[a reason for private methods on the interface](https://stackoverflow.com/questions/37791947/how-to-define-a-private-property-when-implementing-an-interface-in-typescript)

MessagePair.ts

import {Worker, TransferListItem, MessagePort, parentPort} from 'worker_threads';

interface MessagePair<T extends any, U extends any> {
	//THESE SHOULD BE PRIVATE
	//only present in the interface as a contract BETWEEN APPLICABLE (parameterised) parties
	//not ALL PUBLIC PARTIES
	//as they should use the IMPLEMENTATION'S EXPOSED METHODS
	message     :(value :T) => void;
	postMessage :(
		value         :U extends MessagePair<infer V, this> ? V : never,
		transferList ?:TransferListItem[]
	) => void;
}
type Message<T> = T extends {message :(value :infer U) => void} ? U : never;

//bob accepts strings from steve
class Bob implements MessagePair<string[], Steve> {
	message = (strings :string[]) => {
		console.log(strings.join(' '));
	};
	postMessage :(
		steves :Message<Steve>,
		ports ?:TransferListItem[]
	) => void;

	constructor(port :Worker) {
		//with the contract satisfied, this is safe
		this.postMessage = (...args :[any, any]) => { port.postMessage(...args) };
		port.on('message', this.message);
	}

	//PUBLIC
	tellSteveThisPlease(some :number, numbers :number) {
		if (some > numbers)
			throw new Error('I don\'t trust like that');
		this.postMessage([some, numbers]);
	}
}

//steve accepts numbers from bob
class Steve implements MessagePair<number[], Bob> {
	message = (strings :number[]) => {
		this.postMessage(['yo', 'thanks']);
	};
	postMessage :(
		bobs   :Message<Bob>,
		ports ?:TransferListItem[]
	) => void;

	constructor(port :MessagePort) {
		//with the contract satisfied, this is safe
		this.postMessage = (...args :[any, any]) => { port.postMessage(...args) };
		port.on('message', this.message);
	}
}

//you shouldn't be able to send messages to steve/bob YOURSELF
//they should be PRIVATE and you should have to ASK steve/bob to do so on your behalf
//(if that's your IMPLEMENTATION)

//parent
(new Bob(new Worker(''))).tellSteveThisPlease(1, 2);

//worker
new Steve(parentPort!);

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