<!DOCTYPE html>
<html>
 <head>
   <meta charset="utf-8">
   <script>
var one = document.getElementById('one');
function hid() {alert('hello');}
one.addEventListener('click', hid, false);
</script>
 </head>
 <body>
     
 	<div class = "calculator">
     <button id='one' > 1</button>
 	 <button id='two'> 2</button>
 		</div>
 </body>
</html>