For example you are building your own module, the same as Lodash:
my-lodash.d.ts
declare module "lodash" { declare interface FirstFunction { (data: any[]) :any; } declare interface Lodash { first: FirstFunction; } export const _: Lodash: }
Normally you can install @types for popluar 3rd-party libs, if you have to build your own, you need to remember export type defination aas well for Typescript.