SOURCE

var btn0 = document.getElementById("btn0");
var btn1 = document.getElementById("btn1");
var btnEql = document.getElementById("btnEql");
var btnClr = document.getElementById("btnClr");

var a, b, operation;

btn0.onclick = function () {
  res.innerHTML += "0";
};

btn1.onclick = function () {
  res.innerHTML += "1";
};

btnClr.onclick = function () {
  res.innerHTML = "";
};

btnEql.onclick = function () {
  b = res.innerHTML;
  const c = parseInt(a, 2);
  const d = parseInt(b, 2);
  res.innerHTML = eval(`(${c} ${operation} ${d}).toString(2)`);
};

function action(e) {
  var btn = e.target || e.srcElement;
  a = res.innerHTML;
  operation = btn.innerHTML;
  res.innerHTML = "";
}

document.getElementById("btnSum").onclick = action;
document.getElementById("btnSub").onclick = action;
document.getElementById("btnMul").onclick = action;
document.getElementById("btnDiv").onclick = action;

<h1>binary calculator</h1>
<div id="container">
  <div id='res'></div>
  <div id='btns'>
    <button id='btn0'>0</button>
    <button id='btn1'>1</button>
    <button id='btnClr'>C</button>
    <button id='btnEql'>=</button>
    <button id='btnSum'>+</button>
    <button id='btnSub'>-</button>
    <button id='btnMul'>*</button>
    <button id='btnDiv'>/</button>
  </div>
</div>

<p class="description">
  simple calculator that performs the following <span>operations on binary numbers</span>:<br> addition, subtraction, multiplication, and division.
</p>

@import url("https://fonts.googleapis.com/css2?family=Roboto:wght@100;300;400;500;900&display=swap");

* {
  margin: 0;
  padding: 0;
  outline: 0;
  box-sizing: border-box;
  -webkit-font-smoothing: antialiased;
  user-select: none;
}

html,
body {
  width: 100%;
  height: 100%;
}

body {
  display: flex;
  justify-content: center;
  align-items: center;
  flex-direction: column;
  background-color: #f5f5f5;
  font: 400 14px Roboto, sans-serif;
}

h1 {
  margin-bottom: 30px;
  font-size: 28px;
  color: #335c62;
}

#container {
  width: 33%;
}

#res {
  background-color: lightgray;
  height: 48px;
  font-size: 26px;
  border: 1px solid #f5f5f5;
  border-radius: 5px;
  padding: 5px 20px;
  color: #363636;
}

#btns button {
  width: 25%;
  height: 36px;
  font-size: 20px;
  margin: 0px;
  float: left;
  border: 2px solid #f5f5f5;
  border-radius: 5px;
}

#btn0,
#btn1 {
  background-color: #59a2ad;
  color: #f3f3f3;
}

#btnClr,
#btnEql {
  background-color: #335c62;
  color: #f3f3f3;
}

#btnSum,
#btnSub,
#btnMul,
#btnDiv {
  background-color: #363636;
  color: #f3f3f3;
}

.description {
  margin-top: 50px;
  text-align: center;
  color: #999;
  width: 40%;
  line-height: 28px;
  font-size: 16px;
}

span {
  color: #335c62;
}