title: Multiple Variables Javascript map() callback
body: <p>I was doing some reading about es2015, trying out some of the examples on the arrow syntax when I came across this code:</p>
  
  <pre><code>var odds = evens.map(v =&gt; v + 1);//I understand
  var nums = evens.map((v,i) =&gt; v + i);//I don't understand
  </code></pre>
  
  <p>with <code>evens</code> initialized to <code>[0,2,4,6]</code> I get <code>[1,3,5,7]</code> from <code>odds</code>, which I understand.</p>
  
  <p>However I don't understand the result of the the second expression <code>[0,3,6,9]</code>. Essentially I'm not understanding what values are being assigned to the variable <code>i</code> in the second example. Can anyone shed any light on this?</p>