TypeScript Declaration Files (.d.ts)

Declaration Files (.d.ts)

What are Declaration Files?

Declaration files (.d.ts) describe the shape of existing JavaScript code to TypeScript. They contain only type information — no implementation.

Global Declarations

declare const API_VERSION: string;
declare function fetchData(url: string): Promise<unknown>;

interface Window {
  __APP_CONFIG__: { apiUrl: string; debug: boolean };
}

Module Declarations

declare module "my-library" {
  export function doSomething(input: string): number;
  export interface Options {
    timeout: number;
    retries: number;
  }
}

Augmenting Existing Modules

// Add types to existing modules
import "express";

declare module "express" {
  interface Request {
    user?: { id: string; role: string };
  }
}

Generate Declaration Files

{
  "compilerOptions": {
    "declaration": true,
    "declarationDir": "./types",
    "emitDeclarationOnly": true
  }
}

Interview Questions

Easy: What is a .d.ts file?

Medium: How do you create module declarations for a JavaScript library?

Hard: How do you augment an existing module’s types?