javascript中的fadeIn（），fadeOut（）不起作用

Question

I want to use fadeIn and fadeOut in javascript in my webpage;but it doesn't work; it is my code;can you say me where is the problem?it doesn't fadeIn and fadeOut at all!!!

 $(document).ready(function () {

$("#DropDownList1").each(function () {

                $('ListItem', this).each(function () {

                    if ($(this).attr("selected") == true)

                        if (($(this).text() != "a")) {

                            $("div#select").fadeOut();
                                                  }
                        else {
                            $("div#select").fadeIn();

                        }
                });
            });
});

this is my dropdown list:

 <asp:DropDownList ID="DropDownList1" runat="server">
            <asp:ListItem Value=""></asp:ListItem>
            <asp:ListItem Value="m">a</asp:ListItem>
            <asp:ListItem Value="m1">b</asp:ListItem>
            </asp:DropDownList>

this is my actual html:

<td class="style5" width="20%">
situation:&nbsp;
<br>
<select id="DropDownList1" name="DropDownList2">
<option value=""></option>
<option value="m">a</option>
<option value="m1">b</option>
</select>

Answer 1

Turning my comments into an answer...

You should look at the actual HTML that the browser sees (not the ASP template code) because I don't think jQuery sees a ListItem in the HTML. Standard dropdowns in HTML are <option> tags. Your ASP might be generating that or something else.

Do a View/Source in the browser and look at the HTML that the browser actually sees and code your jQuery to that. $('ListItem', this) probably isn't finding anything. You should pretty much never go by the ASP code when writing jQuery to operate on it. You need to know what the browser actually sees after the ASP template has been turned into regular HTML.

FYI, a simplified version of your code that also runs any time the drop down list selection is changed works like this:

$(document).ready(function () {

    function checkDropDownList() {
        if ($("#DropDownList1 option:selected").text() != "a") {
           $("#select").fadeOut();
        } else {
            $("#select").fadeIn();
        }
    }

    checkDropDownList();

    $("#DropDownList1").change(checkDropDownList);

});

There's no reason to use .each() if there can only be one selected item and we already know there's only one DropDownList1 .