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). To create a single line comment in JavaScript, you place two slashes “//” in front of the code or text you wish to have the JavaScript interpreter ignore. When you place these two slashes, all text to the right of them will be ignored, until the next line. These types of comments are great for commenting out single lines of code and writing small notes 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
let x = 5; // Declare x, give it the value of 5 let 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. it can sometimes be burdensome to use when disabling long segments of code or inserting long-winded comments. For this large comments you can use JavaScript’s multi-line comment that begins with /* and ends with */. 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.";
*/
