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Declaration file

Write a .d.ts declaration for the package.

.d.ts for a package

When a library package is published, it includes a .d.ts declaration file alongside the JavaScript code. This file lets TypeScript projects using the package get type information. It contains declare statements; no implementation code.

// library.d.ts
export declare function addBook(name: string): string;
export declare const VERSION: string;

Ambient declarations

Each declare statement in a .d.ts file tells TypeScript that the corresponding value exists at runtime. Functions, variables, classes, and modules can be declared as ambient. The compiler checks them but produces no code.

declare function addBook(name: string): string;
declare const VERSION: string;

Practice Write a declaration file

Write an ambient function declaration for kitapEkle(ad: string): string. Write an ambient const declaration for VERSION: string. Then print "bildirim hazır" with console.log.

Starter code

index.ts
// kitapEkle için ambient fonksiyon bildirimi yaz
// VERSION için ambient const bildirimi yaz
// "bildirim hazır" yazdır

Sample solution

index.ts
declare function kitapEkle(ad: string): string;
declare const VERSION: string;
console.log("bildirim hazır");

Expected output

bildirim 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 package's .d.ts file?

  • Runtime implementation code
  • Only comment lines
  • Compilation configuration
  • Type declarations and declare statements Correct answer

Explanation: A .d.ts file contains declare statements and type declarations; no implementation code.

2 Ambient function declaration

Question 
____ function kitapEkle(ad: string): string;
Answer 
declare function kitapEkle(ad: string): string;

Explanation: The declare keyword is used to tell TypeScript about a function that exists elsewhere.

3 Declaration file concepts

Term Meaning
.d.ts Declaration file extension
declare Ambient value declaration
export declare Package export
ambient Existing in the environment

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