Summary

In this lesson, you learned that:

• A discriminated union is a union of object types that share a tag property, such as kind, with a different literal type in each: type Item = { kind: "potion"; heal: number } | { kind: "weapon"; damage: number };. Checking the tag, if (item.kind === "potion"), narrows the type. TypeScript checks the properties of each kind and catches misspelled kinds.
• switch (item.kind) jumps to the case with the same value; break or return ends it, and without them, the program falls through to the next case. default catches the rest. Every case narrows the type.
• The exhaustiveness check in default, const unhandled: never = item; return unhandled;, makes TypeScript report every switch that has no case for some kind, and it names the kind. never is the type with no values.
• Record<Rarity, number> is an object with exactly the keys of Rarity, and a number for each of them. A missing key is an error, and prices[item.rarity] reads a value.
• const classes = ["warrior", "mage"] as const; is a read-only list of exact values, and type HeroClass = (typeof classes)[number]; turns it into a union, so the list and the type always match.
• A discriminated union can describe the states of one thing too, each with its own data: only a quest with status: "done" has a reward.
• Instead of a boolean with optional properties, write a union, so that impossible combinations can't be written. A boolean can be the tag too: { isLocked: false } | { isLocked: true; keyName: string }.

In the next lesson, Generics, you'll learn to write functions and types that work with any type, while TypeScript still keeps track of which type it is: a function that returns the first element of any array, or a chest that can hold any loot. And you'll see how types with angle brackets, such as Record, work.

Now practice. Kira's journey in the editor is a list of encounters: fights, treasures, and rests.