infer keyword

Learn to extract types with infer.

The infer keyword

infer declares a new type variable inside the extends clause of a conditional type. This variable captures a part of the matched type and can be used in the true branch of the conditional. This lets you extract a sub-type from within a type.

type SarmaliAc<T> =
  T extends { deger: infer V } ? V : never;
type R = SarmaliAc<{ deger: string }>;
// R: string

Where does infer work?

infer is only valid inside the extends clause of conditional types; it cannot appear elsewhere. The declared type variable is visible only in the true and false branches. This mechanism is the foundation for structural inferences like Promise value type, array element type, or function return type.

Exercise Unwrap a wrapper

Write the conditional type SarmaliAc<T> as T extends { deger: infer V } ? V : never. Define type Sonuc as SarmaliAc<{ deger: string }>. Define variable a with type Sonuc and assign "merhaba". Print a with console.log.

Show example solution

type SarmaliAc<T> = T extends { deger: infer V } ? V : never;
type Sonuc = SarmaliAc<{ deger: string }>;
const a: Sonuc = "merhaba";
console.log(a);

Test yourself

What does SarmaliAc<{ deger: number }> produce below?
```
type SarmaliAc<T> = T extends { deger: infer V } ? V : never;
type R = SarmaliAc<{ deger: number }>;
```
- any
- number
- { deger: number }
- never
Show answer

number — When T matches the pattern { deger: infer V }, V becomes the type of the deger property, which is number, and the true branch returns V. never only happens on mismatch; any is never an inference result.

Declare a type variable with infer

type SarmaliAc<T> =
  T extends { deger: ____ V } ? V : never;

Show answer

type SarmaliAc<T> =
  T extends { deger: infer V } ? V : never;

infer parts

infer V

Inferred type variable
{ deger: infer V }

Match pattern
? V

True branch: the inferred type
SarmaliAc<{ deger: string }>

Usage example

Next lesson: Extracting from Promise

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