JavaScript - Comments


Single-line Comments

Single-line comments in JavaScript add short notes to your code. They document the purpose of a line or block of code, making it easier for you and other developers to understand and maintain the codebase.

The syntax for single-line comments in JavaScript is to use two forward slashes (//) at the beginning of the comment. Anything written after the // on the same line is a comment and will be ignored by the JavaScript interpreter during execution.


// This is a single-line comment
let x = 5; // This comment explains the purpose of the variable

// Calculating the sum of two numbers
let sum = a + b;

When using single-line comments, it's important to follow some best practices to keep your code clean and maintainable:

Best Practice Description
Keep comments concise Single-line comments should briefly describe the purpose of the code they accompany.
Place comments on a separate line Put comments just before the code they describe. This makes it easier to read and understand the code.
Use meaningful language Avoid ambiguous or vague comments that do not add value to the code.
Don't overuse comments While comments are helpful, too many comments can clutter your code and make it harder to read. Only add comments when necessary to clarify complex or non-obvious code.
Keep comments up to date If you modify the code, make sure to update the corresponding comments to reflect the changes.

Multi-line Comments

In JavaScript, multi-line comments let you write longer notes or explanations that span multiple lines. They are useful for providing more information about a function, a block of code, or a complex algorithm.

The syntax for multi-line comments in JavaScript is to enclose the comment between /* and */. Any text between these delimiters, including line breaks, will be treated as a comment and ignored by the JavaScript interpreter during execution.

Multi-line comment example

  This is a multi-line comment.
  It can span multiple lines and is useful for providing
  more detailed explanations about the code.

  Function: calculateAverage
  Calculates the average of an array of numbers.

  - numbers: An array of numbers.

  - The average of the numbers in the array.
function calculateAverage(numbers) {
  let sum = 0;
  for (let i = 0; i < numbers.length; i++) {
    sum += numbers[i];
  return sum / numbers.length;

When using multi-line comments, consider the following best practices:

Best Practice Description
Use multi-line comments for longer explanations Use multi-line comments for more detailed descriptions or documentation that cannot be easily expressed in a single line.
Place multi-line comments before the relevant code Put multi-line comments just before the code they describe, making it clear which part of the code the comment refers to.
Format multi-line comments for readability Indent the text within multi-line comments and use line breaks to improve readability.
Keep multi-line comments up to date As with single-line comments, make sure to update multi-line comments if you change the corresponding code.
Don't nest multi-line comments Avoid nesting multi-line comments within each other, as this can make the code harder to read and maintain.

Using Comments for Debugging

Comments are a useful tool when debugging JavaScript code. By adding comments to your code, you can track the values of variables, identify potential issues, and pinpoint the source of errors more easily.

When debugging, you can use comments to temporarily disable parts of your code. If you suspect a particular line or block of code is causing an issue, you can comment it out and run the program again to see if the problem persists. This can help you isolate the problematic code and focus on fixing it.

Debugging example

function calculateTotal(items) {
  let total = 0;
  for (let i = 0; i < items.length; i++) {
    // Debugging: Check the value of each item
    console.log("Item " + i + ": " + items[i]);

    total += items[i];
  return total;

// Debugging: Test the function with sample data
let sampleItems = [10, 20, 30, 40];
let result = calculateTotal(sampleItems);
console.log("Total: " + result);