Extracting from Promise

Learn the value type of a Promise with infer.

Extracting a value from a Promise

The pattern Promise<infer V> extracts the value type of a Promise as V. The pattern T extends Promise<infer V> ? V : T returns the value type when T is a Promise, or T itself otherwise. This is the basis of the Awaited utility type.

type Deger<T> =
  T extends Promise<infer V> ? V : T;
type R = Deger<Promise<number>>;
// R: number

Non-Promise types

When T is not a Promise, Deger<T> falls to the false branch and returns T as-is. For example Deger<number> gives number. This lets you write flexible helpers that work with both Promise and non-Promise types.

Exercise Extract the Promise value type

Write the conditional type Deger<T> as T extends Promise<infer V> ? V : T. Define type Sonuc as Deger<Promise<number>>. Define variable a with type Sonuc and assign 42. Print a with console.log.

Show example solution

type Deger<T> = T extends Promise<infer V> ? V : T;
type Sonuc = Deger<Promise<number>>;
const a: Sonuc = 42;
console.log(a);

Test yourself

What does Deger<Promise<string>> produce?
```
type Deger<T> = T extends Promise<infer V> ? V : T;
type R = Deger<Promise<string>>;
```
- Promise<string>
- any
- never
- string
Show answer

string — Since T is a Promise, the pattern matches and V becomes the Promise's value type, string. The true branch returns V. Promise<string> is the wrong branch; never and any are invalid here.

Extract the Promise value

type Deger<T> =
  T extends Promise<____ V> ? V : T;

Show answer

type Deger<T> =
  T extends Promise<infer V> ? V : T;

Promise infer parts

Promise<infer V>

Captures the Promise value type
? V

True branch: the value type
: T

False branch: T as-is
Deger<Promise<number>>

Usage example

Next lesson: Array element type

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