如何计算列表中未选定的项

Question

在这些代码中，我可以获得所选的项，未选择的项如何？

 <select multiple="multiple" class="multi-select" id="clientIdList" name="clientIdList">
                           @{
                               foreach (var client in Model.ClientLists)
                               {
                                   <option @(client.IsChecked ? "selected" : "") value="@client.ClientId">@client.ClientName</option>
                               }
                           }
                       </select>

     var foo = document.getElementById('clientIdList');
    if (foo) {
        if (foo.selectedIndex < 0) {
            ShowError('Please select client', 'divNotiMailBoxSetting');

            return false;
        }

Answer 1

从一个<option>元素中获取未选择的<select>项数的一种方法：

// get all the option elements from the select element with the given id:
var opts = document.getElementById('clientIdList').options,
// create an object of arrays to contain the selected and unselected elements:
    choices = {
        'selected' : [],
        'unselected' : []
    };

// using Array.prototype.forEach to iterate over the NodeList of option elements:
[].forEach.call(opts, function (optElement) {
    // adding the current optElement to the selected, or unselected, array depending
    // on whether it's selected or unselected:
    choices [optElement.selected ? 'selected' : 'unselected'].push(optElement);
});

// assigning the length of the array of unselected elements to a variable for use,
// or to be returned to the calling context (if this is used inside of a function):
var unselectedElements = choices.unselected.length;

但是，在不了解您正在做什么的情况下，我必须让您在您的函数中实现这一点；然而，这个JS Fiddle演示: http://jsfiddle.net/davidThomas/dxkhtdkn/1/演示了一个简单的用例。

参考文献：

• Array.prototype.forEach()。
• Array.prototype.push()。
• Function.prototype.call()。

Answer 2

您可以使用length属性：

foo.selectedIndex.length