Why is a class field not updated when its function is called by another class?

Question

I'm not sure why this code is not working. The Receiver class instance does not return any messages.

const runExample = (messages) => {
  class Emitter {
    constructor(messages = []) {
      this.messages = messages;
      this.event = () => {};
    }

    setEvent(fn) {
      this.event = fn;
    }

    trigger() {
      this.messages.forEach((message) => this.event(message));
    }
  }

  class Receiver {
    constructor() {
      this.messages = [];
    }

    ping(message) {
      this.messages.push(message);
    }
  }

  const myEmitter = new Emitter(messages);
  const myReceiver = new Receiver();

  myEmitter.setEvent(myReceiver.ping);
  myEmitter.trigger();

  return myReceiver.messages;
};

One simple way to fix this is to pass the Receiver instance to the Emitter constructor, and call the ping() method directly. But I feel like there should be a way to call that function without passing the class.