how to change selected option of dropdown in javascript code example

Example: html get selected option javascript

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

<script>
    var e = document.getElementById("ddlViewBy");
	var strUser = e.value; // 2
	var strUser = e.options[e.selectedIndex].text; //test2
</script>