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.