how to set select options in javascript code example

Example 1: 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>

Example 2: javascript set select option

document.getElementById("mySelectID").value="my_value";
//note: setting value in html directly does not work <select value="my_value">