blocks|key|4411010|text|你可以写更多的代码来帮助我们理解你在做什么。我假设您想要在更改dropdown2时更改dropdown1的值。如果我是对的，您可以使用change+event来完成此操作。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4411011|​|4411012|var+ViewModel+=+function()+{
++var+self+=+this;

++self.data+=+ko.observableArray(["Dog",+"Cat",+"Bird"]);

++self.dropdown1+=+ko.observable(undefined);
++self.dropdown2+=+ko.observable(undefined);

++self.changeDropDownValue+=+function(index)+{
++++if+(index+==+1)
++++++self.dropdown2(undefined);
++++else
++++++self.dropdown1(undefined);
++}
}

var+viewModel+=+new+ViewModel();
ko.applyBindings(viewModel);|code-block|syntax|javascript|4411013|<script+src="https://cdnjs.cloudflare.com/ajax/libs/knockout/3.2.0/knockout-min.js"></script>

<select+data-bind="options:data,+optionsCaption:'Select+a+pet...',value:+$root.dropdown1,+event:{change:$root.changeDropDownValue(1)}"+></select>
<select+data-bind="options:data,+optionsCaption:'Select+a+pet...',value:+$root.dropdown2,+event:{change:$root.changeDropDownValue(2)}"+></select>|4411014|4411015|entityMap^0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|N|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|O|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|P|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|F|7|Q|8|@]|9|@]|A|$G|H]]|$1|K|3|C|5|6|7|R|8|@]|9|@]|A|$]]|$1|L|3|-4|5|6|7|S|8|@]|9|@]|A|$]]]|M|$]]

<p>You could put more code to help us to understand what are you doing. I'm assuming that you want to change the value of dropdown1 when you change the dropdown2. If I'm right, you could use change event to do that.</p>

<p><div class="snippet" data-lang="js" data-hide="false" data-console="true">
<div class="snippet-code">
<pre class="snippet-code-js lang-js prettyprint-override"><code>var ViewModel = function() {
  var self = this;

  self.data = ko.observableArray(["Dog", "Cat", "Bird"]);

  self.dropdown1 = ko.observable(undefined);
  self.dropdown2 = ko.observable(undefined);

  self.changeDropDownValue = function(index) {
    if (index == 1)
      self.dropdown2(undefined);
    else
      self.dropdown1(undefined);
  }
}

var viewModel = new ViewModel();
ko.applyBindings(viewModel);</code></pre>
<pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;script src="https://cdnjs.cloudflare.com/ajax/libs/knockout/3.2.0/knockout-min.js"&gt;&lt;/script&gt;

&lt;select data-bind="options:data, optionsCaption:'Select a pet...',value: $root.dropdown1, event:{change:$root.changeDropDownValue(1)}" &gt;&lt;/select&gt;
&lt;select data-bind="options:data, optionsCaption:'Select a pet...',value: $root.dropdown2, event:{change:$root.changeDropDownValue(2)}" &gt;&lt;/select&gt;</code></pre>
</div>
</div>
</p>


blocks|key|4411251|text|我认为您不应该使用jQuery来编写这种逻辑。下面是一个示例，说明在视图模型中如何在视图不了解任何逻辑的情况下执行此操作。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4411252|​|4411253|var+VM+=+function()+{
++
++var+self+=+this;

++self.items+=+ko.observableArray(["Item1",+"Item2",+"Item3"]);

++self.dropdown1+=+ko.observable();
++self.dropdown2+=+ko.observable();

++self.dropdown1.subscribe(function(newValue)+{
++++if+(newValue)+{
++++++self.dropdown2(null);
++++}
++});

++self.dropdown2.subscribe(function(newValue)+{
++++if+(newValue)+{
++++++self.dropdown1(null);
++++}
++});
}

ko.applyBindings(new+VM());|code-block|syntax|javascript|4411254|<script+src="https://cdnjs.cloudflare.com/ajax/libs/knockout/3.2.0/knockout-min.js"></script>

<select+data-bind="options:items,+optionsCaption:'Select+first+item',value:+dropdown1"></select>

<select+data-bind="options:items,+optionsCaption:'Select+second+item',value:+dropdown2"></select>|4411255|4411256|entityMap^0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|N|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|O|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|P|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|F|7|Q|8|@]|9|@]|A|$G|H]]|$1|K|3|C|5|6|7|R|8|@]|9|@]|A|$]]|$1|L|3|-4|5|6|7|S|8|@]|9|@]|A|$]]]|M|$]]

<p>I think you should not be using jQuery to code that logic. Here is an example of how this can be done in a view model with view not knowing anything about the logic.</p>

