Stack implementation with Array

stack.js

class Stack {
  constructor(val) {
    if (!val) {
      throw new Error('Please set the max for the stack')
    }
    this.max = val
    this.__data__ = []
  }
  get __isFull() {
    return this.__data__.length == (this.max - 1)
  }
}

Stack.prototype.isEmpty = function () {
  return this.__data__.length == 0
}

Stack.prototype.push = function (val) {
  if (this.__isFull)
    throw new Error('Overflow')
  this.__data__.push(val)
}

Stack.prototype.pop = function () {
  if (this.isEmpty) 
    throw new Error('Underflow')
  return this.__data__.pop()
}

Stack.prototype.peek = function () {
  return this.__data__[this.__data__.length]
}