var Obj = document.getElementById('id')
 

 
<select name="dropdown" onchange="control(this)">
	<option value="">Please Select</option>
	<option value="123">test</option>
	<option value="456">test2</option>
</select>
 

 
Obj.value;
 

 
Obj.options[Obj.selectedIndex].text
 

 
Obj.options.add(new Option('text', 'value'))
 

 
Obj.remove(index)
 

 
Obj.options.length = 0
 

 
function move(fromID,toID)
{
	var from = document.getElementById(fromID);
	var to = document.getElementById(toID);
 
	for (var i = 0; i < from.options.length; i++)
	{
		if (from.options[i].selected == true)
		{					
			to.options.add(new Option(from.options[i].text,from.options[i].value))
			from.remove(i);
			i--;
		}
	}
}
 

 
<input type="button" onclick="shift('items', 'up')" value="Up" />
<input type="button" onclick="shift('items', 'down')" value="Down"/>
 

 
function shift(id, direction)
{
	var items = document.getElementById(id);
	var oldIndex = items.selectedIndex;
 
	if (oldIndex != -1)
	{	
		with (items)
		{
			if (direction == 'up')
				var newIndex = (oldIndex == 0) ? options.length - 1 : oldIndex - 1;
			else if (direction == 'down')
				var newIndex = (oldIndex == options.length - 1) ? 0 : oldIndex + 1;
			else
				return false;
 
			var a = new Option(options[oldIndex].text, options[oldIndex].value);
			var b = new Option(options[newIndex].text, options[newIndex].value);
			options[oldIndex] = b;
			options[newIndex] = a;					
			selectedIndex = newIndex;
		}
	}
}