Summary - infer

Write your own Awaited and ElementOf types with infer.

Summary - infer

infer declares a type variable in the extends clause of conditional types, allowing you to extract a sub-type from within a type. Promise<infer V> gives the awaited value type; (infer E)[] gives the array element type. These patterns underpin Awaited and array helpers.

type Beklenen<T> =
  T extends Promise<infer V> ? V : T;
type Eleman<T> =
  T extends (infer E)[] ? E : never;

Exercise Your own Awaited and Eleman types

Write the conditional type Beklenen<T> as T extends Promise<infer V> ? V : T. Write the conditional type Eleman<T> as T extends (infer E)[] ? E : never. Define type P as Beklenen<Promise<string>> and type E as Eleman<number[]>. Define a with type P as "tamam", and b with type E as 7. Print both on separate lines.

Show example solution

type Beklenen<T> = T extends Promise<infer V> ? V : T;
type Eleman<T> = T extends (infer E)[] ? E : never;
type P = Beklenen<Promise<string>>;
type E = Eleman<number[]>;
const a: P = "tamam";
const b: E = 7;
console.log(a);
console.log(b);

Test yourself

Which pattern extracts a Promise's value type?
```
type Beklenen<T> = T extends Promise<infer V> ? V : T;
```
- T extends Promise<V> ? V : T
- T extends infer Promise ? T : never
- T extends Promise<infer V> ? V : T
- T extends (infer V)[] ? V : never
Show answer

T extends Promise<infer V> ? V : T — The Promise<infer V> pattern extracts the Promise's value type as V. The second extracts an array element; the third leaves V undefined; the last is a syntax error.

Extract the array element

type Eleman<T> =
  T extends (____ E)[] ? E : never;

Show answer

type Eleman<T> =
  T extends (infer E)[] ? E : never;

Match the infer patterns

Promise<infer V>

Extracts the Promise value type
(infer E)[]

Extracts the array element type
{ deger: infer V }

Extracts an object property type
? V / ? E

True branch: the inferred type

Next lesson: What is a decorator?

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