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What is a conditional type?

Learn conditional types that branch by type.

What is a conditional type?

A conditional type is a type expression that branches based on a type. In T extends U ? X : Y, if T is assignable to U the type becomes X; otherwise Y. It resembles JavaScript's ternary operator but works at the type level and is evaluated at compile time.

type Etiket<T> =
  T extends string ? "metin" : "sayi";
type A = Etiket<string>;  // "metin"
type B = Etiket<number>;  // "sayi"

Compile-time evaluation

Conditional types are evaluated at compile time and leave no runtime trace. Since the result is a type, it can only appear in type positions (variable declarations, parameters, returns). It cannot be used as a value; a call like console.log(Etiket<number>) is invalid.

Practice Label type

Write the conditional type Etiket<T> as T extends string ? "metin" : "sayi". Define variable a with type Etiket<string> and assign "metin". Define variable b with type Etiket<number> and assign "sayi". Print both with console.log on separate lines.

Starter code

index.ts
// Etiket<T> koşullu türünü tanımla
// T string ise "metin", değilse "sayi"
// a değişkenini Etiket<string> türünde tanımla, "metin" ata
// b değişkenini Etiket<number> türünde tanımla, "sayi" ata
// her ikisini ayrı satırlarda yazdır

Sample solution

index.ts
type Etiket<T> = T extends string ? "metin" : "sayi";
const a: Etiket<string> = "metin";
const b: Etiket<number> = "sayi";
console.log(a);
console.log(b);

Expected output

metin
sayi

Questions and answers

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

1 What does Etiket<number> become below?

index.ts
type Etiket<T> = T extends string ? "metin" : "sayi";
type R = Etiket<number>;
  • never
  • "metin"
  • "sayi" Correct answer
  • number

Explanation: Since number is not assignable to string, the condition is false and the result is the "sayi" literal type. "metin" is the true branch; number and never are wrong inferences.

2 Add the condition

Question 
type Etiket<T> =
  T ____ string ? "metin" : "sayi";
Answer 
type Etiket<T> =
  T extends string ? "metin" : "sayi";

Explanation: To query in a conditional type you use extends. implements is for classes; instanceof checks values; is is not a keyword here in TypeScript.

3 Conditional type parts

Term Meaning
T extends U The condition
? X Type when true
: Y Type when false
Etiket<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