For the objects identified write a constructor.Then create create two methods that act on invoices. One should be addLineItem that adds a line item to an existing invoice. The other should calculateTotal that determines the total billing price of the invoice.
The solution contains 3 classes Customer, Invoice and InvoiceLine. At any one time there will be just one Invoice that must be created by clicking the "Create Invoice" button. To this Invoice, you can add as many line items as you like and calculate it's totals. All code resides in jscript file.