title: How to extend Typescript interface with two another interfaces and add 'null' to one property
body: <p>I have two TS interfaces:</p>
  <pre><code>interface Alpha { name: string; login: string; amount: string; }
  </code></pre>
  <p>and</p>
  <pre><code>interface Beta { id: number; name: string; login: string; amount: string; }
  </code></pre>
  <p>I would like to create the third interface Gamma which extends those two, add new field to it and make amount field can be null as well as a string:</p>
  <pre><code>interface Gamma extends Alpha, Beta { targetAmount: string | null, amount: string | null }
  </code></pre>
  <p>Those <strong>amount: string | null</strong> is not possible to do without extra actions. I've find a few answers here, but they are about extending types or 'nulling' all properties.</p>