class MessageBus {
  constructor() {
    this.channels = {}
  }

  subscribe(object) {
    if (this.channels[object.channel]) {
      if (this.channels[object.channel][object.topic]) {
        this.channels[object.channel][object.topic].push(object.callback);
      } else {
        this.channels[object.channel][object.topic] = [object.callback];
      }
    } else {
      let topic = {};
      topic[object.topic] = [object.callback];
      this.channels[object.channel] = topic;
    }
  }

  publish(object) {
    if (this.channels[object.channel]) {
      if (this.channels[object.channel][object.topic]) {
        for (let i = 0; i < this.channels[object.channel][object.topic].length; i++) {
          this.channels[object.channel][object.topic][i](object.data);
        }
      }
    }
  }
}

let messageBus = new MessageBus;
messageBus.subscribe({ channel: 'hi', topic: 'logging', callback: (payload) => console.log(payload)});
messageBus.subscribe({ channel: 'hey', topic: 'log', callback: (payload) => console.log(payload)});
messageBus.subscribe({ channel: 'hey', topic: 'logging', callback: (payload) => console.log(payload)});
messageBus.subscribe({ channel: 'hey', topic: 'logging', callback: (payload) => console.log(payload, payload)});
messageBus.publish({ channel: 'hey', topic: 'logging', data: 'message'}); // last two subscribers get their stuff run in order