event-emitter.js

module.exports = class Emitter {
  constructor () {
    this.listeners = {}
  }

  on (event, listener) {
    if (!this.listeners[event]) this.listeners[event] = []
    this.listeners[event].push(listener)
  }

  emit (event, ...args) {
    this.listeners[event].forEach(listener => listener(...args))
  }
  
  // BONUS
  removeListener (event, listener) {
    var index = this.listeners[event].indexOf(listener)
    this.listeners[event].splice(index, 1)
  }
  
  // BONUS
  once (event, listener) {
    var self = this
    this.on(event, wrapper)

    function wrapper (...args) {
      self.removeListener(event, wrapper)
      listener(...args)
    }
  }
}

I think there may be an itty bitty bug in once since cb isn't a reference to the actual listener being removed.

@devinivy good catch!

maybe it's just naming, but what about renaming .removeListener to .off?
I've seen that in the past elsewhere already and it's a lot less to type.
By occupying only 3 instead of 14 characters, it gives other code more space to express itself

Next version of Nanobus?