Extracting from Promise
- TypeScript course
- Intermediate
- Lesson 41/89
- 4 min
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: numberNon-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.
Practice 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.
Starter code
// Deger<T> koşullu türünü tanımla
// T bir Promise ise değer türünü çıkar, değilse T'yi döndür
// Sonuc türünü Deger<Promise<number>> olarak tanımla
// a değişkenini Sonuc türünde tanımla, 42 ata
// a'yı console.log ile yazdırSample solution
type Deger<T> = T extends Promise<infer V> ? V : T;
type Sonuc = Deger<Promise<number>>;
const a: Sonuc = 42;
console.log(a);Expected output
42
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 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 Correct answer
Explanation: 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.
2 Extract the Promise value
type Deger<T> =
T extends Promise<____ V> ? V : T;type Deger<T> =
T extends Promise<infer V> ? V : T;Explanation: To extract the Promise's value type you use the Promise<infer V> pattern. type, let and const are for value/type declarations; new is an expression operator; infer does the inference.
3 Promise infer parts
| Term | Meaning |
|---|---|
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 |