โค๊ดตามนี้ครับ
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<script>
function swapCurrency(){
var list_box_a = document.getElementById('list_box_a').value;
var list_box_b = document.getElementById('list_box_b').value;
document.getElementById('list_box_b').value = list_box_a;
document.getElementById('list_box_a').value = list_box_b;
alert(list_box_a);
}
</script>