AI solution for JavaScript Coding Question on BFE.dev
171. create callback on message channel

Here's one possible way to implement BetterChannel:

class BetterChannel {
  constructor() {
    const { port1, port2 } = new SomeChannel()
    this.seqNum = 0 // to keep track of message sequence number
    this.port1Callbacks = {}
    port2.onmessage = (message) => {
      const { data, seq } = message
      const reply = (replyMessage) => {
        port1.postMessage({ replyMessage, replySeq: seq })
      }
      if (this.port1Callbacks[seq]) {
        this.port1Callbacks[seq](data, reply)
        delete this.port1Callbacks[seq]
      }
    }
    this.port1 = {
      postMessage: (message, callback) => {
        const currentSeq = this.seqNum++
        this.port1Callbacks[currentSeq] = callback
        port1.postMessage({ data: message, seq: currentSeq })
      },
    }
    this.port2 = port2
  }
}

Here's how it works:

• BetterChannel is a class that wraps SomeChannel.
• When creating an instance of BetterChannel, it creates SomeChannel and the two ports port1 and port2.
• seqNum is a sequence number that is incremented for each message sent by port1. This is used to match the replies from port2 with the original messages.
• port1Callbacks is an object that maps sequence numbers to callback functions. When port1 sends a message, it stores the callback function in port1Callbacks under the corresponding sequence number.
• port2.onmessage is the handler for messages received by port2. It extracts the message data and sequence number, creates a reply function that sends a reply message back to port1, and calls the callback function associated with the sequence number in port1Callbacks.
• If there is no callback function associated with the sequence number, the message is ignored, since it might be a reply to a previous message that has already been handled. If there is a callback function, it is called with the message data and the reply function as arguments, and then deleted from port1Callbacks.
• port1.postMessage is a function that sends a message to port2, with an optional callback function. It increments seqNum, stores the callback function in port1Callbacks, and sends a message to port2 with the message data and the sequence number.

Note that this implementation assumes that messages sent through SomeChannel will eventually be delivered, but they might be delayed or reordered. If there is a risk of dropped messages, we would need to add some kind of message reliability mechanism (such as ACKs or timeouts) to ensure that messages are not lost.