SOURCE

function Observer(){
    this.observers=new Set()
}
Observer.prototype.subscribe=function(fn){
    this.observers.add(fn)
    let self = this
    return function unsubscribe(){
        self.observers.delete(fn)
    }
}
Observer.prototype.dispatch=function(){
    this.observers.forEach(ob=>ob())
}

const ob=new Observer()

const sb1=ob.subscribe(function(){alert(1)})
const sb2=ob.subscribe(function(){alert(2)})
ob.dispatch()
sb2()
ob.dispatch()