Summary - Conditional types

Write a type filter with conditional types.

Summary - Conditional types

Conditional types are type expressions that branch based on a type. With T extends U ? X : Y for branching, distribution for union filtering, and never for removing members, you can write type filters. Exclude and Extract are built this way.

type MetinAl<T> =
  T extends string ? T : never;
type R = MetinAl<"a" | 1 | "b">;
// "a" | "b"

Exercise Type filter

Write the conditional type MetinAl<T> as T extends string ? T : never. Define type Metinler as MetinAl<"elma" | 42 | "armut">. Define variable dizi with type Metinler[] and assign ["elma", "armut"]. Print dizi[0] and dizi[1] on separate lines.

Show example solution

type MetinAl<T> = T extends string ? T : never;
type Metinler = MetinAl<"elma" | 42 | "armut">;
const dizi: Metinler[] = ["elma", "armut"];
console.log(dizi[0]);
console.log(dizi[1]);

Test yourself

Which conditional type keeps only string members from a union?
```
type MetinAl<T> = T extends string ? T : never;
type R = MetinAl<"a" | 1 | "b">;
```
- T extends string ? T : never
- T extends number ? T : never
- T extends string ? never : T
- T extends string ? string : number
Show answer

T extends string ? T : never — T extends string ? T : never keeps string members as-is and makes the rest never. The first removes strings; the second changes the type; the last keeps numbers.

Type filter body

type MetinAl<T> =
  T extends string ? ____ : never;
type R = MetinAl<"a" | 1 | "b">;
// R: "a" | "b"

Show answer

type MetinAl<T> =
  T extends string ? T : never;
type R = MetinAl<"a" | 1 | "b">;
// R: "a" | "b"

Match conditional type logic

T extends U ? X : Y

Basic conditional type
T extends string ? T : never

Keeps string members
Dağıtıcı koşullu

Evaluates union members one by one
MetinAl<"a" | 1>

Example of a filtered type

Next lesson: infer keyword

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