javascript 取得 select option 複選值

需求:
有二個Selector  selectA & selectB, selectB的值是根據selectA所產生, 且selectA為複選



<div>
<select class="selectpicker"  id="city" onchange="getUserSelect()" title="Multiple Selector" data-actions-box="true" data-live-search="true" multiple></select>
</div>

//get user option on selectA
function getUserSelect() {
    var selectCity = $('#city').val();
    var scity = "";

    if (!selectCity) {
        alert("No");
    } else {
        for (var i = 0; i < selectCity.length; i++) {
            scity = scity + selectCity[i];
        }
    }
    //call selectB function.
}