The examples below used the following in the HEAD section of the document:

<SCRIPT LANGUAGE="Javascript" TYPE="text/javascript" SRC="plugins/overlib.js"></SCRIPT>
<SCRIPT LANGUAGE="Javascript" TYPE="text/javascript">
<!--
function myFunction() {
  var txt = ''
  txt += '<CENTER><FONT FACE="Arial, Helvetica, sans-serif">'
  txt += 'This is an <FONT COLOR="RED">EXAMPLE</FONT> of the <FONT COLOR="RED">FUNCTION</FONT> statement'
  txt += '</FONT></CENTER>'
  return txt
}
ol_function = myFunction;
function myFunc2() {
  var txt = ''
  txt += '<CENTER><FONT FACE="Arial, Helvetica, sans-serif">'
  txt += '<B><I>myFunc2() call</I><B>'
  txt += '</FONT></CENTER>'
  return txt
}
//-->
</SCRIPT>

These examples all have the default function set to some defined javascript function, in this case myFunction. When this function is defined and there isn't any text string on the command line, the popup text will be provided by that function. Now look at the file Function_example1.html which doesn't have a default function defined.