title: In Typescript how to fix Cannot set property 'first' of undefined
body: <p>I'm trying to set a the sub property <code>first</code> that is defined in the <code>Name</code> interface but when do so I always get an error for example:</p>
  
  <pre><code>interface Name{
      first: string,
      last:string,
  }
  
  class Person{
  
      private name:Name
  
      public setName(firstName, lastName){
          this.name.first = firstName;
          this.name.last = lastName;
      }
  
  }
  
  
  var person1  = new Person();
  person1.setName('Tracy','Herrera');
  </code></pre>
  
  <p>When running it i get the error: <code>Cannot set property 'first' of undefined</code></p>
  
  <p>Any one have an idea to work this out?</p>