<script type="text/javascript">  
function timer(){  
var obj=document.getElementById('timer_inp');  
obj.innerHTML--;  
    
if(obj.innerHTML==0){alert('Hello');setTimeout(function(){},1000);}  
  else{setTimeout(timer,1000);}  
}  
setTimeout(timer,1000);  
</script>  
<div id="timer_inp">10</div>