cd_warehouse_test.js

const assert = require('assert');

function Warehouse(catalogue) {
    this.catalogue = catalogue;

    this.getCd = function (artist, title) {
        return this.catalogue.find((cd) => cd.artist == artist && cd.title == title);
    }

    this.updatedCatalogue = function (cd, boughtCd) {
        return Object.assign([], this.catalogue, {
            [this.catalogue.indexOf(cd)]: boughtCd
        });
    }

    this.buyCd = function (artist, title, catalogue) {
        const cd = this.getCd(artist, title, catalogue);
        let boughtCd = {
            ...cd,
            stock: cd.stock - 1
        };
        return {...this, catalogue: this.updatedCatalogue(cd, boughtCd)};
    };
}

function CD(artist, title, stock) {

    this.artist = artist;
    this.title = title;
    this.stock = stock;

    this.getStock = function () {
        return this.stock;
    }

    this.buy = function () {
        return {...this, stock: this.stock - 1};
    };

    this.matches = function (artist, title) {
        return this.artist == artist && this.title == title;
    };
};

describe('CD Warehouse', () => {
    it('reduces stock count by 1 of bought CD', () => {
        let catalogue = [
            new CD('Peter Gabriel', 'So', 1)
        ];

        let warehouse = new Warehouse(catalogue);
        warehouse = warehouse.buyCd('Peter Gabriel', 'So');
        assert.equal(warehouse.getCd('Peter Gabriel', 'So').getStock(), 0);
    })
})