💬 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.

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✍️ Types of Comments in JavaScript

JavaScript supports two types of comments:

🔖 Type	💡 Syntax	📌 Use Case
Single-line	// comment text	For short notes or disabling a single line
Multi-line	/* comment text */	For longer descriptions or block commenting

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🧩 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.

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🧩 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

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🔒 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.

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📌 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.

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🔧 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();

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🧠 Why Comments Matter

Benefit	Description
👓 Readability	Makes your logic clear for future-you or team members
🧪 Debugging Aid	Easily disable or isolate code
📖 Documentation	Acts as inline documentation for your logic
🔄 Maintainability	Helps new developers understand your code faster

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🛡️ 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!

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🧾 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!

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❓ 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.

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