emitter.js

// Publish-Subscribe
// Observable – Observer 
// Publisher - Listener
// Emitter - Handler

/*
Updates:
  0) off
  1) once
  2) raise event on add listener
  3) raise event on remove listener
  4) ...
*/

function Emitter() {
  let handlers = {}
  return {
    on: (eventName, handler) => {
      if (handlers[eventName]) {
        handlers[eventName].push(handler)
      } else {
        handlers[eventName] = [handler]
      }
    },

    emit: (eventName, ...args) => {
      for (handler of handlers[eventName]) {
        handler(...args)
      }
    }
  }
}


let emitter = Emitter()

emitter.on("bar", function handler3(...args) {
  console.log(`handler3 sees <bar> with args=${args.join(", ")}`)
})

emitter.on("foo", function handler1(...args) {
  console.log(`handler1 sees <foo> with args=${args.join(", ")}`)
})

emitter.on("foo", function handler2(...args) {
  console.log(`handler2 sees <foo> with args=${args.join(", ")}`)
})

emitter.on("bar", function handler4(...args) {
  console.log(`handler4 sees <bar> with args=${args.join(", ")}`)
})

emitter.emit("foo", 1, "a")
emitter.emit("bar", 2)
emitter.emit("foo", 2, 0)
emitter.emit("foo", 3)