Slide 11.c: JavaScript how to …
Slide 11.e: JavaScript where to … (cont.)
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JavaScript Where to …

JavaScripts in a page will be executed immediately while the page loads into the browser. This is not always what we want. Sometimes we want to execute a script when a page loads, other times when a user triggers an event.

Scripts in the Head Section
Scripts to be executed when they are called, or when an event is triggered, go in the head section. When you place a script in the head section, you will ensure that the script is loaded before anyone uses it. 
 <html>
  <head>
   <script type="text/javascript">
    alert( "Attention!" );
   </script>
  </head>
  <body> ... </body>
 </html>

Scripts in the Body Section
Scripts to be executed when the page loads go in the body section. When you place a script in the body section it generates the content of the page. 
 <html>
  <head> ... </head>
  <body>
   <script type="text/javascript">
    alert( "Attention!" );
   </script>
  </body>
 </html>

The object window represents a browser window or frame. This is the top-level object for each document, Location, and History object group. One of the window's method alert dsplays an Alert dialog box with a message and an OK button.

†Note that the JavaScript is executed when loaded no matter it is located in the head section or body section.

Demonstration