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 a line, will be ignored by JavaScript (will not be executed).

This example uses a single line comment before each line, to explain the code:

Example

// Change heading:
document.getElementById("myH").innerHTML = "My First Page";
// Change paragraph:
document.getElementById("myP").innerHTML = "My first paragraph.";

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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

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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.";

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Note 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, are 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.";

Try it Yourself »

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.";
*/

Try it Yourself »



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