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                <div id="app">
   <div data-reactroot="" class="calculator">
      <div class="header">
         <div class="header__screen">
            <div class="formula"></div>
            <div id="display">0</div>
      <div class="buttons">
        <button id="seven" class="button number" value="7">7</button>
        <button id="eight" class="button number" value="8">8</button>
        <button id="nine" class="button number" value="9">9</button>
        <button id="clear" class="button button--red">AC</button>
        <button id="four" class="button number" value="4">4</button>
        <button id="five" class="button number" value="5">5</button>
        <button id="six" class="button number" value="6">6</button>
        <button id="div" class="button op" value="/">/</button>
        <button id="one" class="button number" value="1">1</button>
        <button id="two" class="button number" value="2">2</button>
        <button id="three" class="button number" value="3">3</button>
        <button id="mul" class="button op" value="*">*</button>
        <button id="zero" class="button number" value="0">0</button>
        <button id="undo" class="button" value="UNDO">undo</button>
        <button id="add" class="button op" value="+">+</button>
        <button id="sub" class="button op" value="-">-</button>
        <button id="equals" class="button button--full res" value="=">=</button></div>



                @font-face {
  font-family: "Digital";
  src: url("//");
  src: url("//") format("embedded-opentype"), url("//") format("woff2"), url("//") format("woff"), url("//") format("truetype"), url("//") format("svg");
html {
  font-size: 16px;
  font-family: Arial, Verdana, sans-serif;
#app {
  display: flex;
  justify-content: center;
  align-items: center;
  height: 100vh;
.calculator {
  border-radius: 1rem;
  overflow: hidden;
  box-shadow: 0.05rem 0.05rem 0.2rem 0 #54545b;
button {
  cursor : pointer
.header {
  font-family: "Digital";
  text-align: right;
  color: #54545b;
  background: #54545b;
  padding: 0.8rem;
.header .header__screen {
  padding: 0.4rem;
  width: 15.63rem;
  background: #eaecd6;
  box-shadow: inset 0.2rem 0.2rem #c9cbb6;
  border-radius: 0.6rem;
.header .formula {
  font-size: 1.3rem;
  height: 1.5rem;
.header #display {
  font-size: 3rem;
.buttons {
  display: grid;
  grid-template-columns: repeat(4, 1fr);
  grid-template-rows: repeat(5, 1fr);
  background: #f2f2f2;
  padding: 0.6rem;
.buttons .button {
  font-family: inherit;
  font-size: 1rem;
  color: #fff;
  background: #54545b;
  border: 0;
  outline: 0;
  padding: 0.5rem 0;
  margin: 0.6rem;
  border-radius: 0.3rem;
  box-shadow: 0.05rem 0.05rem 0.2rem 0 #54545b;
.buttons .button.button--red {
  background: #eb5151;
.buttons .button.button--full {
  grid-column-start: 1;
  grid-column-end: 5;



                function add(x, y) { return x + y; }
function sub(x, y) { return x - y; }
function mul(x, y) { return x * y; }
function div(x, y) { return x / y; }

//Command Class
class Command {
  constructor (execute, undo, value){
    this.execute = execute;
    this.undo = undo;
    this.value = value;

class AddCommand extends Command {
  constructor (value) {
    super(add, sub, value)
class SubCommand extends Command {
  constructor (value) {
    super(sub, add, value)
class MulCommand extends Command {
  constructor (value) {
    super(mul, div, value)

class DivCommand extends Command {
  constructor (value) {
    super(div, mul, value)

class Calculator {
  constructor (){
    this.current = 0;
    this.history = [];
  execute (command) {    
    this.current =  command.execute(this.current, command.value);
  undo() {
    var command = this.history.pop();
    this.current = command.undo(this.current, command.value);
  setCurrent (val){
    this.current = Number(val)
  getValue (){ 
    return this.current
  reset (){
    this.current = 0;
    this.history = [];

var calculator = new Calculator();
var opMode = false;
var setCommandType = type => val => {
   val = parseInt(val)
   switch (type) {
     case 'add' : return calculator.execute(new AddCommand(val));
     case 'sub' : return calculator.execute(new SubCommand(val));
     case 'div' : return calculator.execute(new DivCommand(val));
     case 'mul' : return calculator.execute(new MulCommand(val));

$('.op').click(function () {
  CURRENT_COMMAND = setCommandType($(this).attr('id'));

  opMode = true
$('.number').click(function () {
  if (!opMode) {
    $('#display').text( $(this).val())
  $('.formula').text($('.formula').text()+' '+$(this).val())

$('#undo').click(() => {

$('#clear').click(() => {
    opMode = false


$('#equals').click(() => {