function checkSelectInstance() {
    var instance;
    $.ajax({
        type: "POST",
        cache: false,
        data: "",
        url: "/ecs/describeInstances",
        success: function(res) {
            instance= res;
        }, error: function(data) {
            instance= "";
        }
    });
    return instance;
}

問題：呼叫這個方法後卻返回undefined，

原因：jq的ajax是非同步請求的，所以大多時候還沒有執行完ajax就return了

解決方法：新增async:false,改為同步請求就ok了

function checkSelectInstance() {
    var instance;
    $.ajax({
        type: "POST",
        cache: false,
        data: "",
        async:false,
        url: "/ecs/describeInstances",
        success: function(res) {
            instance= res;
        }, error: function(data) {
            instance= "";
        }
    });
    return instance;
}