<script type="text/javascript">
  var a = 1;
  console.log(Object.getOwnPropertyDescriptor(window, 'a'));   //false
  aa=1;
  console.log(Object.getOwnPropertyDescriptor(window, 'aa'));  //true
  with({}) {
    var b = 1;
    console.log(Object.getOwnPropertyDescriptor(window, 'b')); //false
    console.log(delete b);
  }
  eval('var c = 1');
  console.log(Object.getOwnPropertyDescriptor(window, 'c'));   //true
</script>