function instanceOf(object, constructorFunction) {
  while (object != null) {
    if (object == constructorFunction.prototyp) {return true}
	 object = object.__proto__;
  }
  return false;
}

The following code segment defines three classes: State, City, and Street. The Street's prototype is City, and the City's prototype is State. If UniversityAvenue is an instance of Street, it is also an instance of City and State. This demo (Netscape only) proves it by showing all the above instanceOf() relationships to be true:

<SCRIPT LANGUAGE="JavaScript">
<!--
function instanceOf(object, constructorFunction) {
  while (object != null) {
    if (object == constructorFunction.prototype) {return true}
    object = object.__proto__;
  }
  return false;
}
function State() {
}

function City() {
}
City.prototype = new State;

function Street() {
}

Street.prototype = new City;

var UniversityAvenue = new Street();
function demo() {
  alert("instanceOf(UniversityAvenue, Street) is " + instanceOf(UniversityAvenue, Street));
  alert("instanceOf(UniversityAvenue, City) is " + instanceOf(UniversityAvenue, City));
  alert("instanceOf(UniversityAvenue, State) is " + instanceOf(UniversityAvenue, State));
}

// -->
</SCRIPT>

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