ReplaceGlobalReferenceWithGetter2.ts

// After applying Subclass & Override Method

class RegisterSale {
    // more code...

    public addItem(code: Barcode): void {
        const newItem = this.getInventory().getItemForBarCode(code);
        this.items.push(newItem);
    }

    // made protected for testing
    protected getInventory(): Inventory {
        return Inventory.getInstance();
    }

    // more code...
}

// In some test
describe("RegisterSale", () => {
    it('adds an item', () => {
        // some more setup code
        // We make the ForTestingRegisterSale class return a test double of Inventory
        const inventory: jest.Mocked<Inventory> = { getItemForBarCode: jest.fn() };
        inventory.getItemForBarCode.mockReturnValue(anItem().withBarcode(code).Build());
        const registerSale = new ForTestingRegisterSale(inventory);

        // rest of the test...
    });

    // more tests...

    // we subclass & override the getter
    class ForTestingRegisterSale extends RegisterSale {
        private readonly inventory: Inventory;

        constructor(inventory: Inventory) {
            super();
            this.inventory = inventory;
        }

        // overridden to separate
        protected getInventory(): Inventory {
            return this.inventory;
        }
    }
});