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.

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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

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.

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.";

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.";

*/

Video: JavaScript Comments