title: Extend interface with another
body: <p>Basically I want to extend userinterface as shown below, is this the way to do it? everything besides the data field get's filled properly and using "UserInterface extends DataInterFace" just throws a lot of errors.</p>
  
  <p>There's nothing official in the documentation about this so do any of you have an idea?</p>
  
  <pre><code>export interface UserInterface {
    username: string,
    firstname: string,
    lastname: string,
    data: DataInterface
  }
  
  export interface DataInterface {
    test: string,
    test2: string
  }
  
  let test = { username: "test", firstname: "test", lastname: "test", data: {
    test: "hello",
    test2: "hello2"
  }};
  
  let DoesItWork:UserInterface = &lt;UserInterface&gt;test;
  console.log(DoesItWork);
  </code></pre>