Simple Javascript / html to alert

Question

I'm currently playing around with javascript and looking for whenever I press a html button that it will read the value of a html drop down menu and create an alert with it.

Drop down :

select id="valueDropdown">
  <option value="lessThanFive">below 5</option>
  <option value="betweenSixAndTen">6-10</option>
  <option value="tenPlus">10+</option>
 </select>

when a button is pressed (this part is omitted because it works) then it will call this function :

 var e = document.getElementById("valueDropdown");
if(e=='lessThanFive'){
alert(e);
}