title: Arrary map strange behavor
body: <p>I am tring to use parseInt in a <code>map</code>, but some strange behavor happened,
  here is code:</p>
  
  <pre><code>["1","2"].map(parseInt)
  </code></pre>
  
  <p>the result is:</p>
  
  <pre><code>[1, NaN]
  </code></pre>
  
  <p>but when I wrap the <code>parseInt</code> function:</p>
  
  <pre><code>["1","2"].map(function(n){ return parseInt(n)})
  </code></pre>
  
  <p>it works fine, the result is:</p>
  
  <pre><code>[1, 2]
  </code></pre>
  
  <p>why?</p>