1.6 JS Comments

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 1: More JavaScript Comments

// 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 2: JavaScript Comments part 2

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 3: JavaScript Comments part 3

/*
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 4: JavaScript Comments part 4

//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 5: JavaScript Comments part 5

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