keyof และชนิดการเข้าถึงแบบดัชนี

interface User { id: number; name: string; email: string; }
type UserKeys = keyof User; // 'id' | 'name' | 'email'

type IdType = User['id'];   // number
type NameType = User['name']; // string

function getProperty<T, K extends keyof T>(obj: T, key: K): T[K] {
  return obj[key];
}
const name = getProperty(user, 'name'); // string

const config = { host: 'localhost', port: 3000 };
type ConfigKey = keyof typeof config; // 'host' | 'port'

type NumericValues = User['id' | 'age']; // number | number = number

type ArrKey = keyof string[]; // number | 'length' | 'push' | ...
type Element = string[][number]; // string

type Address = { city: string; zip: string };
type Profile = { address: Address };
type City = Profile['address']['city']; // string

function setProperty<T, K extends keyof T>(obj: T, key: K, value: T[K]): void {
  obj[key] = value;
}

type Readonly2<T> = { readonly [K in keyof T]: T[K] };

type StringKeys<T> = { [K in keyof T]: T[K] extends string ? K : never }[keyof T];