Distributive conditional

Learn distribution and unions.

Distributive conditional

When a conditional type is used on a union type, distribution occurs. Etiket<string | number> is evaluated separately for each member: Etiket<string> | Etiket<number>. The result is the union of each branch.

type Etiket<T> =
  T extends string ? "metin" : "sayi";
type R = Etiket<string | number>;
// "metin" | "sayi"

The basis of distribution

Distribution happens automatically when the type parameter on the left of a conditional is a union. The condition is evaluated separately for each member and the results combine. This is the core mechanism for filtering specific types from a union (Exclude, Extract).

Exercise Label a union

Write the conditional type Etiket<T> as T extends string ? "metin" : "sayi". Define type Sonuclar as Etiket<string | number>. Define variable dizi with type Sonuclar[] and assign ["metin", "sayi"]. Print dizi[0] and dizi[1] on separate lines.

Show example solution

type Etiket<T> = T extends string ? "metin" : "sayi";
type Sonuclar = Etiket<string | number>;
const dizi: Sonuclar[] = ["metin", "sayi"];
console.log(dizi[0]);
console.log(dizi[1]);

Test yourself

What does Etiket<string | number> produce with a distributive conditional?
```
type Etiket<T> = T extends string ? "metin" : "sayi";
type R = Etiket<string | number>;
```
- string | number
- never
- "metin"
- "metin" | "sayi"
Show answer

"metin" | "sayi" — Distribution evaluates each member separately: "metin" for string, "sayi" for number. The result is the union of these literal types. string | number or never is wrong; just "metin" is incomplete.

Distribution with a union

type Etiket<T> = T extends string ? "metin" : "sayi";
type R = Etiket<____>;
// R: "metin" | "sayi"

Show answer

type Etiket<T> = T extends string ? "metin" : "sayi";
type R = Etiket<string | number>;
// R: "metin" | "sayi"

Distributive conditional parts

Etiket<string | number>

Distributive call
Etiket<string>

First member evaluation
Etiket<number>

Second member evaluation
"metin" | "sayi"

Distribution result

Next lesson: Logic with conditional

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