kangax

/*

Checks that `toString` of constructor's prototype has "special" behavior (i.e. is `Object.prototype.toString`) as per 15.2.4.2

 Cons: 
    - Relies on `constructor` property of an object (as well as `toString` of constructor's prototype) being present and not augmented
      (will fail on host objects in IE that lack constructor, but could be guarded against)
    - Does not differentiate between Object objects created implicitly via function constructor and those created explicitly via Object

 Pros:

kangax

/* can you spot what is happening here */
/* the alert("X") should never execute */
<script>
onload = function() {

  var fn = function() {
    alert("X");
  }

  (function() { })();