JavaScript Comments

JavaScript comments can be used to explain the code, and make the code more readable. JavaScript comments can also be used to prevent execution, when testing alternative code.

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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).

The following example uses a single line comment in front of 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.";

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.

The following 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.";

 

Using Comments to Prevent Execution

Using comments to prevent execution of code, can be very suitable for testing.

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

The next 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."; 

The following 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.";
*/

 Sumber : w3schools