title: 'undefined' variable works as key to object with 'undefined' property name
body: <p>Consider the following...</p>
  
  <pre><code>var x = {};
  x.undefined = "Hello World!";
  var y;
  
  //Prints "Hello World!"
  console.log(x[y]);
  </code></pre>
  
  <p><a href="http://jsfiddle.net/TuWfK/" rel="noreferrer">Working jsFiddle</a></p>
  
  <p>Why does this happen? Is it because of <a href="https://stackoverflow.com/questions/776950/javascript-undefined-undefined">this</a>, where it is returning a string instead of the actual undefined?</p>