...  

Elm.d.ts

declare const Elm: Elm

interface Elm {
  Main: AppConstructor<unknown, { port1: SubscribePort<unknown>; port2: SendPort<unknown> }>
}

interface AppConstructor<Flags, Ports> {
  init(options: { node?: HTMLElement; flags: Flags }): { ports: Ports }
}

interface SendPort<A> {
  send(value: A): void
}

interface SubscribePort<A> {
  subscribe(callback: (value: A) => void): void
  unsubscribe(callback: (value: A) => void): void
}