JavaScript Comments

Adding notes and documentation to your code

💬 What are JavaScript Comments?

Comments are text notes in your code that are ignored by the JavaScript engine. They help explain what your code does and make it easier to understand. 


// This is a single-line comment
let name = "John"; // Comment at end of line

/* This is a 
   multi-line comment */
let age = 25;

Output:

name = "John"
age = 25

Types of Comments

//

Single-line

Comments that span one line 

// This is a single-line comment
let x = 5;
/* */

Multi-line

Comments that span multiple lines 

/* This comment
   spans multiple
   lines */
📝

Inline

Comments at the end of code lines 

let y = 10; // Set y to 10
🚫

Code Disable

Temporarily disable code 

// console.log("Debug");
let z = 15;

🔹 Single-line Comments

Single-line comments start with // and continue to the end of the line: 

// This is a comment
let firstName = "John";

let lastName = "Doe"; // Another comment

// You can use multiple single-line comments
// to create a block of comments
let fullName = firstName + " " + lastName;

Output:

firstName = "John"
lastName = "Doe"
fullName = "John Doe"

🔹 Multi-line Comments

Multi-line comments start with /* and end with */: 

/*
This is a multi-line comment.
It can span multiple lines.
Everything between /* and */ is ignored.
*/

let score = 100;

/* 
You can also use multi-line comments
to temporarily disable blocks of code
let tempVar = "disabled";
console.log(tempVar);
*/

Output:

score = 100

🔹 Best Practices for Comments

Here are some guidelines for writing good comments: 

// Good: Explain WHY, not WHAT
let taxRate = 0.08; // Sales tax rate for California

// Good: Explain complex logic
if (age >= 18 && hasLicense && !hasViolations) {
  // Allow driving only if all conditions are met
  allowDriving = true;
}

// Avoid: Obvious comments
let x = 5; // Set x to 5 (unnecessary)

// Good: Document function purpose
function calculateTotal(price, tax) {
  // Calculate final price including tax
  return price + (price * tax);
}

Output:

Well-documented code is easier to understand and maintain

🧠 Test Your Knowledge

Which symbol starts a single-line comment in JavaScript?