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What are declaration files (.d.ts) and when do you need them?

#tooling#core

Declaration Files (.d.ts)

Declaration files describe the types of existing JavaScript code without containing any implementation.

What they contain

ts
// types/analytics.d.ts
declare function trackEvent(name: string, data?: object): void;
declare const VERSION: string;

declare module 'some-js-library' {
  export function doSomething(value: string): number;
  export interface Options {
    timeout: number;
    retries: number;
  }
}

When you need them

Third-party JS libraries without built-in types (installed from @types/...).
Your own JS code that you want to use in a TS project.
Global variables injected by scripts (e.g. window.analytics).
Ambient module declarations for non-TS files (CSS, SVG imports).

@types packages

bash
npm install -D @types/lodash
npm install -D @types/node

These are community-maintained declaration files from the DefinitelyTyped repository.

Augmenting existing types

ts
// extend the global Window interface
declare global {
  interface Window {
    analytics: Analytics;
  }
}

When you install a library and TypeScript says Could not find a declaration file - you either need @types/... or to write a .d.ts yourself.

updated Mar 8, 2026

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