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<!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>