title: Empty slots in JavaScript objects?
body: <p>Lately I started seeing this in firefox' console</p>
  
  <pre><code>Object [ &lt;6 empty slots&gt;, false, &lt;3 empty slots&gt;, 1 more… ]
  </code></pre>
  
  <p>When I have an object like </p>
  
  <pre><code>{
    6: false,
    10: true
  }
  </code></pre>
  
  <p>I simply want an object with numeric keys that I can access, but I am worried by this because if it keeps track of empty slots then this must mean that some memory is wasted?</p>
  
  <p>Are my concerns valid and if yes what would be a correct way to define such an object?</p>