<p><div class="snippet" data-lang="js" data-hide="false" data-console="true">
<div class="snippet-code">
<pre class="snippet-code-js lang-js prettyprint-override"><code>var VM = function() {
  
  var self = this;

  self.items = ko.observableArray(["Item1", "Item2", "Item3"]);

  self.dropdown1 = ko.observable();
  self.dropdown2 = ko.observable();

  self.dropdown1.subscribe(function(newValue) {
    if (newValue) {
      self.dropdown2(null);
    }
  });

  self.dropdown2.subscribe(function(newValue) {
    if (newValue) {
      self.dropdown1(null);
    }
  });
}

ko.applyBindings(new VM());</code></pre>
<pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;script src="https://cdnjs.cloudflare.com/ajax/libs/knockout/3.2.0/knockout-min.js"&gt;&lt;/script&gt;

&lt;select data-bind="options:items, optionsCaption:'Select first item',value: dropdown1"&gt;&lt;/select&gt;

&lt;select data-bind="options:items, optionsCaption:'Select second item',value: dropdown2"&gt;&lt;/select&gt;</code></pre>
</div>
</div>
</p>


blocks|key|4411323|text|OP问题是：“如何为knockout+JS变量分配未定义的值？”我在这里读到的其他答案并没有明确回答这个问题。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4411324|我认为在过去，someObservable(undefined)的作用可能与someObservable()完全相同，返回的是可观察到的值。然而，在最近的Knockout版本(在3.4.2上测试)中，该语法确实可以在可观察值上进行重置。|offset|length|style|CODE|4411325|通过运行此脚本亲自查看。|4411326|​|4411327|let+x+=+ko.observable();
console.log(x());
x(1);
console.log(x());
x(undefined);
console.log(x());|code-block|syntax|javascript|4411328|<script+src="https://cdnjs.cloudflare.com/ajax/libs/knockout/3.4.2/knockout-min.js"></script>|4411329|4411330|entityMap^0|0|7|P|12|G|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|V|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|W|8|@$D|X|E|Y|F|G]|$D|Z|E|10|F|G]]|9|@]|A|$]]|$1|H|3|I|5|6|7|11|8|@]|9|@]|A|$]]|$1|J|3|K|5|6|7|12|8|@]|9|@]|A|$]]|$1|L|3|M|5|N|7|13|8|@]|9|@]|A|$O|P]]|$1|Q|3|R|5|N|7|14|8|@]|9|@]|A|$O|P]]|$1|S|3|K|5|6|7|15|8|@]|9|@]|A|$]]|$1|T|3|-4|5|6|7|16|8|@]|9|@]|A|$]]]|U|$]]

<p>The OP question was: "How to assign undefined value to a knockout JS variable?". The other answers I read here do not explicitly answer that question.</p>

<p>I think it might have been true in the past that <code>someObservable(undefined)</code> would do exactly the same as <code>someObservable()</code>, returning the observable value that is. However in recent Knockout versions (tested on 3.4.2) that syntax does work to reset on observable value.</p>

<p>See for yourself by running this script.</p>

<p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
<div class="snippet-code">
<pre class="snippet-code-js lang-js prettyprint-override"><code>let x = ko.observable();
console.log(x());
x(1);
console.log(x());
x(undefined);
console.log(x());</code></pre>
<pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;script src="https://cdnjs.cloudflare.com/ajax/libs/knockout/3.4.2/knockout-min.js"&gt;&lt;/script&gt;</code></pre>
</div>
</div>
</p>


<p>I have 2 dropdownlists and one list of result with a binding using computed observable function something like this:</p>

<pre><code>     self.Records = ko.computed(function() {
                    if (self.dropdown1 !== undefined) {
 ...
                        return Collection[0];
                    }

                    if (self.dropdown2() != undefined) {
                     ...
                        return Collection[0];
                    }
                });
</code></pre>

<p>But my problem is that I need to set in undefined  the value on self.dropdown1 when I change the valie of the dropdownlist2, for  my Collection of records can change.</p>

<p>I'm changing the value of the first dropdown with jquery like this:</p>

<pre><code>  $('#reportTypeSelection').prop('selectedIndex', 0);
</code></pre>

<p>But the binding is not refreshing. Any advices please.</p>


How assign undefined value in a knockout JS variable?

jQuery

我有两个下拉列表和一个带有绑定的结果列表，使用计算的可观察函数，如下所示：     self.Records = ko.computed(function() {                    if (self.dropdown1 !== undefined) { ...                    ...

问如何在knockout JS变量中分配未定义的值？
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