isPrototypeOf.html

<script src="simpletest.js"></script>
<script>

  /*
  The function isPrototypeOf should work just like Object.prototype.isPrototypeOf

  Syntax:
    Formula: isPrototypeOf(parentObject, childObject)

    Arguments: 
      parentObject
      childObject

    Return Value: A Boolean indicating whether the calling object lies in the prototype chain of the specified object

  Tests:
    It should throw a typeError if childObject or parentObject are not objects [DONE]
    It should return true if childObject is a prototype of parentObject [DONE]
    It should return false if childObject is not a prototype of parentObject [DONE]
    It should work for any number of prototype links
  */

  function isPrototypeOf(parentObject, childObject) {

    // Return false if you have reached the top of the prototype chain
    if (childObject === null) {
      return false;
    }

    // Check if parent and child objects are of type object
    if (typeof parentObject !== 'object' || typeof childObject !== 'object') {
      throw new TypeError('Parent or child is not object type');
    }

    // Return true if childObject is prototype of parentObject
    if (Object.getPrototypeOf(childObject) === parentObject) {
      return true;
    }

    // Recurse up the prototype chain to check ifPrototypeOf
    else {
      return isPrototypeOf(parentObject, Object.getPrototypeOf(childObject));
    }
  }


  tests({
    'It should throw a typeError if childObject or parentObject are not objects': function () {
      var isTypeError = false;
      try {
        isPrototypeOf();
      }
      catch (e) {
        isTypeError = e instanceof TypeError;
      }
      eq(isTypeError, true);
    },

    'It should return true if childObject is a prototype of parentObject': function() {
      var parentObject = {name: 'xavier'};
      var childObject = Object.create(parentObject);

      eq(isPrototypeOf(parentObject, childObject), true);
    },

    'It should return false if childObject is not a prototype of parentObject': function() {
      var parentObject = {name: 'xavier'};
      var childObject = {name: 'ally'};

      eq(isPrototypeOf(parentObject, childObject), false);
    },

    'It should work for any number of prototype links': function() {
      var canine = {
        bark: function() {
          console.log('bark');
        }
      };  

      var dog = Object.create(canine);
      dog.fetch = function() {
        console.log('fetch');
      };

      var myDog = Object.create(dog);

      eq(isPrototypeOf(canine, myDog), true);
    }


  });
</script>