You can write custom type definitions for a module in .d.ts files, for example:

// some-name.d.ts

// my-module will have the type `any`
declare module "my-module";

// specify the exports
declare module "my-other-module" {
  export function foo(bar: number): string;
}

.d.ts files should be in the src directory.

With tsc, this file is included globally.

Starting with version 7 of ts-node, .d.ts files are not included automatically.

Injecting TS_NODE_FILES=true tells ts-node to include the files as well.

See here for more information.