JSFiddle - React, Tailwind, and code Playground

HTML

<body>

  <div class="calculator">


    <div id="output"> </div>
    <table>

      <tr>
        <td>
          <button id="button7" class="squareButtons">7</button>
        </td>
        <td>
          <button id="button8" class="squareButtons">8</button>
        </td>
        <td>
          <button id="button9" class="squareButtons">9</button>
        </td>
        <td>
          <button id="buttonSubt" class="squareButtons">-</button>
        </td>
      </tr>
      <tr>
        <td>
          <button id="button4" class="squareButtons">4</button>
        </td>
        <td>
          <button id="button5" class="squareButtons">5</button>
        </td>
        <td>
          <button id="button6" class="squareButtons">6</button>
        </td>
        <td>
          <button id="buttonPlus" class="squareButtons">+</button>
        </td>
      </tr>
      <tr>
        <td>
          <button id="button1" class="squareButtons">1</button>
        </td>
        <td>
          <button id="button2" class="squareButtons">2</button>
        </td>
        <td>
          <button id="button3" class="squareButtons">3</button>
        </td>
        <td>
          <button id="buttonMult" class="squareButtons">x</button>
        </td>
      </tr>
      <tr>
        <td>
          <button id="buttonClr" class="squareButtons">Clr</button>
        </td>
        <td>
          <button id="button0" class="squareButtons">0</button>
        </td>
        <td>
          <button id="buttonDiv" class="squareButtons">÷</button>
        </td>
        <td>
          <button id="buttonEqu" class="squareButtons">=</button>
        </td>
      </tr>

    </table>

  </div>

CSS

.calculator {
  position: fixed;
  border: 2px solid black;
  background-color: beige;
  width: 220px;
  height: 250px;
  padding: 2px;
}

#output {
  border: 2px solid black;
  background-color: white;
  height: 28px;
  text-align: right;
}

.squareButtons {
  style: none;
  color: green;
  border: none;
  width: 50px;
  height: 50px;
  background-color: lightblue;
  display: inline-block;
  padding: 10px;
}

JavaScript

var storedNumber = document.getElementById("output").innerHTML;
var sign = 0;
var storedNumberTwo = document.getElementById("output").innerHtml;
var calc;




document.getElementById("button1").onclick = onePress;
document.getElementById("button2").onclick = twoPress;
document.getElementById("button3").onclick = threePress;
document.getElementById("button4").onclick = fourPress;
document.getElementById("button5").onclick = fivePress;
document.getElementById("button6").onclick = sixPress;
document.getElementById("button7").onclick = sevenPress;
document.getElementById("button8").onclick = eightPress;
document.getElementById("button9").onclick = ninePress;
document.getElementById("button0").onclick = zeroPress;
document.getElementById("buttonPlus").onclick = plusPress;
document.getElementById("buttonEqu").onclick = equalPress;


function onePress() {
  document.getElementById("output").innerHTML += "1";
}

function twoPress() {
  document.getElementById("output").innerHTML += "2";
}

function threePress() {
  document.getElementById("output").innerHTML += "3";
}

function fourPress() {
  document.getElementById("output").innerHTML += "4";
}

function fivePress() {
  document.getElementById("output").innerHTML += "5";
}

function sixPress() {
  document.getElementById("output").innerHTML += "6";
}

function sevenPress() {
  document.getElementById("output").innerHTML += "7";
}

function eightPress() {
  document.getElementById("output").innerHTML += "8";
}

function ninePress() {
  document.getElementById("output").innerHTML += "9";
}

function zeroPress() {
  document.getElementById("output").innerHTML += "0";

}

function plusPress() {
  storedNumber = document.getElementById("output").innerHTML;
  document.getElementById("output").innerHTML = "";
  var sign = 1;
 // alert(storedNumber);

}

function equalPress() {
 
  storedNumberTwo = document.getElementById("output").innerHTML;
 // if(sign === 1) {
   calc = +storedNumberTwo + +storedNumber;
  ...