title: what's the result of 1 + undefined
body: <pre><code>1 + undefined = ?  
  </code></pre>
  
  <ol>
  <li>first, String(undefined) get string "undefined"</li>
  <li>second, 1 + "undefined" = "1undefined"</li>
  </ol>
  
  <p>what's wrong?</p>
  
  <p>I run it in chrome console ,it return <code>NaN</code>.</p>
  
  <p>can you pls explain the result?</p>
  
  <p>I think it should be "1undefined".
  tks</p>