Stack in ES6

stackNew.js

'use strict'

class Stack {
  constructor(capacity) {
    this._capacity = capacity || Infinity
    this._storage = {}
    this._count = 0
  }

  push(value) {
    if (this._count < this._capacity) {
      this._storage[this._count++] = value
      return this._count
    }
    return 'Max capacity reached. Remove an element before adding a new one'
  }

  pop() {
    let value = this._storage[--this._count]
    delete this._storage[this._count]
    if (this._count < 0) {
      this._count = 0
    }
    return value
  }

  peek() {
    return this._storage[this._count-1]
  }

  count() {
    return this._count
  }
}

var myStack = new Stack(3)
console.log(myStack.push('a'))
console.log(myStack.push('b'))
console.log(myStack.push('c'))
console.log(myStack.push('d'))
console.log(myStack.pop())
console.log(myStack.count())
console.log(myStack.peek())
console.log(myStack.count())

class MinStack {
  constructor(capacity) {
    this._capacity = capacity 
    this._storage = {}
    this._count = 0
    this._min = new Stack()
  }

  push() {
    if(this._count < this._capacity) {
      if(this._min.peek() < value) {
        this._min.push(this._min.peek())
      } else {
        this._min.push(value)
      }
      this._storage[this._count++] = value
      return this._count
    }
    return 'Max capacity reached Remove element before adding a new one'
  }

  pop() {
    this._min.pop()
    let value = this._storage[--this._count]
    delete this._storage[this._count]
    if (this._count < 0) {
      this._count = 0
    }
    return value
  }

  peek() {
    return this._storage[this._count-1]
  }

  count() {
    return this._count
  }

  min() {
    return this._min.peek()
  }
}