How to get the selected value of dropdownlist using JavaScript

Html Code

<select id="ddlViewBy">
            <option value="1">test1</option>
            <option value="2" selected="selected">test2</option>
            <option value="3">test3</option>
 </select>

Javascript code

var e = document.getElementById("ddlViewBy");

var strUser = e.options[e.selectedIndex].value;