<div class="custom-select" id="js__combox">
  <label for="combox">语言：</label>
  <div class="combo-wrap" id="js__select">
    <input type="text" class="combo__box" id="combox" />
    <i class="arrow"></i>
    <ul class="listbox">
      <li class="option selected">CSS</li>
      <li class="option">HTML</li>
      <li class="option">JavaScript</li>
      <li class="option">React</li>
      <li class="option">Vue</li>
    </ul>
  </div>
</div>

@import url("https://fonts.googleapis.com/css?family=Gochi+Hand");

* {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
}

body {
  margin: 0;
  padding: 0;
  background-color: #291642;
  font-family: "Gochi Hand", sans-serif;
  color: #fff;
  font-size: 130%;
  letter-spacing: 0.1rem;
  display: flex;
  align-items: center;
  justify-content: center;
  width: 100vw;
  min-height: 100vh;
}

// custom style for select
label {
  padding: 5px;
  font-size: 18px;
  font-weight: 500;
  display: block;
}

.combo-wrap {
  display: inline-flex;
  align-items: center;
  position: relative;
}

.combo__box {
  width: 30vw;
  padding: 10px 35px 10px 5px;
  border: 1px solid #fff;
  box-sizing: border-box;
  font-size: 14px;
  background-color: #fff;
  color: #212e59;
  box-shadow: 0 0 5px #000;
}

.arrow {
  position: absolute;
  top: 0;
  right: 0;
  bottom: 0;
  width: 32px;
  background-color: #212e59;

  &::after {
    content: "";
    position: absolute;
    top: calc(50% + 4px);
    left: 50%;
    transform: translate(-50%, -50%);
    border: 6px solid #fff;
    border-color: #fff transparent transparent;
  }
}

.listbox {
  display: none;
  position: absolute;
  list-style: none outside none;
  z-index: 99;

  max-height: 30vh;
  overflow-y: auto;
  background-color: #fff;
  top: calc(100% + 0px);
  left: 0;
  right: 0;
  box-shadow: 1px 2px 2px #000;
  color: #333;

  li {
    background-color: #fff;
    padding: 4px 6px;

    &.selected,
    &:hover {
      background-color: #212e59;
      color: #fff;
    }
  }
}

.combo-wrap.open .listbox,
.combo-wrap:focus-within .listbox {
  display: block;
}

const combox = document.getElementById("js__combox");

const options = document.querySelectorAll(".option");
const comboxVal = document.getElementById("combox");

combox.addEventListener("click", function() {
  console.log(this);
  this.querySelector(".combo-wrap").classList.toggle("open");
});

window.addEventListener("click", function(e) {
  const select = document.getElementById("js__select");

  if (!select.contains(e.target)) {
    select.classList.remove("open");
  }
});

for (const option of options) {
  option.addEventListener("click", function() {
    if (!this.classList.contains("selected")) {
      this.parentNode
        .querySelector(".option.selected")
        .classList.remove("selected");
      this.classList.add("selected");
      comboxVal.value = this.textContent;
    }
  });
}