Tryit Editor v1.8

Source Code:
<!DOCTYPE html> <html> <head> <script> var c=0; var t; var timer_is_on=0; function timedCount() { document.getElementById('txt').value=c; c=c+1; t=setTimeout("timedCount()",1000); } function doTimer() { if (!timer_is_on) { timer_is_on=1; timedCount(); } } </script> </head> <body> <form> <input type="button" value="Start count!" onClick="doTimer()"> <input type="text" id="txt"> </form> <p>Click on the button above. The input field will count forever, starting at 0.</p> </body> </html>
Result:
W3Schools.com - Try it yourself