beast-5-recursive-version.html

<script src="simpleTest.js"></script>
<script>
  /**
   * Purpose of isPrototypeOf:
   * Checks if an object exists in another object's prototype chain. 
   * 
   * Signature:
   * function isPrototypeOf(searchedForObject, objectToSearchIn)
   * 
   * Parameters:
   * searchFor = The object to look for.
   * searchIn = The object whose prototype chain will be searched.
   * 
   * Return: Boolean - true/false 
   * First argument does or doesn't exist in second arguments prototype chain.
   * 
   * Errors: 
   * TypeError is thrown when first argument is null or undefined.
   */

function isPrototypeOf(searchFor, searchIn) {
  // searchFor is the prototype object search for in SearchIn's prototype chain
  if (searchFor === null || searchFor === undefined) {
    throw new TypeError('First argument cannot be null or undefined');
  }

  if (searchIn === null) {
    return false;
  }

  if (typeof searchIn === 'object' || typeof searchIn === 'function') {
    var nextLevelUpSearchInChain = Object.getPrototypeOf(searchIn);
    if (nextLevelUpSearchInChain === searchFor) {
      return true;
    } else {
      if (nextLevelUpSearchInChain instanceof Object) {
        return isPrototypeOf(searchFor, nextLevelUpSearchInChain);
      } else {
        return false;
      }  
    }
  } 

  return false; 
  
}
// Testing
var canine = {
      bark: function() {
        console.log('bark');
      }
    };

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

    var myDog = Object.create(dog);
    var empty = Object.create(null);

tests({
  'It should return true when first-argument object is found in second-argument object\'s prototype chain': function() {
    eq(isPrototypeOf(dog, myDog), true);
    eq(isPrototypeOf(canine, myDog), true);
  },
  'It should return false when first object is not found in second object\'s prototype chain': function() {    
    eq(isPrototypeOf(dog, empty), false);
  },
  'It should throw a TypeError when first argument is null or undefined': function() {
    var isTypeError = false;
     try {
       isPrototypeOf(null, myDog);
     } catch(e) {
       isTypeError = (e instanceof TypeError);
     }
     eq(isTypeError, true);
  },
  'It should work for any number of prototype links.': function () {
    eq(isPrototypeOf(canine, myDog), true);
  },
  // Chujun's tests for parameter edgecases
  'It should return false when second argument is null or undefined.': function () {
    eq(isPrototypeOf(canine, null), false);
    eq(isPrototypeOf(canine, undefined), false);
  },

  'It should return true if first argument is any kind of object.': function () {
    eq(isPrototypeOf(Object.prototype, {}), true);
    eq(isPrototypeOf(Array.prototype, []), true);
    eq(isPrototypeOf(Function.prototype, function () {}), true);
  },

  'It should return false if first argument is the prototype of a primitive data type.': function () {
    eq(isPrototypeOf(String.prototype, ''), false);
    eq(isPrototypeOf(Boolean.prototype, true), false);
    eq(isPrototypeOf(Number.prototype, 1), false);
    eq(isPrototypeOf(Symbol.prototype, Symbol()), false);
  },
});


  
</script>