JAVASCRIPT ONCHANGE EVENT
Run 
<!doctype html>
	<head>
		<title>JavaScript onchange Event</title>
	</head>
	<body>
	    <select id="dropdown">
	        <option value="1">1</option>
	        <option value="2">2</option>
	        <option value="3">3</option>
	    </select>

		<script>
			/* JS comes here */
            function eventHandler() {
                // do something
                alert("Value selected");
            } 
            // setup event handler on HTML element
            document.getElementById("dropdown").onchange = eventHandler;
		</script>
	</body>
</html>