how to select a select option with javascript code example

Example 1: make select option selected javascript

document.getElementById('personlist').getElementsByTagName('option')[11].selected = 'selected'

Example 2: js select option in form

<form name="AddAndEdit">
   <select name="list" id="personlist">
      <option value="P1">Person1</option>
      <option value="P2">Person2</option>
      <option value="P3">Person3</option>
   </select>
</form>