Linked List Javascript implementation (node.js)

linked_stack.js

var assert = require('assert');

var LinkedStack = (function(){
  function Item(data){
    this.data = data;
    this.next = null;
  }

  Item.prototype = {
    tail: function(){
      return this.next ? this.next : this;
    }
  };

  function LinkedStack(){
    this.head = null;
  }

  LinkedStack.prototype = {
    push: function(data){
      var oldHead = this.head;
      this.head = new Item(data);
      this.head.next = oldHead;
    },
    pop: function(){
      if (this.head){
        var popped = this.head;
        this.head = popped.next;
        return popped.data;
      } else {
        return null;
      }
    },
    tail: function(){
      if (this.head){
        return this.head.tail().data;
      } else {
        return null;
      }
    }
  };

  return LinkedStack;
})();

describe('LinkedStack', function(){
  var stack;
  beforeEach(function(){
    stack = new LinkedStack();
  });

  describe('#head', function(){
    it('should return null for an empty stack', function(){
      assert.equal(null, stack.head);
    });
  });

  describe('#push', function(){
    it('should push an item onto the top of the stack', function(){
      stack.push('Hello');
      assert.equal('Hello', stack.head.data);
      stack.push('Hi');
      assert.equal('Hi', stack.head.data);
    });
  });

  describe('#pop', function(){
    it('should return null on an empty stack', function(){
      assert.equal(null, stack.pop());
    });

    it('should remove and return the top item on the stack', function(){
      stack.push('Hello');
      stack.push('Hi');
      assert.equal('Hi', stack.pop());
      assert.equal('Hello', stack.head.data);
    });
  });

  describe('#tail', function(){
    it('should return null for an empty stack', function(){
      assert.equal(null, stack.tail());
    });

    it('should return the last item in the stack', function(){
      stack.push('Hi');
      stack.push('Buddy');
      assert.equal('Hi', stack.tail());
    });
  });
});