title: How the check if a value is NULL or unassigned in knockout.js?
body: <p>Suppose in this example, <code>firstName</code> is not set and  <code>lastName</code> is assigned a value. How to check if the value is assigned or not.</p>
  
  <pre><code>function AppViewModel() {
      this.firstName = ko.observable();
      this.lastName = ko.observable('Smith');
  }
  </code></pre>
  
  <p>Which one is the best approach? Will these work?</p>
  
  <pre><code>   if(lastName  == '')
         //do something
  </code></pre>
  
  <p>or</p>
  
  <pre><code>   if(lastName)
         //do something
  </code></pre>
  
  <p>or</p>
  
  <pre><code>   if(lastName  == null)
         //do something
  </code></pre>
  
  <p>Please help.</p>