title: How to join two interfaces declarations into one?
body: <p>I have two properties of an interface:</p>
  
  <pre><code>export interface IInvoicesData {
      invoice: IInvoice;
      invoiceWithTotals: IInvoice &amp; IInvoiceTotals;
  }
  </code></pre>
  
  <p>This works fine and I make <code>invoiceWithTotals</code> property to contain properties from both <code>IInvoice</code> &amp; <code>IInvoiceTotals</code> interfaces.</p>
  
  <p>How can I move declaration <code>IInvoice &amp; IInvoiceTotals</code> to its own interface so that replace <code>IInvoice &amp; IInvoiceTotals</code> with something like <code>IInvoiceWithTotals</code> besides using <code>extends</code>?</p>