[06] JavaScript Comments

JavaScript Comments

 

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.

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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 line, to explain the code:

 

// 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.";

Using Comments to Prevent Execution

Using comments to prevent execution of code, is 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:

 

<script> //document.getElementById("myH").innerHTML = "My First Page"; document.getElementById("myP").innerHTML = "My first paragraph."; </script> <p><strong>Note:</strong> The comment is not executed.</p>