Just done google search found following script
<html>
<body>
<p>Click the button to wait 3 seconds, then alert "Hello".</p>
<p>After clicking away the alert box, an new alert box will appear in 3 seconds. This goes on forever...</p>
<button onclick="myFunction()">Try it</button>
<script>
function myFunction()
{
setInterval(function(){alert("Hello")},3000);
}
</script></body>
</html>
