Countdown

Kode 

<script>
function countdown(callback,n){
    if(typeof n != 'number') {
        var n = 3;
    }
    alert(n);
    if(n > 0) {
        n--;
        var f = function() { countdown(callback,n); };
        setTimeout(f,1000);
    } else {
        callback();
    }
}
var f = function(){ alert('Done'); };
countdown(f,null);
</script>