let stack = (function () {
  let data = []

  function push (el) {
    data.push(el)
  }

  function pop () {
    let el = data.pop()
    return el
  }

  function peek () {
    return data[data.length - 1]
  }

  return { push, pop, peek }
})()

stack.push(1)
stack.push(2)
stack.push(3)
console.log(stack.peek())
stack.pop()
console.log(stack.peek())