keyof operator

Learn the keys of a type with keyof.

The keyof operator

The keyof operator gives the union of all property names of a type. When you write keyof Ayar, each property name of Ayar becomes a string literal type in a union. The result is a type, not a value; it can only appear in type positions.

interface Ayar {
  tema: string;
  dil: string;
}
type Anahtar = keyof Ayar;
// "tema" | "dil"

Type alias from keyof

The union produced by keyof can be stored as a type alias and used as a parameter type. The function then accepts only valid property names; a wrong name is a compile error. This is useful for functions that accept setting or configuration keys.

Exercise Print with a key type

Define interface Ayar with tema: string and dil: string. Define type Anahtarlar as keyof Ayar. Write function yaz(a: Anahtarlar): void that prints the key via console.log(a). Call yaz("tema").

Show example solution

interface Ayar {
  tema: string;
  dil: string;
}
type Anahtarlar = keyof Ayar;
function yaz(a: Anahtarlar): void {
  console.log(a);
}
yaz("tema");

Test yourself

What does the Anahtarlar type become below?
```
interface Ayar { tema: string; dil: string; }
type Anahtarlar = keyof Ayar;
```
- string | number
- { tema: string; dil: string }
- string
- The union "tema" | "dil"
Show answer

The union "tema" | "dil" — keyof gives a union of property name literals. Since Ayar's properties are tema and dil, the result is "tema" | "dil". It does not give value types or the object itself.

Add keyof

interface Ayar { tema: string; dil: string; }
type Anahtar = ____ Ayar;

Show answer

interface Ayar { tema: string; dil: string; }
type Anahtar = keyof Ayar;

keyof parts

keyof Ayar

Union of Ayar's property names
type Anahtarlar = keyof Ayar

Type alias of key names
function yaz(a: Anahtarlar)

Key-type parameter
yaz("tema")

Call with a valid key

Next lesson: typeof operator

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