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JavaScript Web development

JavaScript Comments

Comments are annotations in the source code of a program that are ignored by the interpreter, and therefore have no effect on the actual output of the code. Comments can be immensely helpful in explaining the intent of what your code is or should be doing.

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.

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:

<!DOCTYPE html>
<html>
<body>

<h1 id="myH"></h1>
<p id="myP"></p>

<script>
// Change heading:
document.getElementById("myH").innerHTML = "JavaScript Comments";
// Change paragraph:
document.getElementById("myP").innerHTML = "My first paragraph.";
</script>

</body>
</html>

Output:

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

<!DOCTYPE html>
<html>
<body>

<h2>JavaScript Comments</h2>

<p id="demo"></p>

<script>
var x = 5;    // Declare x, give it the value of 5
var y = x + 2;  // Declare y, give it the value of x + 2 
// Write y to demo:
document.getElementById("demo").innerHTML = y;
</script>


</body>
</html>

Output:

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:

<!DOCTYPE html>
<html>
<body>

<h1 id="myH"></h1>
<p id="myP"></p>

<script>
/*
The code below will change
the heading with id = "myH"
and the paragraph with id = "myP"
*/
document.getElementById("myH").innerHTML = "JavaScript Comments";
document.getElementById("myP").innerHTML = "My first paragraph.";
</script>

</body>
</html>

Output:

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:

<!DOCTYPE html>
<html>
<body>

<h2>JavaScript Comments</h2>

<h1 id="myH"></h1>

<p id="myP"></p>

<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>

</body>
</html>

Output:

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

<!DOCTYPE html>
<html>
<body>

<h2>JavaScript Comments</h2>

<h1 id="myH"></h1>

<p id="myP"></p>

<script>
/*
document.getElementById("myH").innerHTML = "Welcome to my Homepage";
document.getElementById("myP").innerHTML = "This is my first paragraph.";
*/
document.getElementById("myP").innerHTML = "The comment-block is not executed.";
</script>


</body>
</html>

Output:

Enjoy coding!

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