JS Comments

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 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 5var 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 the 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.";
*/

---