Message Popop on no input

[admin]

Hi, if someone can help me with this.

Im trying to display HTML when it is inserted into this Textbox,
this works, but I also want a massage popup olny if the user does not input nothing.

How colud I do it?

<SCRIPT LANGUAGE="JavaScript">
<!-- Begin
function displayHTML(form) {
var inf = form.htmlArea.value;
win = window.open(", ", 'popup', 'toolbar = no, status = no');
win.document.write("" + inf + "");
}
// End -->
</script>

<form>
<textarea name="htmlArea" cols=45 rows=6></textarea>
<br>
<input type="button" value=http://www.webdeveloper.com/forum/archive/index.php/" view " onclick="displayHTML(this.form)">
</form>