<main id="main">
      <form id="calc_form">
        <header class="calc_header">
          <input
            type="text"
            disabled
            id="output"
            class="calc_output"
            value="0"
          />
        </header>
        <footer class="calc_footer">
          <div class="btn_group">
            <button
              data-type="clear"
              type="reset"
              value="clear"
              class="btn btn-grey"
            >
              AC
            </button>
            <button data-type="operator" value="invert" class="btn btn-grey">
              +/-
            </button>
            <button data-type="operator" value="%" class="btn btn-grey">
              %
            </button>
            <button data-type="operator" value="/" class="btn btn-orange">
              ÷
            </button>
          </div>
          <div class="btn_group">
            <button
              data-type="operand"
              value="7"
              class="btn btn-dark-grey"
              id="7"
            >
              7
            </button>
            <button data-type="operand" value="8" class="btn btn-dark-grey">
              8
            </button>
            <button data-type="operand" value="9" class="btn btn-dark-grey">
              9
            </button>
            <button data-type="operator" value="*" class="btn btn-orange">
              x
            </button>
          </div>
          <div class="btn_group">
            <button data-type="operand" value="4" class="btn btn-dark-grey">
              4
            </button>
            <button data-type="operand" value="5" class="btn btn-dark-grey">
              5
            </button>
            <button data-type="operand" value="6" class="btn btn-dark-grey">
              6
            </button>
            <button data-type="operator" value="-" class="btn btn-orange">
              -
            </button>
          </div>
          <div class="btn_group">
            <button data-type="operand" value="1" class="btn btn-dark-grey">
              1
            </button>
            <button data-type="operand" value="2" class="btn btn-dark-grey">
              2
            </button>
            <button data-type="operand" value="3" class="btn btn-dark-grey">
              3
            </button>
            <button data-type="operator" value="+" class="btn btn-orange">
              +
            </button>
          </div>
          <div class="btn_group">
            <button
              data-type="operand"
              value="0"
              class="btn btn-grow btn-dark-grey"
            >
              0
            </button>
            <button data-type="operand" value="." class="btn btn-dark-grey">
              .
            </button>
            <button value="=" data-type="operator" class="btn btn-orange">
              =
            </button>
          </div>
        </footer>
      </form>
    </main>

*,
*::after,
*::before {
  padding: 0px;
  margin: 0px;
  font-family: inherit;
}

body {
  background-color: #333333;
  display: flex;
  align-items: center;
  justify-content: center;
  font-size: 16px;
}
body,
html {
  height: 100vh;
  font-family: sans-serif;
}
button {
  cursor: pointer;
  border: 0px;
}
input[type="text"] {
  background-color: transparent;
  text-align: end;
  width: 100%;
  color: #d2d2d2;
  border: 0px;
  font-size: 4rem;
}
#main {
  border: 2px solid #ededed;
  border-radius: 50px;
  width: 100%;
  max-width: 280px;
  background-color: #000000;
  padding: 2rem 1rem;
}
.calc_header {
  margin-top: 90px;
  padding: 12px;
}

.calc_footer > * + * {
  margin-top: 1rem;
}
.calc_footer > *:last-child {
  gap: 1rem;
}
.btn_group {
  display: flex;
  justify-content: space-between;
  align-items: center;
  gap: 0.3rem;
}

.btn {
  display: flex;
  justify-content: center;
  align-items: center;
  border-radius: 50%;
  width: 60px;
  height: 60px;
  color: #ffffff;
  font-size: 20px;
  transition: all 0.3s cubic-bezier(0.19, 1, 0.22, 1);
}
.btn.btn-grow {
  flex-grow: 1;
  border-radius: 40px;
}
.btn:hover {
  transform: translate(-2px, -3px);
}
.btn.active {
  background-color: #ffffff;
  color: #f69906;
}

.btn-grey {
  background-color: #9f9f9f;
  color: #000;
}
.btn-grey:hover {
  background-color: #ededed;
}

.btn-dark-grey {
  background-color: #313131;
}
.btn-dark-grey:hover {
  background-color: #999999;
}
.btn-orange {
  background-color: #f69906;
}
.btn-orange:hover {
  background-color: orange;
}

const output = document.getElementById("output");
const form = document.getElementById("calc_form");
const operand_btns = document.querySelectorAll("button[data-type=operand]");

form.addEventListener("submit", (e) => {
  e.preventDefault();
});

let is_operator = false;
operand_btns.forEach((btn) => {
  btn.addEventListener("click", (e) => {
    if (output.value == "0") {
      output.value = e.target.value;
    } else if (is_operator) {
      is_operator = false;
      output.value = e.target.value;
		} else if (output.value.includes(".")) {
      output.value = output.value + "" + e.target.value.replace(".", "");
    } else {
      output.value = output.value + "" + e.target.value;
    }
  });
});