title: Knockout JS - not a function error
body: <p>My HTML Goes like Following:</p>
  
  <pre><code>&lt;td&gt;Total Amount to be Paid: &lt;/td&gt;
  &lt;td data-bind="text: invoiceAmount() + ' Kr'"&gt;&lt;/td&gt;
  &lt;td&gt;bill:&lt;/td&gt;
  &lt;td data-bind="text: billAmount()"&gt;&lt;/td&gt;
  &lt;td&gt;Chosen Provision: &lt;/td&gt;
  &lt;td&gt;&lt;input type="text" data-bind="value: chosenProvision()"/&gt;  Kr&lt;/td&gt;
  </code></pre>
  
  <p>JS:</p>
  
  <pre><code>self.invoiceAmount = ko.computed(function(){
          var total = 0;
          if(self.selectedInvoices().length &gt; 0){
              for(var p = 0; p &lt; self.selectedInvoices().length; ++p)
              {
                  total += Number(self.selectedInvoices()[p].invoicedArticleFinal);
              }
          }
  
          return total;
      });
  
      self.billAmount = ko.computed(function(){
          return self.invoiceAmount() + self.chosenProvision();
      });
  
      self.chosenProvision = ko.computed(function(){
          if(self.invoiceAmount() &lt; 750){
              return "50";
          } else {
              return (self.invoiceAmount()*0.1);
          }
      });
  </code></pre>
  
  <p>I keep getting error: self.chosenProvision is not a function.</p>
  
  <p>I also want to be able to modify the chosenProvision from browser that is why it's binding is to a input tag.</p>