Code Kata #9 'Checkout' in JavaScript (see http://codekata.com/kata/kata09-back-to-the-checkout/)

checkout.js

var _ = require('lodash');

module.exports = Checkout;

function Checkout(pricingRules) {
  this.pricingRules = pricingRules;
  this.basket = {};
}

Checkout.prototype.scan = function(item) {
  this.basket[item] = ++this.basket[item] || 1;
};

Checkout.prototype.getTotal = function() {
  var total = 0
    , self = this;
  _.forIn(this.basket, function(quantity, item) { total += self.pricingRules[item](quantity) });
  return total;
};

pricingRules.js

module.exports = {
  'A': bundledPricing({singlePrice: 50, bundledPrice: 130, bundleSize: 3}),
  'B': bundledPricing({singlePrice: 30, bundledPrice: 45, bundleSize: 2}), 
  'C': singlePricing({singlePrice: 30})
};

function bundledPricing(options) {
  return function(quantity) {
    var bundeledQuantity = (quantity - quantity %options.bundleSize) / options.bundleSize;
    quantity = quantity %options.bundleSize;
    return bundeledQuantity * options.bundledPrice + quantity * options.singlePrice;
  }
}

function singlePricing(options) {
  return function(quantity) {
    return quantity * options.singlePrice;
  }
}

testCheckout.js

var chai = require('chai')
  , Checkout = require('./checkout')
  , PricingRules = require('./pricingRules');

chai.should();

describe('testCheckout.js', function() {

  describe('When item A is scanned once and item B is scanned once', function() {
    it('should return total price of 80', function() {
      var checkout = new Checkout(PricingRules);

      checkout.scan('A');
      checkout.scan('B');

      checkout.getTotal().should.equal(80);
    });
  });

  describe('When item A is scanned three times and item B is scanned once', function() {
    it('should return total price of 160', function() {
      var checkout = new Checkout(PricingRules);

      checkout.scan('A');
      checkout.scan('A');
      checkout.scan('A');
      checkout.scan('B');

      checkout.getTotal().should.equal(160);
    });
  });

  describe('When item A is scanned four times and item B is scanned two times', function() {
    it('should return total price of 225', function() {
      var checkout = new Checkout(PricingRules);

      checkout.scan('A');
      checkout.scan('A');
      checkout.scan('A');
      checkout.scan('A');
      checkout.scan('B');
      checkout.scan('B');

      checkout.getTotal().should.equal(225);
    });
  });
});

testPricingRules.js

var chai = require('chai')
  , PricingRules = require('./pricingRules');

chai.should();

describe('testCheckout.js', function() {

  describe('When item A is scanned once', function() {
    it('should return price for item A', function() {
      PricingRules['A'](1).should.equal(50);
    });
  });

  describe('When item A is scanned twice', function() {
    it('should return price for 2 items A', function() {
      PricingRules['A'](2).should.equal(100);
    });
  });

  describe('When item A is scanned three times', function() {
    it('should return bundel price for 3 items A', function() {
      PricingRules['A'](3).should.equal(130);
    });
  });

  describe('When item A is scanned four times', function() {
    it('should return bundle price for 3 items A plus 1 single item price', function() {
      PricingRules['A'](4).should.equal(180);
    });
  });

  describe('When item B is scanned once', function() {
    it('should return price for item B', function() {
      PricingRules['B'](1).should.equal(30);
    });
  });

  describe('When item B is scanned five times', function() {
    it('should return bundle price for 4 items B plus 1 single item price', function() {
      PricingRules['B'](5).should.equal(120);
    });
  });
});