What is a conditional type?

Learn conditional types that branch by type.

What is a conditional type?

A conditional type is a type expression that branches based on a type. In T extends U ? X : Y, if T is assignable to U the type becomes X; otherwise Y. It resembles JavaScript's ternary operator but works at the type level and is evaluated at compile time.

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

Compile-time evaluation

Conditional types are evaluated at compile time and leave no runtime trace. Since the result is a type, it can only appear in type positions (variable declarations, parameters, returns). It cannot be used as a value; a call like console.log(Etiket<number>) is invalid.

Exercise Label type

Write the conditional type Etiket<T> as T extends string ? "metin" : "sayi". Define variable a with type Etiket<string> and assign "metin". Define variable b with type Etiket<number> and assign "sayi". Print both with console.log on separate lines.

Show example solution

type Etiket<T> = T extends string ? "metin" : "sayi";
const a: Etiket<string> = "metin";
const b: Etiket<number> = "sayi";
console.log(a);
console.log(b);

Test yourself

What does Etiket<number> become below?
```
type Etiket<T> = T extends string ? "metin" : "sayi";
type R = Etiket<number>;
```
- never
- "metin"
- "sayi"
- number
Show answer

"sayi" — Since number is not assignable to string, the condition is false and the result is the "sayi" literal type. "metin" is the true branch; number and never are wrong inferences.

Add the condition

type Etiket<T> =
  T ____ string ? "metin" : "sayi";

Show answer

type Etiket<T> =
  T extends string ? "metin" : "sayi";

Conditional type parts

T extends U

The condition
? X

Type when true
: Y

Type when false
Etiket<string>

Usage example

Next lesson: Distributive conditional

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