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Array element type

Learn to extract the element type of an array with infer.

Array element type

The pattern (infer E)[] extracts the element type of an array as E. The pattern T extends (infer E)[] ? E : never gives the element type when T is an array. This is useful in generic helpers that need to know an array's element type.

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

Why the parentheses matter

In (infer E)[] the parentheses are mandatory. Writing infer E[] tries to declare an array of element types named E and is a syntax error. The parentheses say: first infer E, then require the type to be an array of E.

Practice Extract the array element type

Write the conditional type Eleman<T> as T extends (infer E)[] ? E : never. Define type Sonuc as Eleman<string[]>. Define variable a with type Sonuc and assign "elma". Print a with console.log.

Starter code

index.ts
// Eleman<T> koşullu türünü tanımla
// T bir dizi ise eleman türünü çıkar, değilse never
// Sonuc türünü Eleman<string[]> olarak tanımla
// a değişkenini Sonuc türünde tanımla, "elma" ata
// a'yı console.log ile yazdır

Sample solution

index.ts
type Eleman<T> = T extends (infer E)[] ? E : never;
type Sonuc = Eleman<string[]>;
const a: Sonuc = "elma";
console.log(a);

Expected output

elma

Questions and answers

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

1 What does Eleman<number[]> produce?

index.ts
type Eleman<T> = T extends (infer E)[] ? E : never;
type R = Eleman<number[]>;
  • number Correct answer
  • never
  • any[]
  • number[]

Explanation: Since T is an array, the pattern matches and E becomes the element type, number. The true branch returns E. number[] is the matched type itself; never is the wrong branch; any[] is not an inference result.

2 Add the array brackets

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

Explanation: After inferring the element type you add [] to require the array condition. {} is an object type; [T] forms a tuple; E alone does not form the array condition.

3 Array infer parts

Term Meaning
(infer E) Inferred element type
(infer E)[] Array match pattern
? E True branch: the element type
Eleman<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