title: JavaScript: parseInt's radix mystery with Array.map
body: <p>When I execute the following statements:</p>
  
  <pre class="lang-js prettyprint-override"><code>var string = "1 1 1 1 1 1 0 1 1"
  console.log(string)
  var strings = string.split(" ")
  console.log(strings)
  var numbers1 = strings.map(parseInt)
  console.log(numbers1)
  var numbers2 = strings.map(function(i){ return parseInt(i, 10) })
  console.log(numbers2)
  </code></pre>
  
  <p>I get the following output in the console:</p>
  
  <pre><code>1 1 1 1 1 1 0 1 1
  ["1", "1", "1", "1", "1", "1", "0", "1", "1"]
  [1, NaN, 1, 1, 1, 1, 0, 1, 1]
  [1, 1, 1, 1, 1, 1, 0, 1, 1]
  </code></pre>
  
  <p>I wonder why the second element in <code>numbers1</code> is NaN. Why does it work on all the other elements except this one?</p>