Simpelt exempel:
<script type="text/javascript">
function f(){
var cb;
while (!(cb = parseInt(window.prompt("Hur många checkboxar önskas?"))));
var ta;
while (!(ta = parseInt(window.prompt("Hur många textareor önskas?"))));
var form = "<form name='f' action='sida.asp'>";
while (cb--) form += "<input type='checkbox' name='cb"+ cb +"'>";
form += "<br>";
while (ta--) form += "<textarea name='ta"+ ta +"'><\/textarea>";
form += "<br><input type='submit' value='Skicka'><\/form>";
document.getElementById("form").innerHTML = form;
}
</script>
<a href="javascript:void(0)" onclick="f()">Popup</a>
<div id="form"></div>