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    <p>i don't think you're doing anything wrong. the problem is the "if" binding to the root.isEmployee is being applied before the binding to the "with" is being updated. so the code is seeing the update to the isEmployee and then re-evaluating the view from there down, but the current context is still the old person (as that subscription hasn't fired).</p> <p>this is proven via a custom binding in <a href="http://jsfiddle.net/drdamour/X6pC9/2/" rel="nofollow">http://jsfiddle.net/drdamour/X6pC9/2/</a> notice the update receives 2 events, once with the old value cause the isEmployee was updated, and second time with the updated new value. this second update comes from the "with" binding subscription being triggered. The subscription of the 'with' binding happens during the applyBindings call, which happens AFTER your model does a subscription.</p> <p>you can use $data.PropertyName trick to deal with undefined not causing issues. Ala: <a href="http://jsfiddle.net/drdamour/X6pC9/1/" rel="nofollow">http://jsfiddle.net/drdamour/X6pC9/1/</a></p> <pre><code>&lt;span data-bind="with: person"&gt; &lt;span data-bind="text: firstName"&gt;&lt;/span&gt; &lt;!-- ko if: $root.isEmployee --&gt; &lt;span data-bind="text: $data.employeeId"&gt;&lt;/span&gt; &lt;span data-bind="text: $data.employer"&gt;&lt;/span&gt; &lt;!-- /ko --&gt; &lt;/span&gt; </code></pre> <p>the RIGHT way to solve this is to have a PersonVM that has the isEmployee computed, that way you don't bind to the root. see: <a href="http://jsfiddle.net/drdamour/eVXTF/1/" rel="nofollow">http://jsfiddle.net/drdamour/eVXTF/1/</a></p> <pre><code>&lt;span data-bind="with: person"&gt; &lt;span data-bind="text: firstName"&gt;&lt;/span&gt; &lt;!-- ko if: isEmployee --&gt; &lt;span data-bind="text: $data.employeeId"&gt;&lt;/span&gt; &lt;span data-bind="text: $data.employer"&gt;&lt;/span&gt; &lt;!-- /ko --&gt; &lt;/span&gt; </code></pre> <p>and</p> <pre><code>var ViewModel = function(){ var self = this; self.person = ko.observable(); }; var PersonVM = function() { var self = this; this.firstName = ko.observable(); this.employeeId = ko.observable(); this.employer = ko.observable(); self.isEmployee = ko.computed(function(){return self.employer() != null}); } var vm = new ViewModel(); var customer = new PersonVM(); customer.firstName("John"); var employee = new PersonVM(); employee.firstName("Bill"); employee.employeeId(123); employee.employer("ACME"); vm.person(customer); ko.applyBindings(vm); setTimeout(function(){ vm.person(employee); }, 3000); </code></pre> <p>computed is preferred to subscribe methods as it deals with the subscription chain for you and abstracts you away from having to manage all that.</p>
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    1. USChris DaMour
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    1. COThank you very much. Any idea why the $data trick works?
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      1. USMike Davis
    2. CObecause $data can be resolved by the binder always, and you can always get a non-existent property of an object, it just returns undefined, and the text binding treats undefined as empty text. It would only not work if $data was null...i think.
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    3. CO+1. My head hurts trying to get my head around ko bindings.
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