typeof operator

Learn the type of a value with typeof.

The typeof operator

The typeof operator infers a value's type and yields it as a type. typeof ayarlar produces the type of the ayarlar variable. You can store it as a type alias and reuse it elsewhere. Unlike JavaScript's runtime typeof, this works at the type level.

const ayarlar = { tema: "koyu", ses: true };
type AyarTip = typeof ayarlar;
// { tema: string; ses: boolean }

Deriving a type from a value

typeof is especially handy for deriving a type from a const object. When the object's shape changes, the type produced by typeof updates automatically. So you obtain the type from an existing value instead of writing the interface by hand.

Exercise Derive a type from a value

Assign the object { tema: "koyu", ses: true } to a variable ayarlar. Define type AyarTip as typeof ayarlar. Write function goster(a: AyarTip): void whose body prints tema via console.log(a.tema). Call goster(ayarlar).

Show example solution

const ayarlar = { tema: "koyu", ses: true };
type AyarTip = typeof ayarlar;
function goster(a: AyarTip): void {
  console.log(a.tema);
}
goster(ayarlar);

Test yourself

What is AyarTip the type of in the code below?
```
const ayarlar = { tema: "koyu", ses: true };
type AyarTip = typeof ayarlar;
```
- "koyu" | true
- any
- { tema: string; ses: boolean }
- string
Show answer

{ tema: string; ses: boolean } — A const object is still mutable, so its properties infer wide types: tema is string, ses is boolean. typeof gives this object's type.

Add typeof

const ayarlar = { tema: "koyu", ses: true };
type AyarTip = ____ ayarlar;

Show answer

const ayarlar = { tema: "koyu", ses: true };
type AyarTip = typeof ayarlar;

typeof parts

typeof ayarlar

The type of the ayarlar value
type AyarTip = typeof ayarlar

Type alias from a value
const ayarlar = { tema: "koyu" }

The source value for typeof
function goster(a: AyarTip)

Derived type as parameter

Next lesson: Indexing with keyof

We Are Enhancing Your Experience

Cookie Preferences

Strictly Necessary Cookies

Analytical Cookies