javascript - Multiple Select 限制选择数量

Question

我希望用户最多只能从多个选择选项中选择三个选项。到目前为止我试过这段代码:

<select id="userRequest_activity" required name="activity[]" class="multiselect form-control" multiple="multiple">
    <option value="2">Bungee Jumping</option>
    <option value="3">Camping</option>
    <option value="5">Mountain Biking</option>
    <option value="6">Rappelling</option>
    <option value="7">Rock Climbing / Bouldering</option>
    <option value="8">Skiing</option>
    <option value="10">Wild Life (Safari)</option>
    <option value="11">Canoeing &amp; Kayaking</option>
    <option value="12">Rafting</option>
    <option value="13">Sailing</option>
    <option value="14">Scuba Diving</option>
    <option value="15">Snorkeling</option>
    <option value="16">Surfing</option>
    <option value="18">Hang Gliding</option>
    <option value="19">Hot-air Ballooning</option>
    <option value="20">Micro-light Aircrafts</option>
    <option value="21">Paragliding</option>
    <option value="22">Paramotoring</option>
    <option value="23">Parasailing</option>
    <option value="24">Skydiving / Parachuting</option>
    <option value="25">Zip-line / Flying Fox</option>
    <option value="26">Caving</option>
    <option value="27">Cycling</option>
    <option value="28">Fishing &amp; Angling</option>
    <option value="29">Motorbike trips</option>
    <option value="30">Nature Walks</option>
    <option value="31">Road Trips</option>
    <option value="32">Zorbing</option>
    <option value="33">Trekking Hiking and Mountaineering</option>
    <option value="34">Backpacking</option>
    <option value="61">Water</option>
</select>

JavaScript 代码:

<script type="text/javascript">
$("select").change(function() {
    if ($("select option:selected").length > 3) {
        $(this).removeAttr("selected");
        alert('You can select upto 3 options only');
    }
});
</script>

此代码在选择超过 3 个选项时显示警告框，但仍允许访问选择第 4、5、6 等选项并出现警告框。如何验证这一点？

Answer 1

试试这个...

检查长度并在达到最大选择后取消选择

<select id="userRequest_activity" required name="activity[]" class="multiselect form-control" multiple="multiple">
    <option value="2">Bungee Jumping</option>
    <option value="3">Camping</option>
    <option value="5">Mountain Biking</option>
    <option value="6">Rappelling</option>
    <option value="7">Rock Climbing / Bouldering</option>
    <option value="8">Skiing</option>
    <option value="10">Wild Life (Safari)</option>
    <option value="11">Canoeing &amp; Kayaking</option>
    <option value="12">Rafting</option>
    <option value="13">Sailing</option>
    <option value="14">Scuba Diving</option>
    <option value="15">Snorkeling</option>
    <option value="16">Surfing</option>
    <option value="18">Hang Gliding</option>
    <option value="19">Hot-air Ballooning</option>
    <option value="20">Micro-light Aircrafts</option>
    <option value="21">Paragliding</option>
    <option value="22">Paramotoring</option>
    <option value="23">Parasailing</option>
    <option value="24">Skydiving / Parachuting</option>
    <option value="25">Zip-line / Flying Fox</option>
    <option value="26">Caving</option>
    <option value="27">Cycling</option>
    <option value="28">Fishing &amp; Angling</option>
    <option value="29">Motorbike trips</option>
    <option value="30">Nature Walks</option>
    <option value="31">Road Trips</option>
    <option value="32">Zorbing</option>
    <option value="33">Trekking Hiking and Mountaineering</option>
    <option value="34">Backpacking</option>
    <option value="61">Water</option>
</select>

    <script type="text/javascript">


        $(document).ready(function() {

          var last_valid_selection = null;

          $('#userRequest_activity').change(function(event) {

            if ($(this).val().length > 3) {

              $(this).val(last_valid_selection);
            } else {
              last_valid_selection = $(this).val();
            }
          });
        });
        </script>

演示: http://jsfiddle.net/9c3sevuv/