ReplaceGlobalReferenceWithGetter.ts

class Inventory {
    private static inventory: Inventory | null = null;

    public static getInstance(): Inventory {
        if (this.inventory === null) {
            this.inventory = new Inventory();
        }
        return this.inventory;
    }

    public getItemForBarCode(code: Barcode): Item {
        // get item with barcode from persistence
        //...
    }
}

class RegisterSale {
    // more code...

    public addItem(code: Barcode): void {
        // using the Singleton!! x(
        const newItem = Inventory.getInstance().getItemForBarCode(code);
        this.items.push(newItem);
    }

    // more code...
}