Tryit Editor v2.9

Edit This Code:
<!DOCTYPE html>
<html>
<body>

<button onClick="setInterval(myCounter, 1000)">Start counter!</button>

<p id="demo">Click on the button above and I will count forever.</p>

<script>
var c = 0;
function myCounter() {
    document.getElementById("demo").innerHTML = ++c;
}
</script> 

</body>
</html>