With jQuery it's super easy:
var lastValue = $('#idOfSelect option:last-child').val();
With plain Javascript it's not much worse:
var theSelect = document.getElementById('idOfSelect');
var lastValue = theSelect.options[theSelect.options.length - 1].value;
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…