checkout.js

var Checkout =  function Checkout(inventory) {

    inventory = inventory || [];

    this.basket = {};

    this.inventory = {};
    inventory.forEach(function (product) {
        this.inventory[product.code] = new Product(product);
    }.bind(this));

}

Checkout.prototype = {
    scan: function (code) {
        this.basket[code] = this.basket[code] || 0;
        this.basket[code]++;
    },
    total: function () {
        var total = 0;
        for (var i in this.basket) {
            if (this.basket.hasOwnProperty(i) && this.inventory[i]) {
                total += this.inventory[i].getPrice(this.basket[i]);
            }
        }
        return total;
    }
}

var Product = function (data) {
    this.price = data.price;
    this.offer = data.offer;
}
Product.prototype = {
    getPrice: function (n) {
        if (typeof this.offer === 'function') {
            return this.offer(n);
        } else {
            return n * this.price;
        }
    }
}

module.exports = Checkout;

checkout.test.js

var Checkout = require('../checkout');

global.should = require('chai')
    .should();

describe('Checkout Class', function () {

    it('has a scan method', function () {
        var checkout = new Checkout();
        checkout.scan.should.be.a('function');
    });
    it('has a total method', function () {
        var checkout = new Checkout();
        checkout.total.should.be.a('function');
    });

    describe('total', function () {

        var checkout;

        beforeEach(function () {

            checkout = new Checkout([
                { code: 'FR1', name: 'Tea', price: 311, offer: function (n) {
                    return this.price * (Math.floor(n / 2) + n % 2);
                } },
                { code: 'SR1', name: 'Strawberries', price: 500, offer: function (n) {
                    if (n >= 3) {
                        return n * 450;
                    } else {
                        return n * this.price;
                    }
                } },
                { code: 'CF1', name: 'Coffee', price: 1123 }
            ]);

        });



        it('test basket 1', function () {
            checkout.scan('FR1');
            checkout.scan('SR1');
            checkout.scan('FR1');
            checkout.scan('FR1');
            checkout.scan('CF1');
            checkout.total().should.equal(2245);
        });

        it('test basket 2', function () {
            checkout.scan('FR1');
            checkout.scan('FR1');
            checkout.total().should.equal(311);
        });

        it('test basket 3', function () {
            checkout.scan('SR1');
            checkout.scan('SR1');
            checkout.scan('FR1');
            checkout.scan('SR1');
            checkout.total().should.equal(1661);
        });

    });

});