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JavaScript Comments

JavaScript comments describe the JavaScript code and make it more readable.

JavaScript comments prevent execution when testing alternative code.

Single-Line Comments

Single line comments

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

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

Example

// Change heading:

document.getElementById(“myH”).innerHTML = “JavaScript Comments”;

// Change paragraph:

 

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

Example

let x = 5;      // Declare x, give it the value of 5

let y = x + 2;  // Declare y, give it the value of x + 2

// Write y to demo:

Multi-line Comments

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

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

This example defines a multi-line comment (a comment block) that explains the code:

Example

/*

The code below will change

the heading with id = “myH”

and the paragraph with id = “myP”

*/

Using Comments to Prevent Execution

The comments are used to prevent the execution of code suitable for code testing.

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

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

Example

<script>

//document.getElementById(“myH”).innerHTML = “My First Page”;

document.getElementById(“myP”).innerHTML = “My first paragraph.”;

</script>

<p>The line starting with // is not executed.</p>

Example

<!DOCTYPE html>

<html>

<body>

<h1>JavaScript Variables</h1>

<p>In this example, x, y, and z are variables.</p>

<p id=”demo”></p>

<script>

var x = 5;

var y = 6;

var z = x + y;

document.getElementById(“demo”).innerHTML =

“The value of z is: ” + z;

</script>

</body>

</html>