We have SomeChannel which allows us to send messages between 2 ports.

const {port1, port2} = new SomeChannel()

port2.onmessage = (message) => {
  console.log(message) // hi
}
port1.postMessage('hi')

It looks like MessageChannel, but SomeChannel is not in good condition, it has random delay in sending the messages, which means the order messages are sent might be different from the order they are received.

Now your job is to create a BetterChannel that enables communication between ports with callback and reply handle, but on top of SomeChannel.

const {port1, port2} = new BetterChannel()

port2.onmessage = (message, reply) => {
  if (message === 'ping?') {
    reply('pong!')
  }
  if (message === 'pong?') {
    reply('ping!') 
  }
}

port1.postMessage('ping?', (data) => {
  console.log(data) // 'pong!'
})

you should avoid global state outside of the classes.
internally SomeChannel must be used.
though there might be delays, messages are guaranteed to be delivered through SomeChannel, but it is worthy for you to think about the case in which messages might be dropped.

動画リスト - JavaScript コーディング問題171. create callback on message channel

動画リスト - JavaScript コーディング問題
171. create callback on message channel