Learn about JavaScript and Case Sensitivity.

You have to be extremely careful when keying in the text of your JavaScripts. JavaScript is a case sensitive programming language (unlike HTML, which enables you to use different capitalization when defining HTML tags). Case sensitivity means that you must type JavaScript elements exactly as they appear in this book in order for them to work. For example, as far as JavaScript is concerned, the words document and Document refer to two different things, so pay special attention when typing your scripts. If I had accidentally typed a capital D in the document.write() statement used in the preceding example, my script would have experienced an error. As a result, the Hello World message would not have been displayed as I intended.