Javascript Comments

Question 1

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:

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

Answer 1

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

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;

Question 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:

Answer 2

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>

It is most common to use single line comments.
Block comments are often used for formal documentation.

Question 3

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:

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

Answer 3

script>
//document.getElementById(“myH”).innerHTML = “My First Page”;
document.getElementById(“myP”).innerHTML = “My first paragraph.”;
/script>

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>