Careeroza

The typeof Type Operator

Types From Values

In a type position, typeof value gives the TypeScript type of a variable (not the runtime string from JavaScript typeof). It lets you write the value first and derive the type from it.

Example

const defaultConfig = {
  apiUrl: "https://api.shopease.in",
  timeoutMs: 5000,
  retries: 3,
  features: { wishlist: true, reviews: false },
};

type Config = typeof defaultConfig;

function withOverrides(overrides: Partial<Config>): Config {
  return { ...defaultConfig, ...overrides };
}

const cfg = withOverrides({ timeoutMs: 8000 });
console.log(cfg.timeoutMs, cfg.features.wishlist);
console.log(typeof cfg.retries); // this is the JavaScript typeof

Output

8000 true
number

typeof with as const

Example

Combined with as const and keyof, you can derive a union from an object of constants.

const ROLES = { ADMIN: "admin", SELLER: "seller", BUYER: "buyer" } as const;

type RoleKey = keyof typeof ROLES;             // "ADMIN" | "SELLER" | "BUYER"
type Role = (typeof ROLES)[RoleKey];           // "admin" | "seller" | "buyer"

function homePage(role: Role): string {
  return role === "admin" ? "/admin" : role === "seller" ? "/seller/dashboard" : "/";
}

const keys = Object.keys(ROLES) as RoleKey[];
keys.forEach((k) => console.log(k.padEnd(6), homePage(ROLES[k])));

Output

ADMIN  /admin
SELLER /seller/dashboard
BUYER  /