手写eventhub

function EventHub () {
  this.events = {}
}

EventHub.prototype.$on = function(name, fn) {
  (this.events[name] || (this.events[name] = [])).push(fn)
}

EventHub.prototype.$emit = function (name, ...args) {
  this.events[name] && this.events[name].map(dn => fn.call(undefined, ...args))
}

EventHub.prototype.$off = function (name, fn) {
  const events = this.events[name] && this.events[name].filter(v => v !== fn)
  if (events) {
    if (events.length === 0) {
      delete this.events[name]
    } else {
      this.events[name] = events
    }
  }
}