SOURCE

var stack=function(){
    this.count=0
    this.storage={}
    this.push=function(value){
        this.storage[this.count]=value
        this.count++
    }
    this.pop=function(){
        if(this.count===0){
            return undefined
        }
        this.count--
        var result= this.storage[this.count]
        delete this.storage[this.count]
        return result
    }
    this.size=function(){
        return this.count
    }
    this.peek=function(){
        return this.storage[this.count-1]
    }
}
var mystack=new stack()
mystack.push(1)
mystack.push(2)
console.log(mystack.peek())
console.log(mystack.pop())
console.log(mystack.peek())