JavaScript Comments

JavaScript comments are used to explain the code to make it more readable. It can be used to prevent the execution of a section of code if necessary. JavaScript comments are ignored while the compiler executes the code. 

Single Line Comments

In Javascript, single-line comments start with //. Any code written after // will be ignored by Javascript.

Example 1: This example illustrates the single-line comment using //. 

Javascript

// A single line comment
console.log("Hello Geeks!");

Output

Hello Geeks!

Example 2: In this example, we will assign values to some variables and explain them with single-line comments.

Javascript

// Declaring a variable and assign value to it
let geek = 'Computer science portal';
console.log(geek)

// Perform operation of addition of two numbers
let sum = 5 + 8
console.log(sum)

Output

Computer science portal
13

Multi-line Comments

In Javascript, multi-line comments start with /* and end with */. Any text or code written between /* and */ will be ignored by JavaScript at the time of execution.

Example: This example illustrates the multi-line comment using /* … */ 

Javascript

/* It is multi line comment.
It will not be displayed upon
execution of this code */

console.log("Multiline comment in javascript");

Output

Multiline comment in javascript

JavaScript Comments to Prevent Execution

We can use // or /*…*/ to change the JavaScript code execution using comments. JavaScript Comments are used to prevent code execution and are considered suitable for testing the code.

Example 1: JavaScript comments are used to prevent execution of selected code to locate problems of code or while testing new features. This example illustrates that commented code will never execute. 

Javascript

function add() {
    let x = 10;
    let y = 20;
    let z = x + y;
    // console.log(x + y);
    console.log(z);
}

add();

Output: The sum is calculated but is only displayed once because the code to display the sum is written in a comment in the other line and will not be displayed.

30

Example 2: This example uses multi-line comments to prevent the execution of addition code and perform subtraction operations.

Javascript

function sub() {
    let x = 10;
    let y = 20;
    /* let z = x + y;
    console.log(z); */

    let z = x - y;
    console.log(z);
}

sub();

Output

-10