March 3, 2001 - Finding the Object's Superclass | WebReference

March 3, 2001 - Finding the Object's Superclass

Yehuda Shiran March 3, 2001
Finding the Object's Superclass
Tips: March 2001

Yehuda Shiran, Ph.D.
Doc JavaScript

Sometimes, you need to find out if a certain object is an instance of a given class (constructor function). In other languages, this operation is called instanceOf(). JavaScript does not support the instanceOf() method, but we can write one ourselves, using the internal __proto__ (two underscores on each side) property. The algorithm is based on searching the object's constructor along the inheritance chain, using __proto__:

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>