concatenative-inheritance.js

window.test = tape;

// Concatenative inheritance example; 
var Animal = {
  animalName: 'animal',
  
  describe: function() {
    return 'This is an ' + this.animalName + '. ';
  }
};

var ElephantFactory = (function() {
   // default properties of elephant
   var weight = {weight: 2000};
   var animalName = {animalName: 'elephant'};
   var elephant = Object.create(Animal);
   
   // mutators of private fields
   elephant.getWeight = function() {
      return weight.weight;
   };
   
   elephant.setWeight = function(weight) {
      weight.weight = weight;
   };
   
   // @overridden method
   elephant.describe = function () {
     return Animal.describe.call(this) 
             + 'Which weighs is ' 
             + elephant.getWeight();
   }
   
   // Create closure for separating private fields from free access
   return function(options) {
     weight.weight = options.weight || weight.weight;
     delete options.weight;
     
     return Object.assign(elephant, animalName, options);
   }
})();

test('check inheritance', function(assert) {
  var elephantWeight = 5000;
  var expectedDescribe = 'This is an elephant. Which weighs is ' + elephantWeight;

  var elephant = ElephantFactory({
    animalName: 'elephant',
    weight: elephantWeight
  });
  
  assert.equal(elephant.weight, undefined);
  assert.equal(elephant.getWeight(), elephantWeight);
  assert.equal(elephant.describe(), expectedDescribe);
  assert.end();
});