<!DOCTYPE html>
<html>
<body>

<input id="textfield" type="text" name="fname" value="">
<input type="submit" value="Submit">

<script type="text/javascript">
var input_text = "a";
var id = setInterval(function() {
  document.getElementById("textfield").value = input_text;
  input_text = input_text + "a";
}, 1);
</script>

</body>
</html>