SOURCE

class EventEmitter {
  /* TODO */
   listeners = {}
  on(name, fn) {
    if (this.listeners.hasOwnProperty(name)) {
      this.listeners[name].push(fn)
    }else{
      this.listeners[name] = [fn]
    }
  }
  emit(name) {
    const args = [].slice.call(arguments, 1);
    
    if(this.listeners.hasOwnProperty(name)){
      this.listeners[name].forEach((ele)=>{
        ele.apply(null,args)
      })
    }
  }
  off(name, fn) {
    if (this.listeners.hasOwnProperty(name)) {
      this.listeners[name].splice(this.listeners[name].indexOf(fn),1)
    }
  }
}
const emitter = new EventEmitter()
const sayHi = (name) => console.log(`Hello ${name}`)
const sayHi2 = (name) => console.log(`Good night, ${name}`)

emitter.on('hi', sayHi)
emitter.on('hi', sayHi2)
emitter.emit('hi', 'ScriptOJ')
// => Hello ScriptOJ
// => Good night, ScriptOJ

emitter.off('hi', sayHi)
emitter.emit('hi', 'ScriptOJ')
// => Good night, ScriptOJ

const emitter2 = new EventEmitter()
emitter2.on('hi', (name, age) => {
  console.log(`I am ${name}, and I am ${age} years old`)
})
emitter2.emit('hi', 'Jerry', 12)
// => I am Jerry, and I am 12 years old