What is the difference between single-line and multi-line comments in JavaScript?

JavaScript supports two types of comments: single-line comments using // and multi-line comments using /* */. Comments help document code and are ignored during execution.

Single-Line Comments

Single-line comments start with // and continue until the end of the line. Everything after // on that line is treated as a comment.

Syntax

// This is a single-line comment

Example

// This is a comment explaining the variable
let userName = "John Doe";

let age = 25; // Age of the user

console.log(userName); // Output the user's name
console.log(age);

John Doe
25

Multi-Line Comments

Multi-line comments start with /* and end with */. They can span multiple lines and are useful for longer explanations or temporarily disabling code blocks.

Syntax

/*
This is a multi-line comment
that spans several lines
*/

Example

/*
This function calculates the area of a rectangle
Parameters: length and width
Returns: area as a number
*/
function calculateArea(length, width) {
    return length * width;
}

let area = calculateArea(5, 3);
console.log("Area:", area);

/*
The following code is temporarily disabled
let oldVariable = "not needed";
console.log(oldVariable);
*/

Area: 15

Comparison

Nested Comments

Multi-line comments cannot be nested. If you try to nest /* */ comments, the first */ will close the comment.

/*
This is the outer comment
/* This inner comment will cause problems */
This text will not be commented out!
*/

console.log("This might cause an error");

Conclusion

Use single-line comments for brief explanations and multi-line comments for longer documentation or temporarily disabling code blocks. Both are essential tools for writing maintainable JavaScript code.