Dropdown menu item reversal

Dropdown menu item reversal

I am new to javascript and my code looks as below
In HTML:
<select name="dropdown" onChange="javascript: alertUser()" >
<option value="0" selected>Eat</option>
<option value="1">Work</option>
<option value="2">Sleep</option>
<option value="3" >Enjoy</option>
In javascript:
function alertUser()
{
confirm("Are you sure?");
}



Now, here is my problem.
When I select an element from dropdown, it will pop up confirmation
dialogue "Are you Sure?". If I click Ok, the selection should change to
selected value. If I cancel, the selected value should reset back to
previously selected value in dropdown list.
Could anyone please help me to solve the problem.
Thank you