is a single-line comment
let name = "Michael"; // You can also put a comment at the end of a line

console.log(name);

Multi-line Comments

Multi-line comments start with /* and end with */. Everything between them is ignored, even if it spans many lines.

/*
  This is a multi-line comment.
  It can span as many lines as you need.
  Useful for longer explanations.
*/
let age = 28;
console.log(age);

Commenting Out Code

A very common use of comments is to temporarily disable a line or block of code without deleting it.

let score = 100;
console.log("Score:", score);

// console.log("This line is temporarily disabled");
/*
console.log("These lines are also disabled");
console.log("They will not run");
*/

Documentation Comments (JSDoc)

JSDoc-style comments start with /** and are used to document functions, parameters, and return values. Many editors use them to provide better autocomplete and tooltips.

/**
 * Adds two numbers together.
 * @param {number} a - The first number.
 * @param {number} b - The second number.
 * @returns {number} The sum of a and b.
 */
function add(a, b) {
    return a + b;
}

console.log(add(5, 3));

Comment Syntax Overview

When Should You Use Comments?

Good comments explain the why behind a piece of code, not just the what. The code itself already shows what is happening; a good comment adds context that isn’t obvious from the code alone.

// Not very useful: restates the obvious
let age = 28; // set age to 28

// More useful: explains the reasoning
let retryLimit = 3; // API is unreliable, so allow up to 3 retries

console.log(age, retryLimit);

Practical Example

Let’s combine different comment styles in a small practical example.

/**
 * Calculates the final price after applying a discount.
 * @param {number} price - The original price.
 * @param {number} discountPercent - Discount percentage (0-100).
 * @returns {number} The discounted price.
 */
function applyDiscount(price, discountPercent) {
    // Convert the percentage into a decimal multiplier
    const multiplier = 1 - (discountPercent / 100);
    return price * multiplier;
}

console.log(applyDiscount(200, 25)); // Expected: 150

Common Mistakes With Comments

• Trying to nest multi-line comments, which causes a syntax error.
• Writing comments that simply repeat what the code already says, adding no real value.
• Leaving outdated comments that no longer match the current code.
• Forgetting to close a multi-line comment with */.
• Overusing comments instead of writing clear, self-explanatory code.

Best Practices for Comments

• Explain why the code does something, not just what it does.
• Keep comments short and to the point.
• Update or remove comments when the related code changes.
• Use JSDoc-style comments for functions that are reused or shared with other developers.
• Avoid commenting every single line; reserve comments for places that genuinely need clarification.

Quick Summary

• Comments are ignored by JavaScript and exist only for humans reading the code.
• Single-line comments start with //.
• Multi-line comments are wrapped between /* and */.
• Multi-line comments cannot be nested inside each other.
• JSDoc-style comments (/** */) document functions and can power editor autocomplete.
• Good comments explain reasoning, not just restate the code.

Try It Yourself

Add your own single-line and multi-line comments to the code below, and try commenting out the second console.log line.

// Try adding a comment above this line
let name = "Michael";
console.log(name);
console.log("Hello, " + name + "!");

Tip: A good comment tells future readers of your code (including future you) something the code itself cannot: the reasoning behind a decision, a warning about a tricky edge case, or a note about why a simpler approach wasn’t used.

Understanding comments is a small but important step before moving on to conditionals, loops, functions, and more advanced JavaScript concepts.