Select An Item On A Dropdown List

I have have some values stored in javascript variables. I have a
<select> dropdown list whose options correspond to these values. I want
to be able to select an item on the dropdown list based on the value of
the javascript variable.

Let's say this is my list and my variable:

<select id='popup'>
<option value="default">--Please Choose a saying--</option>
<option value="hello">Hello</option>
<option value="goodbye">Goodbye</option>
</select>

var input = "hello";

Is there a way to select the 2nd option using using that variable
value? Something like:

var popup = document.getElementById("popup");
popup.selectedItem = input;