Comments

Comments are notes you write in your code to explain what it does. They're for humans to read—the computer completely ignores them!

What Are Comments?

Imagine you're writing a recipe and you add little notes like "Mom's secret ingredient!" or "Don't skip this step!" Those notes help you (and others) understand the recipe better, but they're not part of the actual cooking instructions.

Comments in code work the same way. They're messages to yourself and other programmers that explain what the code does and why it works that way.

Single-Line Comments

In JavaScript, you create a single-line comment with two forward slashes (//). Everything after // on that line is a comment.

// This entire line is a comment

let age = 25;  // You can also add comments at the end of a line

// Comments can explain complex logic
// or remind you of things to fix later

Multi-Line Comments

For longer comments that span multiple lines, use /* to start and */ to end:

/*
  This is a multi-line comment.
  It can span as many lines as you need.
  Great for explaining complex sections of code!
*/

/*
  Author: Alex
  Date: January 2024
  Purpose: Calculate user scores
*/

Try It Yourself

When to Use Comments

Good Uses for Comments

• Explain complex logic: When code isn't obvious, explain your thinking
• Document functions: Describe what a function does and what it expects
• Leave TODO notes: Mark things you need to come back and fix
• Explain business rules: Why does the discount apply only on Tuesdays?
• Temporarily disable code: Comment out code while debugging

// TODO: Add error handling for invalid input

// Apply 20% discount for premium members (business rule from marketing)
let discount = isPremium ? 0.2 : 0;

/*
  Binary search algorithm
  Assumes the array is already sorted
  Returns -1 if not found
*/
function binarySearch(arr, target) {
  // ...
}

When NOT to Comment

• Don't state the obvious: let count = 0; // set count to zero
• Don't write novels: Keep comments concise and relevant
• Don't leave outdated comments: Update comments when you change code

// This increments i (obvious!)
i++;

// Add one to x
x = x + 1;

// Loop through array (the code already says this)
for (let item of items) {
  // ...
}

Comments in Other Languages

Different programming languages use different comment syntax:

Commenting Out Code

One useful technique is "commenting out" code—temporarily turning code into a comment so it won't run. This is helpful when debugging:

function calculateTotal(items) {
  let total = 0;

  for (let item of items) {
    total += item.price;
  }

  // Temporarily disable discount while debugging
  // total = total * 0.9;

  return total;
}

Quick Quiz

Key Takeaways

• Comments are notes for humans—computers ignore them completely
• Use // for single-line comments in JavaScript
• Use /* */ for multi-line comments
• Good comments explain why, not what
• Keep comments up to date when you change code
• Don't over-comment obvious code

What's Next?

Now that you know how to document your code, let's dive deeper into working with strings—text data that powers everything from usernames to messages!