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;

Exercise 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.

Show example solution

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

Test yourself

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
Show answer

Type declarations and declare statements — A .d.ts file contains declare statements and type declarations; no implementation code.

Ambient function declaration

____ function kitapEkle(ad: string): string;

Show answer

declare function kitapEkle(ad: string): string;

Declaration file concepts

.d.ts

Declaration file extension
declare

Ambient value declaration
export declare

Package export
ambient

Existing in the environment

Next lesson: Integration

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