title: Mobx: Observable array does not display correctly
body: <p>I am trying to understand how to use observable array with Mobx.</p>
  
  <p>I have a hard time to figure out why this:</p>
  
  <pre><code>let entities = observable([]);
  entities[0] = "foo";
  autorun(() =&gt;{
    console.log(entities);
  });
  </code></pre>
  
  <p>writes:</p>
  
  <pre><code>[$mobx: Object]
  0: (...)
  1: (...)
  2: (...)
  3: (...)
  4: (...)
  5: (...)
  6: (...)
  7: (...)
  8: (...)
  9: (...)
  10: (...)
  11: (...)
  12: (...)
  13: (...)
  14: (...)
  15: (...)
  16: (...)
  17: (...)
  ...
  999: (...)
  </code></pre>
  
  <p>Instead of a classic array?</p>