To prevent old browsers from displaying your JS code, you can do the following:
Immediately after the opening tag, put a one-line HTML-style comment without the closing characters, so that the first two lines of your script would look like this:
<script type="text/javascript" language="JavaScript">
<!--
At the end of your script, put the following two lines:
//-->
</script>
Thus, your HTML file will contain the following fragment:
<script language="JavaScript">
<!--
Here you put your JS code.
Old browsers will treat it
as an HTML comment.
//-->
</script>
Old browsers will treat your JS code as one long HTML comment. On the other hand, new JavaScript-aware browsers will normally interpret JS code between the tags and (the first and last lines of your JS code will be treated by the JavaScript interpreter as one-line comments).