Let's look at an example to practice working with data types and see how we can use variables in action. Copy the following code to a file and save it with the extension .html.
Load the page in IE and this is what you will get:
"The variable aNumber = "+ undefined + "<br>"
Then it concatenate all the values into one big string:
"The variable aNumber = undefined<br>"
Then the interpreter sends this value to the browser. Note that in the Web page we don't see the value <br> because it's an HTML tag, and the browser will understand that you want to put <br> after the string value. To prove that, we will replace the doument.write() functionality with alert() because alert displays a message box to the user. Message boxes don't understand what <br> means, and it will include them in the string values. Replace the following code, save the file, and then load the page.