keyof and Record

keyof: the property names as a type

keyof T is a union of T's property names as literal types:

interface Point {
  x: number;
  y: number;
}
// keyof Point  is  "x" | "y"

function getCoord(p: Point, key: keyof Point): number {
  return p[key];
}

getCoord(p, "x") compiles, getCoord(p, "z") does not. In plain JavaScript, p[key] with a misspelled key silently returns undefined. With keyof, property access by name is checked like everything else. You already saw Pick<User, "id" | "name"> in the last lesson, that K is constrained with keyof behind the scenes.

Record: a typed lookup table

Record<K, V> builds an object type with keys K and values V. Pair it with a union of literals for an exhaustive table:

type Fruit = "apple" | "banana" | "cherry";

const stock: Record<Fruit, number> = {
  apple: 4,
  banana: 0,
  cherry: 12,
};

Two guarantees at once: every fruit must appear (forget cherry and it will not compile), and no stray keys sneak in. Compare the JavaScript habit of plain object maps where a typo creates a new key instead of an error. Record<string, number> is also legal when the key set is open-ended.