[Dilettante_Pro]

<ul>
   <li  value="1">Возрастание</li>
   <li  value="-1">Убывание</li>
   <li  value="0">Стоп</li>
</ul>
<input type="text" id = "count" value = "1">
<script>
   var loop,
        variant = document.querySelectorAll("li");
   for(var i = 0;i<variant.length;i++) {
         variant[i].onclick = foo;
   }
   function foo() {
       clearInterval(loop);
       var step = +this.value;
       loop = setInterval(function() {
           count.value = (+count.value + step).toString();
       }, 1000);     
    }
</script>