Write a JavaScript program to show alert message on button click.
<html>
<head>
<title>JavaScript event example</title>
<script type="text/javascript">
function sayHello() {
alert("Hello World!")
}
</script>
</head>
<body>
<p>Click the button to see result</p>
<form>
<input type="button" onclick="sayHello()" value="Say Hello" />
</form>
</body>
</html>
