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Exporting types

Export public types from a barrel file.

What is a barrel file?

A barrel file (usually index.ts) re-exports multiple modules from a single place. The consumer imports from one path only: import { x } from "./lib". This hides the internal file structure and simplifies the API.

// index.ts (barrel)
export { Book } from "./types";
export { addBook } from "./helpers";

Type-only re-export

For type-only exports, export type is used. This tells the compiler that the value does not exist at runtime, only as type information. When isolatedModules is on, types must be specified separately.

export type { Book } from "./types";
export { addBook } from "./helpers";

Practice Library barrel object

The function kitapEkle(ad: string): string returns ad + " eklendi". The function kitapAra(ad: string): string returns ad + " bulundu". The Kutuphane object contains both. Print Kutuphane.kitapEkle("TS") with console.log.

Starter code

index.ts
// kitapEkle ve kitapAra fonksiyonlarını tanımla
// Kutuphane nesnesi ikisini de içersin
// Kutuphane.kitapEkle("TS") sonucunu yazdır

Sample solution

index.ts
function kitapEkle(ad: string): string {
  return ad + " eklendi";
}
function kitapAra(ad: string): string {
  return ad + " bulundu";
}
const Kutuphane = { kitapEkle, kitapAra };
console.log(Kutuphane.kitapEkle("TS"));

Expected output

TS eklendi

Questions and answers

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

1 What is the main purpose of a barrel file?

  • To aggregate multiple module exports in one place Correct answer
  • To hide TypeScript errors
  • To increase compilation speed
  • To reduce runtime code

Explanation: A barrel file aggregates exports from multiple modules in one file; the consumer uses a single import path.

2 Re-export

Question 
// index.ts
____ { kitapEkle } from "./helpers";
Answer 
// index.ts
export { kitapEkle } from "./helpers";

Explanation: To re-export a value from another module, use the export { } from syntax.

3 Barrel concepts

Term Meaning
barrel Aggregated re-export
index.ts Barrel file name
export { } from Re-export
export type Type-only export

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