从属性元素

Question

我有javascript函数来控制按钮的可见性，这些按钮的设置取决于参数的值。

function checkStateDropDownLists(avaiable,selected) {
    if (avaiable < 1) {
        document.getElementById("btnAddHouse").style.visibility = 'hidden';
        document.getElementById("btnShowDescription").style.visibility = 'hidden';
    }
    else {
        document.getElementById("btnAddHouse").style.visibility = 'visible';
        document.getElementById("btnShowDescription").style.visibility = 'visible';
    }
    if (selected < 1) {
        document.getElementById("btnRemoveSelected").style.visibility = 'hidden';
        document.getElementById('btnConfirmHouses').style.visibility = 'hidden'
    }
    else {
        document.getElementById("btnRemoveSelected").style.visibility = 'visible';
        document.getElementById('btnConfirmHouses').style.visibility = 'visible'
    }
}

我在标记脚本中调用此函数

checkStateDropDownLists(document.getElementById("ReservationAvaiableHouses").length, document.getElementById("ReservationSelectedHouses").length);

元素： ReservationAvaiableHouses 和 ReservationSelectedHouses 是下拉列表，参数是每个选项的数量。以这种方式打电话没有任何结果。是格式给定参数还是这种方法调用应该在head section之外？方法的定义在头部。

Html代码

<div class="form-horizontal">
        <h4>Domek</h4>
        <hr />
        @Html.ValidationSummary(true, "", new { @class = "text-danger" })

        <div id="housesDropdownLists">

            <div class="form-group">
                @Html.Label("Rodzaje domków", htmlAttributes: new { @class = "control-label col-md-2" })
                <div class="col-md-10">
                    @Html.DropDownList("ReservationAvaiableHouses", Model.AvaiableHouses, new { id = "ReservationAvaiableHouses" })
                    <input type="button" id="btnShowDescription" value="Opis" />
                    <div id="description" style="display:inline-flex">

                    </div>
                </div>

            </div>

            <div class="form-group">
                @Html.Label("Wybrane domki", htmlAttributes: new { @class = "control-label col-md-2" })
                <div class="col-md-10">
                    @Html.DropDownList("ReservationSelectedHouses", Model.SelectedHouses, new { id = "ReservationSelectedHouses" })
                    <input type="button" id="btnRemoveSelected" value="Usuń zaznaczenie" onclick="handler(event)" />
                </div>
            </div>

        </div>
        <div class="form-group">
            <div class="col-md-10">
                <input type="button" id="btnAddHouse" value="Dodaj domek" class="btn btn-default"  />
                <input type="submit" id="btnConfirmHouses" value="Potwierdź domki" class="btn btn-default" style="visibility:hidden" />
            </div>
        </div>
    </div>