DontDelete Flag -JS

  /* create global property via variable declaration; property has DontDelete */
  var num = 1;

  /* create global property via undeclared assignment; property has no DontDelete */
  prop = 2;

  delete num; // false
  typeof num; // "number"

  typeof prop; // "number"
  delete prop; // true
  typeof prop; // "undefined"

  /* `func` is created as a property with DontDelete */
  function func(){}

  /* Later assignments do not modify attributes. DontDelete is still there! */
  func = 1;
  delete func; // false
  typeof func; // "number"

  /* But assigning to a property that doesn't exist,
     creates that property with empty attributes (and so without DontDelete) */

  this.bar = 1;
  delete bar; // true
  typeof bar; // "undefined"