Indexed Access Types

T[K] accesses a type’s property by key — at the type level.

A Single Key

type User = { name: string; age: number };

type Name = User["name"];   // string
type Age = User["age"];      // number

User["name"] is the type of User’s name field — string.

Multiple Keys (Union)

type StringOrNumber = User["name" | "age"];   // string | number

A union of keys gives a union of value types.

All Keys With keyof

type Values = User[keyof User];   // string | number

User[keyof User] is the union of every value type. Useful for “some value from this object’s properties”.

Array Element Types

type Tags = string[];
type Tag = Tags[number];     // string

Tags[number] says “the type at any number index” — the element type.

For tuples, T[0] and T[1] give specific positions.

A Useful Pattern — Drive Types From Const Data

const ROUTES = ["/", "/about", "/blog"] as const;
type Route = typeof ROUTES[number];
// "/" | "/about" | "/blog"

as const keeps the literal types; [number] extracts the union of elements. A small, powerful combo.

Object Values

const STATUS = {
  ok: 200,
  notFound: 404,
  error: 500,
} as const;

type StatusCode = typeof STATUS[keyof typeof STATUS];   // 200 | 404 | 500

A bit dense. Reading it inside out:

• typeof STATUS — { readonly ok: 200; readonly notFound: 404; readonly error: 500 }
• keyof typeof STATUS — "ok" | "notFound" | "error"
• typeof STATUS[keyof typeof STATUS] — 200 | 404 | 500

A common idiom for deriving enum-like types from a const object.

Up Next

Building new types by walking a type’s properties — mapped types.