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)
    }
  }
}

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