Using Items

Items are there to be used: a potion heals Kira, a weapon goes into her hand, and a key opens something. Using an item changes the hero, so the function gets her as a parameter and changes her properties. Objects aren't copied when you pass them to a function, so the changes happen to kira herself.

Where will Kira keep the weapon in her hand? In a new property, weapon. At the start of the adventure, her hand is empty, so the property is optional, as in the lesson on object types: weapon?: Weapon. For example, a ranger and her pet:

type Pet = { name: string; species: string };

interface Ranger {
    name: string;
    pet?: Pet;
}

function adopt(ranger: Ranger, pet: Pet): string {
    ranger.pet = pet;
    return `${ranger.name} adopts ${pet.name} the ${pet.species}`;
}

const ema: Ranger = { name: "Ema" };
console.log(ema.pet?.name ?? "no pet");
console.log(adopt(ema, { name: "Shadow", species: "wolf" }));
console.log(ema.pet?.name ?? "no pet");

This prints:

no pet
Ema adopts Shadow the wolf
Shadow

useItem is a switch on item.kind, like describeItem, but some of its cases first change the hero and only then return the text:

• The potion adds item.heal to her health. If that's more than maxHealth, the health is set to maxHealth: Kira has 34 health, the potion heals 20, and 54 would be more than her 50.
• The weapon goes into her hand: hero.weapon = item;. That works only inside case "weapon", where TypeScript knows that the item is a Weapon. Above the switch, the item could be of any kind, and TypeScript would report:

  main.ts(46,5): error TS2322: Type 'Item' is not assignable to type 'Weapon | undefined'.
    Property 'damage' is missing in type 'Key' but required in type 'Weapon'.

• The key doesn't change anything; it only returns Kira opens the crypt.

Write the return type string here too. TypeScript then checks that every case returns a text. And as you'll see in the last step, it also reports a kind of item that has no case, even without the never check: the function would end without returning a text.

In the main program, kira.inventory[0] is the first item (the potion) and kira.inventory[1] the second one (the sword). Kira's name is read-only, but that's fine: useItem only reads it.