Really simple Javascript custom event system (slightly extended to keep listeners and allow whether stuff should be called immediately)

event.js

var Event = function () {
  var self = this

  self.queue = {}
  self.fired = []

  return {
    fire: function (event) {
      var queue = self.queue[event]

      if (typeof queue === 'undefined') { return }

      queue.forEach((callback) => callback())

      self.fired[event] = true
    },

    on: function (event, callback, callIfFired) {
      if (callIfFired && self.fired[event] === true) { callback() }

      if (typeof self.queue[event] === 'undefined') {
        self.queue[event] = []
      }

      self.queue[event].push(callback)
    },

    off: function (event, callback) {
      var queue = self.queue[event]
      
      if (typeof queue === 'undefined') { return }

      if (callback) {
        var index = queue.indexOf(callback)
        if (index > -1) { queue.splice(index, 1) }
      } else {
        delete self.queue[event]
      }
    }
  }
}()


// Basic usage

Event.on('counted.to::1000', function () {
  doSomething()
})

for (i = 0; i <= 1000; i++) {
  // Count to 1000...
}

Event.fire('counted.to::1000'); // doSomething() is called

// doThisImmediately() is called immediately, since we provide callIfFired as true
Event.on('counted.to::1000', function () { doThisImmediately() }, true)
// dontDoThisImmediately() is *not* called, since we do *not* provide callIfFired
Event.on('counted.to::1000', function () { dontDoThisImmediately() })