JavaScript Comments
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:
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.
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.";
»
It is most common to use
single line comments.
Block comments are often
used for formal documentation.
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:
Example
//document.getElementById("myH").innerHTML
= "My First Page";
document.getElementById("myP").innerHTML
= "My first paragraph.";
»
This 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.";
*/
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