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What is .d.ts?

Learn what declaration files are.

What is a .d.ts?

A .d.ts file is a TypeScript declaration file. It contains only type information; no executable JavaScript code. It is used to add types to JavaScript libraries on the TypeScript side. The declare keyword tells TypeScript about a value that already exists outside.

// global.d.ts
declare const VERSION: string;

Difference between .d.ts and .ts

A .ts file holds both types and implementation; when compiled it produces .js. A .d.ts file holds only type declarations and is not turned into .js by the compiler. That is why you use .d.ts for JS libraries that need types.

// app.d.ts (declaration)
export const PI: number;
// app.ts (implementation)
export const PI = 3.14;

Practice Declare topla ambient function

Declare an ambient function topla that takes two number parameters and returns number. Then print "topla hazır" with console.log.

Starter code

index.ts
// topla fonksiyonunu declare ile bildir
// (a: number, b: number) -> number

console.log("topla hazır");

Sample solution

index.ts
declare function topla(a: number, b: number): number;
console.log("topla hazır");

Expected output

topla hazır

Questions and answers

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

1 Which of the following is found in a .d.ts file?

  • Only comment lines
  • Type declarations Correct answer
  • Type declarations and implementation
  • Only JavaScript code

Explanation: A .d.ts file contains only type declarations; it does not host any implementation or executable code.

2 Declare the VERSION global

Question 
// VERSION globalini ambient olarak bildir
____ const VERSION: string;
Answer 
// VERSION globalini ambient olarak bildir
declare const VERSION: string;

Explanation: The `declare` keyword is used to declare an ambient value; it tells TypeScript that the value already exists elsewhere.

3 .d.ts concepts

Term Meaning
.d.ts Declaration file extension
declare Declares an existing value
tür bilgisi Content without implementation
.ts Both type and implementation

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