Declaration Files

Overview

Declaration files (.d.ts) contain only type information—no runtime code—and describe the shape of JavaScript libraries, global variables, or module APIs. TypeScript consumes them to type-check code that imports untyped packages or interacts with globals. Bundled libraries ship their own; for others you write or install ambient declarations.

Syntax / Usage

Use declare to describe things that exist at runtime but have no TypeScript source. Module and global augmentation extend existing types.

// globals.d.ts — describe a runtime-injected global
declare global {
  interface Window {
    analytics: {
      track(event: string, props?: Record<string, unknown>): void
    }
  }
}

// Declare the shape of an untyped module
declare module "legacy-math" {
  export function add(a: number, b: number): number
  export const PI: number
}

// Needed so the file is treated as a module
export {}

Examples

Provide types for a module that only ships JavaScript.

// slugify.d.ts
declare module "slugify" {
  interface Options {
    lower?: boolean
    strict?: boolean
  }
  function slugify(input: string, options?: Options): string
  export = slugify
}

Let non-code imports type-check by declaring wildcard modules.

declare module "*.svg" {
  const content: string
  export default content
}

Augment an existing library's types without forking it.

import "express"

declare module "express" {
  interface Request {
    userId?: string
  }
}

Common Mistakes

• Adding runtime code to a .d.ts file, which is not allowed
• Forgetting export {}, so declare global is not applied
• Using export default where the JS uses CommonJS export =
• Redeclaring types that the package already ships, causing conflicts
• Placing .d.ts files outside the include paths in tsconfig.json

See Also

interfaces typescript-decorators basic-types