JavaScript Comments

The JavaScript comments are a meaningful way to deliver a message. It is used to add information about the code, warnings or suggestions so that end user can easily interpret the code.

The JavaScript comment is ignored by the JavaScript engine i.e. embedded in the browser.

Advantages of JavaScript comments

There are mainly two advantages of JavaScript comments.

  1. To make code easy to understand It can be used to elaborate the code so that the end user can easily understand the code.
  2. To avoid the unnecessary code It can also be used to avoid the code being executed. Sometimes, we add the code to perform some action. But after some time, there may need to disable the code. In such a case, it is better to use comments.

 

Types of JavaScript Comments

There are two types of comments in JavaScript.

Single Line Comments

Single line comments start with //.

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

This example uses a single-line comment before each code line:

Example

// Change heading:
document.getElementById("myH").innerHTML = "My First Page";

// Change paragraph:
document.getElementById("myP").innerHTML = "My first paragraph.";

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

 

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.";

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 the execution of code is suitable for code testing.

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

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

Example

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

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

Example

/*
document.getElementById("myH").innerHTML = "My First Page";
document.getElementById("myP").innerHTML = "My first paragraph.";
*/