Declaration Files (.d.ts)

Types for plain JS

A .d.ts file contains only type declarations — no implementation. It describes the shape of a .js library so TS can check code that calls it.

A .d.ts snippet

This describes a JS function's signature without containing its body.

// mathLib.d.ts
declare function square(n: number): number;
declare const PI: number;

export { square, PI };

@types packages

For popular JS libraries, someone has usually already written the .d.ts — install it with npm install --save-dev @types/library-name.