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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]

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).

@misc{iil_note_3xaqcxgt,
  author = {{iinventedlight}},
  title = {{TypeScript declaration files (.d.ts)}},
  howpublished = {iinventedlight Library},
  year = {2026},
  date = {2026-11-04},
  url = {https://iinventedlight.com/library/notes/typescript-declaration-files/},
  note = {Permanent ID iil:note:3xaqcxgt. License: CC-BY-4.0.},
}

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