Summary - Declaration files

Add types to a JS library by writing a .d.ts file.

Summary - Declaration files

In this group you learned .d.ts files. They add types to JS libraries. You use declare for existing values, declare module for ambient modules, and /// <reference> to reference other files.

declare module "x" {
  export function fn(): void;
}
/// <reference path="./x.d.ts" />

Exercise Declare the "araclar" ambient module

Declare an ambient module "araclar". Export a function topla(a: number, b: number): number inside it. Then print "araclar hazır" with console.log.

Show example solution

declare module "araclar" {
  export function topla(a: number, b: number): number;
}
console.log("araclar hazır");

Test yourself

What is used to add types to a JS library on the TypeScript side?
- A .d.ts declaration file
- A package.json package file
- A tsconfig.json configuration file
- A JavaScript file (.js)
Show answer

A .d.ts declaration file — .d.ts files are used to add type information to JS libraries; tsconfig and package.json do not define types.

topla return type

declare function topla(a: number, b: number): ____;

Show answer

declare function topla(a: number, b: number): number;

Declaration files summary

.d.ts dosyası

Contains types, not code
declare anahtar kelimesi

Used for declarations
declare module

Gives types to a JS module
reference yönergesi

References another .d.ts file

Next lesson: abstract class

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