title: Is null considered zero and undefined not a number on arithmetic expressions?
body: <p>Is <code>null</code> evaluated to <code>0</code> and <code>undefined</code> to <code>NaN</code> on arithmetic expressions?</p>
  
  <p>According to some testing it seems so:</p>
  
  <pre><code>&gt; null + null
  0
  
  &gt; 4 + null
  4
  
  &gt; undefined + undefined
  NaN
  
  &gt; 4 + undefined
  NaN
  </code></pre>
  
  <p>Is it safe or correct to assume this? (a quote from a documentation would be A+).</p>