使用原生js写一个下拉框且可以搜索

下面是一个使用原生JS编写的带搜索功能的下拉框代码示例：

HTML：

<div class="dropdown">
  <input type="text" id="searchInput" placeholder="Search...">
  <button class="dropbtn">Select item</button>
  <div class="dropdown-content">
    <a href="#">Item 1</a>
    <a href="#">Item 2</a>
    <a href="#">Item 3</a>
    <a href="#">Item 4</a>
    <a href="#">Item 5</a>
    <a href="#">Item 6</a>
    <a href="#">Item 7</a>
    <a href="#">Item 8</a>
    <a href="#">Item 9</a>
    <a href="#">Item 10</a>
  </div>
</div>

CSS：

.dropdown {
  position: relative;
  display: inline-block;
}

#searchInput {
  padding: 5px;
  font-size: 16px;
  border: none;
  border-bottom: 1px solid #ccc;
}

.dropdown-content {
  display: none;
  position: absolute;
  z-index: 1;
  background-color: #f9f9f9;
  min-width: 160px;
  max-height: 200px;
  overflow-y: auto;
  border: 1px solid #ddd;
}

.dropdown-content a {
  color: black;
  padding: 12px 16px;
  text-decoration: none;
  display: block;
}

.dropdown-content a:hover {
  background-color: #f1f1f1;
}

.show {
  display: block;
}

JS：

// Get the button and dropdown div
const btn = document.querySelector(".dropbtn");
const dropdown = document.querySelector(".dropdown-content");

// Show/hide dropdown when button is clicked
btn.addEventListener("click", function() {
  dropdown.classList.toggle("show");
});

// Search the dropdown when text is entered in the input field
const searchInput = document.getElementById("searchInput");
searchInput.addEventListener("keyup", function(event) {
  const filter = event.target.value.toUpperCase();
  const items = dropdown.getElementsByTagName("a");
  for (let i = 0; i < items.length; i++) {
    const txtValue = items[i].textContent || items[i].innerText;
    if (txtValue.toUpperCase().indexOf(filter) > -1) {
      items[i].style.display = "";
    } else {
      items[i].style.display = "none";
    }
  }
});

这个下拉框将显示一个输入框和一个按钮。当用户点击按钮时，下拉框中的项目将显示。当用户在输入框中输入文本时，下拉框将根据用户的搜索过滤项目。