What are d.ts Files

A declaration file (.d.ts) contains only types: no implementation. It describes JavaScript code that lives somewhere else. When you publish a library, tsc --declaration generates .d.ts files next to the .js files so TypeScript users get types.

src/price.ts

export function withGst(amount: number, rate = 0.18): number {
  return Math.round(amount * (1 + rate));
}

Generated dist/price.js and dist/price.d.ts:

function withGst(amount, rate = 0.18) {
  return Math.round(amount * (1 + rate));
}
exports.withGst = withGst;

export declare function withGst(amount: number, rate?: number): number;