# JavaScript Comments

« Previous
Next Chapter »

JavaScript comments can be used to explain JavaScript code, and to make it more readable.

JavaScript comments can also be used to prevent execution, when testing alternative code.

## Single Line Comments

Single line comments start with //.

Any text between // and the end of a line, will be ignored by JavaScript (will not be executed).

This example uses a single line comment before each line, to explain the code:

## Example

// Change heading:
document.getElementById("myH").innerHTML = "My First Page";
// Change paragraph:
document.getElementById("myP").innerHTML = "My first paragraph.";
Try it Yourself »

This example uses a single line comment at the end of each line, to explain the code:

## Example

var x = 5; // Declare x, give it the value of 5
var y = x + 2; // Declare y, give it the value of x + 2
Try it Yourself »

## Multi-line Comments

Multi-line comments start with /* and end with */.

Any text between /* and */ will be ignored by JavaScript.

This example uses a multi-line comment (a comment block) to explain the code:

## Example

/*
The code below will change
the heading with id = "myH"
and the paragraph with id = "myP"
in my web page:
*/
document.getElementById("myH").innerHTML = "My First Page";
document.getElementById("myP").innerHTML = "My first paragraph.";
Try it Yourself »

| It is most common to use single line comments. Block comments are often used for formal documentation.

## Using Comments to Prevent Execution

Using comments to prevent execution of code, are suitable for code testing.

Adding // in front of a code line changes the code lines from an executable line to a comment.

This example uses // to prevent execution of one of the code lines:

## Example

//document.getElementById("myH").innerHTML = "My First Page";
document.getElementById("myP").innerHTML = "My first paragraph.";
Try it Yourself »

This example uses a comment block to prevent execution of multiple lines:

## Example

/*
document.getElementById("myH").innerHTML = "My First Page";
document.getElementById("myP").innerHTML = "My first paragraph.";
*/
Try it Yourself »
« Previous
Next Chapter »

### WEB HOSTING

UK Reseller Hosting

### WEB BUILDING

FREE Website BUILDER Free HTML5 Templates

### W3SCHOOLS EXAMS

HTML, CSS, JavaScript, PHP, jQuery, XML, and ASP Certifications

### SHARE THIS PAGE

### Color Picker

### HTML/CSS

HTML Tutorial HTML5 Tutorial CSS Tutorial CSS3 Tutorial Bootstrap Tutorial

### JavaScript

JavaScript Tutorial jQuery Tutorial jQuery Mobile Tutorial AngularJS Tutorial AJAX Tutorial JSON Tutorial

### Graphics

Canvas Tutorial SVG Tutorial Google Maps Tutorial

### Server Side

SQL Tutorial PHP Tutorial ASP Tutorial ASP.NET Tutorial VBScript Tutorial AppML Tutorial

### Web

Web Building Web Statistics Web Certification

### XML

XML Tutorial DTD Tutorial Schema Tutorial XML DOM Tutorial XPath Tutorial XSLT Tutorial XQuery Tutorial XSL-FO Tutorial RSS Tutorial WSDL Tutorial

### HTML/CSS

HTML Tag Reference HTML Event Reference HTML Color Reference CSS 1,2,3 Reference Bootstrap Reference

### JavaScript

JavaScript Reference HTML DOM Reference jQuery Reference jQuery Mobile Reference Google Maps Reference

### Server Side

PHP Reference SQL Reference ASP Reference VBScript Reference Razor Reference ASP.NET Reference

### XML

XML DOM Reference XSLT Reference XSL-FO Reference XPath Reference XQuery Reference Schema Reference RSS Reference SVG Reference

### HTML Charsets

HTML Character Sets HTML ASCII HTML ANSI HTML Windows-1252 HTML ISO-8859-1 HTML Symbols HTML UTF-8

### HTML/CSS

HTML Examples CSS Examples

### Browser Scripting

JavaScript Examples HTML DOM Examples jQuery Examples jQuery Mobile Examples AngularJS Examples AJAX Examples

### Server Scripting

PHP Examples ASP Examples VBScript Examples Razor Examples .NET Examples

### XML Languages

XML Examples DTD Examples XML DOM Examples XSL Examples XSLT Examples XPath Examples XQuery Examples Schema Examples RSS Examples SVG Examples