d.ts files

Declaration files describe types for code that exists at runtime but is not written in TS. They are used for type checking only and emit no JS.

Basics

// globals.d.ts
declare const VERSION: string;
declare function logMessage(msg: string): void;

// main.ts
console.log(VERSION);
logMessage("Hello!");

External libraries

• Install community types when available.

npm i --save-dev @types/lodash

• Or write your own declarations.

declare module "lodash" {
  export function chunk<T>(arr: T[], size?: number): T[][];
}

Notes

• Use declare for external entities without implementations.
• Common for globals, untyped packages, and shared type packages.