---
source: https://iinventedlight.com/library/notes/typescript-declaration-files.md
title: "TypeScript declaration files (.d.ts)"
license: CC-BY-4.0
license_url: https://creativecommons.org/licenses/by/4.0/
source_license: "CC BY 4.0 (microsoft/TypeScript-Website); prose restates cited source lines"
attribution: "Summary by iinventedlight (AI-assisted). Sources: TypeScript (Microsoft), TypeScript: Documentation - Introduction, CC BY 4.0 (microsoft/TypeScript-Website); TypeScript (Microsoft), TypeScript: Documentation - Declaration Reference, CC BY 4.0 (microsoft/TypeScript-Website)."
author: "iinventedlight (synthesis of cited sources); text drafted with AI assistance"
date: 2026-11-04
status: published
checked_on: 2026-11-04
as_of: 2026-11-04
confidence: medium
untested: "No code was run for this note; only the fetched documentation pages were read."
category: coding-references
topic_area: "code-ai-eng: TypeScript"
tags: typescript, declaration-files, d-ts
sources:
  - id: S1
    title: "TypeScript: Documentation - Introduction"
    publisher: TypeScript (Microsoft)
    url: https://www.typescriptlang.org/docs/handbook/declaration-files/introduction.html
    accessed: 2026-11-04
    license: "CC BY 4.0 (microsoft/TypeScript-Website)"
  - id: S2
    title: "TypeScript: Documentation - Declaration Reference"
    publisher: TypeScript (Microsoft)
    url: https://www.typescriptlang.org/docs/handbook/declaration-files/by-example.html
    accessed: 2026-11-04
    license: "CC BY 4.0 (microsoft/TypeScript-Website)"
---
# TypeScript declaration files (.d.ts)

## Short answer
For JavaScript library users, the Consumption section offers a few simple steps to locate and install corresponding declaration files. (S1) The most common reason to write one is typing an npm package that ships no types. [S1]

```ts
declare function getWidget(n: number): Widget;
declare function getWidget(s: string): Widget[];
interface GreetingSettings { greeting: string; color?: string }
```
[S2]

## Key points
- The handbook's sections cover library structures, templates, do's and don'ts, publishing and consumption. [S1]

## Claims
- id: C1
  claim: The most common case is typing an npm package with no types.
  source: S1
  evidence: "you’re typing an npm package with no types"
- id: C2
  claim: The Library Structures guide covers common library formats.
  source: S1
  evidence: "helps you understand common library formats"
- id: C3
  claim: Use declare namespace for values accessed by dotted notation.
  source: S2
  evidence: "Use declare namespace to describe types or values accessed by dotted notation."
- id: C4
  claim: Use an interface to define a type with properties.
  source: S2
  evidence: "Use an interface to define a type with properties."

## Sources
- [S1] TypeScript: Documentation - Introduction. TypeScript (Microsoft). https://www.typescriptlang.org/docs/handbook/declaration-files/introduction.html . License: CC BY 4.0 (microsoft/TypeScript-Website). Accessed 2026-11-04.
- [S2] TypeScript: Documentation - Declaration Reference. TypeScript (Microsoft). https://www.typescriptlang.org/docs/handbook/declaration-files/by-example.html . License: CC BY 4.0 (microsoft/TypeScript-Website). Accessed 2026-11-04.

License: this note is CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Summary by iinventedlight (AI-assisted).