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";

Exercise 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.

Show example solution

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"));

Test yourself

What is the main purpose of a barrel file?
- To aggregate multiple module exports in one place
- To hide TypeScript errors
- To increase compilation speed
- To reduce runtime code
Show answer

To aggregate multiple module exports in one place — A barrel file aggregates exports from multiple modules in one file; the consumer uses a single import path.

Re-export

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

Show answer

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

Barrel concepts

barrel

Aggregated re-export
index.ts

Barrel file name
export { } from

Re-export
export type

Type-only export

Next lesson: Generic helpers

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