Regular Customers

The blacksmith has regular customers, and some of them get a discount: a number of gold coins off every order. They're in the new array regulars, whose elements have the type Customer. Not every regular has a discount, so discount is an optional property.

A regular with a discount pays the price minus the discount, and the line of the order ends with the discount in parentheses. A regular without a discount pays the full price. A customer who isn't among the regulars pays the full price too, but the line ends with (new customer). The total is now what the customers really pay:

Kira: forging, Arrowheads (steel), 30 gold (5 off)
Bruno: repair, Battle Axe, 15 gold (new customer)
Lena: sharpening, Dagger, 8 gold
Bruno: forging, Shield (bronze), 20 gold (new customer)
Total: 73 gold

Kira's discount is 5 gold, so she pays 30 for her arrowheads. Lena is a regular without a discount, and Bruno isn't a regular at all.

A tip: when you look for a customer with find, remember from the Unions and Narrowing lesson that it may find nothing. TypeScript makes you handle that case, and the missing discount too.