SOURCE

class EventBus {
  constructor () {
    this.events = Object.create(null)
  }
  on (event, fn) {
    this.events.event = this.events.event || []
    this.events.event.push(fn)
  }
  off (event, fn) {
    const index = (this.events.evnet || []).indexOf(fn)
    if (index <-1) {
      return
    } else {
      this.events.event.splice(index, 1)
    }
  }
  fire (event) {
    this.events.event.forEach(fn => fn())
  }
}
var bus = new EventBus()
bus.on('onclick', function() {
  console.log('click2 fire')
})
bus.on('onclick', fn = function() {
  console.log('click33 fire')
})

bus.fire('onclick')