title: Angular: Defining models and declaring objects with empty values
body: <p>I am building an angular application and I have defined couple os models in model.ts file. 
  I have created an instance of this object in my component. But when I am trying to directly assign a value to a property, it says cannot assign value to undefined. </p>
  
  <p>models.ts</p>
  
  <pre><code>export class Staff {
      id: string;
      name: string;
      address: Address;
  }
  
  export class Address{
  streetName: string;
  zipCode: string;
  }
  </code></pre>
  
  <p>component.ts</p>
  
  <pre><code>  public staff: Staff =  &lt;Staff&gt;{};
  
  staff.address.zipCode = "28657";
  </code></pre>
  
  <p>Here when I try to directly assign a value to the zipcode, it throws an error. 
  Is there a way to declare the object with default empty values '' ?</p>