Javascript: How to copy all options from one select element to another?

One of the easiest ways without looping, is using jquery (select1 = id of select 1, select2 = id of select 2):

$('#select1 option').clone().appendTo('#select2');

Without jquery:

var select1 = document.getElementById("select1");
var select2 = document.getElementById("select2");
select2.innerHTML = select2.innerHTML+select1.innerHTML;

html:

<select id="selector_a">
    <option>op 1</option>
    <option>op 2</option>
</select>

<select id="selector_b">
    <option>op 3</option>
    <option>op 4</option>
</select>

javascript:

var first = document.getElementById('selector_a');
var options = first.innerHTML;

var second = document.getElementById('selector_b');
var options = second.innerHTML + options;

second.innerHTML = options;