typed-messaging.ts

interface Message<Type extends string | number, Data = never> {
  type: Type;
  data: Data;
}

type MessageData<
  Msg extends Message<any, any>,
  Type extends Msg['type']
> = Extract<Msg, { type: Type }>['data'];

type ClientMessage =
  | Message<'ERROR', { message: string }>
  | Message<'CONNECT'>
  | Message<'RECONNECT', string>
  | Message<'OPEN', { server: string; port: number }>;

function sendMessage<Type extends ClientMessage['type']>(
  type: Type,
  data: MessageData<ClientMessage, Type>,
) {}

function receiveMessage<Type extends ClientMessage['type']>(
  type: Type,
  listener: (data: MessageData<ClientMessage, Type>) => void,
) {}

sendMessage('ERROR', { message: 'potato' });
receiveMessage('ERROR', x => x.message);