In JavaScript, comments are used to annotate your code with explanations or notes, making it easier to understand and maintain. Comments are ignored by the JavaScript engine during execution, meaning they have no impact on the behavior of the code.

Types of Comments

JavaScript supports two types of comments:

1. Single-line Comments
2. Multi-line Comments

1. Single-line Comments

Single-line comments are used for brief notes or explanations that fit on a single line. They start with //.

Syntax

// This is a single-line comment

Example

let x = 10; // This is a single-line comment explaining that x is initialized to 10

• Explanation: The // marks the beginning of a single-line comment. Everything following // on that line is considered a comment.

2. Multi-line Comments

Multi-line comments are used for longer comments that span multiple lines. They start with /* and end with */.

Syntax

/*
  This is a multi-line comment.
  It can span multiple lines.
*/

Example

/*
  The following code calculates the area of a rectangle:
  - width: 5
  - height: 10
*/
let width = 5;
let height = 10;
let area = width * height;
console.log(area); // Outputs: 50

• Explanation: Everything between /* and */ is treated as a comment, regardless of how many lines it spans.

Key Points

1. Purpose: Comments are used to:

   • Explain code functionality.
   • Provide context or rationale for certain code decisions.
   • Temporarily disable code for debugging or development purposes.

2. Best Practices:

   • Clarity: Write comments that clearly explain what the code does, especially if the logic is complex.
   • Relevance: Keep comments relevant and up-to-date with code changes.
   • Avoid Redundancy: Don’t comment on obvious code; comments should add value.

3. Disabling Code: You can use comments to temporarily disable code during development.

   // let debugMode = true; // Disabled debug mode for production
   

4. Documentation: Use comments to document functions, parameters, and return values.

   /**
    * Calculates the area of a rectangle.
    * @param {number} width - The width of the rectangle.
    * @param {number} height - The height of the rectangle.
    * @returns {number} The area of the rectangle.
    */
   function calculateArea(width, height) {
     return width * height;
   }
   

   • Explanation: The /** ... */ syntax is often used in documentation comments, which can be processed by tools like JSDoc to generate documentation.

Summary

• Single-line Comments: Start with // and are used for short notes.
• Multi-line Comments: Start with /* and end with */, used for longer explanations.
• Usage: Helps in documenting code, explaining logic, and making code more readable and maintainable.