<!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>