title: How to define an interface for objects with dynamic keys?
body: <p>I have an Object like this that is created by underscore's <code>_.groupBy()</code> method.</p>
  
  <pre><code>myObject = {
    "key" : [{Object},{Object2},{Object3}],
    "key2" : [{Object4},{Object5},{Object6}],
    ...
  }
  </code></pre>
  
  <p>How would I define that as an Interface with TypeScript?
  i don't want to simply define it as <code>myObject:Object = { ...</code> but rather have an own type for it.</p>