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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.

Practice 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.

Starter code

index.ts
// SarmaliAc<T> koşullu türünü tanımla
// T { deger: V } kalıbına uyuyorsa V'yi çıkar, değilse never
// Sonuc türünü SarmaliAc<{ deger: string }> olarak tanımla
// a değişkenini Sonuc türünde tanımla, "merhaba" ata
// a'yı console.log ile yazdır

Sample solution

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

Expected output

merhaba

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 What does SarmaliAc<{ deger: number }> produce below?

index.ts
type SarmaliAc<T> = T extends { deger: infer V } ? V : never;
type R = SarmaliAc<{ deger: number }>;
  • any
  • number Correct answer
  • { deger: number }
  • never

Explanation: 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.

2 Declare a type variable with infer

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

Explanation: To declare a type variable in the extends clause of a conditional type you use infer. type, let and const are for value/type declarations elsewhere; they cannot infer.

3 infer parts

Term Meaning
infer V Inferred type variable
{ deger: infer V } Match pattern
? V True branch: the inferred type
SarmaliAc<{ deger: string }> Usage example

All lessons in Intermediate

1 Classes

  1. Class basics
  2. Constructor and fields
  3. Methods
  4. Static members
  5. Summary - Classes

2 Access modifiers

  1. public
  2. private
  3. protected
  4. Parameter properties
  5. Summary - Access modifiers

3 Readonly properties

  1. readonly fields
  2. Readonly parameter
  3. Readonly vs const
  4. Summary - Readonly

4 Generics

  1. What is a generic?
  2. Generic functions
  3. Generic classes
  4. Multiple type parameters
  5. Summary - Generics

5 Generic constraints

  1. extends constraint
  2. keyof constraint
  3. Default type parameter
  4. Summary - Generic constraints

6 Utility types

  1. Partial
  2. Required and Pick
  3. Omit and Record
  4. Summary - Utility types

7 keyof and typeof

  1. keyof operator
  2. typeof operator
  3. Indexing with keyof
  4. Summary - keyof and typeof

8 Mapped types

  1. What is a mapped type?
  2. Readonly mapped type
  3. Optional mapped type
  4. Summary - Mapped types

9 Conditional types

  1. What is a conditional type?
  2. Distributive conditional
  3. Logic with conditional
  4. Summary - Conditional types

10 infer

  1. infer keyword
  2. Extracting from Promise
  3. Array element type
  4. Summary - infer

11 Decorators

  1. What is a decorator?
  2. Method decorator
  3. Property decorator
  4. Decorator factories
  5. Summary - Decorators

12 Modules

  1. export and import
  2. Default export
  3. Re-export
  4. Type-only import
  5. Summary - Modules

13 Namespaces

  1. What is a namespace?
  2. Nested namespace
  3. Namespace vs module
  4. Summary - Namespaces

14 Declaration files

  1. What is .d.ts?
  2. Ambient module
  3. Triple-slash directive
  4. Summary - Declaration files

15 Advanced classes

  1. abstract class
  2. implements
  3. Get and set
  4. Summary - Advanced classes

16 Type assertion

  1. as keyword
  2. angle bracket syntax
  3. Type assertion vs casting
  4. Casting from unknown
  5. Summary - Type assertion

17 Project: Library package

  1. Package design
  2. Exporting types
  3. Generic helpers
  4. Declaration file
  5. Integration

18 Project: Quiz app

  1. Question model
  2. Question pool
  3. Answer check
  4. Score calculation
  5. Integration

19 Project: Weather app

  1. Weather model
  2. API response type
  3. Data transform
  4. Display
  5. Integration

20 Intermediate final

  1. Challenge 1: Classes
  2. Challenge 2: Generics
  3. Challenge 3: Type systems
  4. Challenge 4: Mini app