TypeScript · Chapter 38 of 44

TypeScript Declaration Files

Declaration files, ending in `.d.ts`, contain only type information without any actual implementation code. They let TypeScript understand the shape of JavaScript code, including third-party libraries.

Many popular JavaScript libraries ship their own .d.ts files, or have community-maintained ones available through the DefinitelyTyped project, installed via packages like `@types/library-name`.

Syntax

declare function greet(name: string): string;

What's inside a .d.ts file

A declaration file contains type declarations like interfaces, type aliases, and function signatures, using the `declare` keyword for values that exist elsewhere at runtime.

Using @types packages

For JavaScript libraries without built-in types, you can often install community type definitions with `npm install --save-dev @types/library-name`, giving you full type checking and autocomplete.

Example 1 (typescript)

// greet.d.ts
declare function greet(name: string): string;

Output

(no runtime output — a declaration file)

This declares that a greet function exists elsewhere, describing its type signature only.

Example 2 (bash)

npm install --save-dev @types/lodash

Output

added 1 package

Installs community-maintained type definitions for the lodash library so TypeScript understands its API.

Key points

Declaration files end in .d.ts and contain only type information.
They let TypeScript type-check plain JavaScript libraries.
The `declare` keyword describes values that exist without providing implementation.
@types packages provide community-maintained types for popular JS libraries.

💡 Note: You rarely need to write .d.ts files by hand unless you're publishing a library or typing an untyped dependency.

TypeScript Declaration Files

What's inside a .d.ts file

Using @types packages

Key points

📝 Quick Quiz