JavaScript Comments Explained – Single-Line, Multi-Line & Best Practices

Ever seen lines in JavaScript code that don’t execute?
Those are comments — your way to explain, annotate, or temporarily disable code for debugging.

Comments are essential for:

• Improving readability
• Debugging or disabling code
• Collaborating with others

Let’s explore how to use them effectively.

Types of Comments in JavaScript

JavaScript supports two types of comments:

1. Single-Line Comments

Use // to create a comment that spans just one line.

code// This is a single-line comment
let name = "John"; // Set the user's name

Explanation:

• // This is a single-line comment — ignored by the JavaScript engine.
• let name = "John"; — actual code.
• // Set the user's name — describes what the line is doing.

Best Practice: Use single-line comments to describe simple actions above or beside the code.

2. Multi-Line Comments

Use /* ... */ to span comments over multiple lines.

code/* This block of code
   fetches user data from the API
   and displays it in the UI */
fetchUserData();

Explanation:

• This block of comment helps you document more complex logic.
• Especially useful when commenting out multiple lines for debugging.

Best Practice: Use for:

• Describing complex logic
• Commenting out large blocks temporarily

Commenting Out Code

You can temporarily disable execution of code using comments.

code// console.log("This will not run");
console.log("This will run");

This is especially useful during debugging or testing different blocks.

Nested Comments – Not Allowed

JavaScript does not support nested multi-line comments.

code/*
   /* This will cause an error */
*/

This will throw a syntax error!

Tip: Don’t nest block comments. Instead, break them apart or use single-line syntax where needed.

Comment Formatting Tips

To keep your code clean and maintainable:

• Use consistent spacing in comments
• Add TODOs or FIXMEs to track pending tasks
• Keep comments concise and meaningful
• Avoid stating the obvious (e.g., // increment i on i++)

//  GOOD: Explains why something is done
// Using a timeout to prevent rate-limiting issues
setTimeout(makeRequest, 3000);

//  BAD: Too obvious
// Call the function
makeRequest();

Why Comments Matter

Performance and Comments

Comments are stripped out during JavaScript minification for production builds — they don’t affect performance or load times.

So feel free to comment generously during development!

Summary

• Use // for single-line, /* */ for multi-line comments.
• Comments improve readability, debugging, and maintainability.
• Avoid over-commenting or stating obvious facts.
• JavaScript doesn’t support nested comments — be careful with block nesting!

FAQ – JavaScript Comments

What are comments in JavaScript used for?

Comments are used to add notes, explain code, or temporarily disable execution for debugging purposes.

Do comments affect JavaScript performance?

No. Comments are ignored by the JavaScript engine and stripped out in production builds during minification.

Can you nest multi-line comments?

No. JavaScript doesn’t allow nested block comments and doing so will result in a syntax error.

How do I comment out multiple lines in JavaScript?

Use /* ... */ to comment out multiple lines at once.

What’s the difference between // and /* */ in JavaScript?

// is for single-line comments, while /* */ is for multi-line or block comments.