<script>

function fn()
{
  var a = 0;
  var x = document.forms[0].elements['x'].value;
  x += 1;

  alert('x: ' + typeof x + ': ' + x);
  for (var i = 0; i < x; i++){ a+=i; }
  alert('i: ' + i);

  document.getElementById('qqq').innerHTML = 'Результат ='+a   ;
}

</script>
<form>
  <input name="x" id="x" onchange="fn()">
</form>
<pre id="qqq"></pre>