Comments are lines in your code that JavaScript completely ignores when running your program — they exist purely for the humans reading the code, whether that's a teammate, your future self, or you tomorrow morning when you've forgotten exactly why you wrote something a certain way.

Single-Line Comments

A double slash // turns the rest of that line into a comment:

// Calculate the total price including tax
let price = 100;
let tax = 0.08;
let total = price + price * tax; // 8% sales tax

console.log(total);

Output:

108

Multi-Line Comments

For longer notes, or to temporarily disable a block of code, use /* to start and */ to end — everything in between is ignored, even across multiple lines:

/*
 * This function calculates a discounted price.
 * discountPercent should be a number between 0 and 100.
 */
function applyDiscount(price, discountPercent) {
  return price - price * (discountPercent / 100);
}

console.log(applyDiscount(200, 25));

/*
console.log("This whole line is disabled");
console.log("So is this one");
*/

Output:

150

This "commenting out" trick is extremely common while debugging — temporarily disabling a line or block without deleting it, so you can test whether removing that logic fixes (or causes) a problem.

When to Comment — and When Not To

Good comments explain why something is done a certain way, not what the code does — if the "what" isn't obvious from reading the code itself, that's often a sign the code could be written more clearly instead.

// Bad: states the obvious, adds no value
let x = 5; // set x to 5

// Good: explains a non-obvious reason
let retryLimit = 3; // API occasionally fails once or twice under load, so we retry

As you write more real code — starting with JavaScript functions — you'll naturally develop a sense for which lines genuinely need an explanation and which are already clear on their own.