Lesson 19 — keyof & indexed access

Types from an object's keys

keyof T is the union of an object type's property names: for { name: string; age: number } it is "name" | "age". T[K] is an indexed access type — the type of property K. Together they let a function accept only real keys and return the correct type for each one.

type User = { name: string; age: number };
type Keys = keyof User;      // "name" | "age"
type Age = User["age"];      // number

Why it matters: generic getters, form helpers and sort-by-field functions all use this to stay safe.