如何为可扩展的搜索栏编写代码，其中的导航链接会变成一个框？

<div class="search-bar">
  <input type="text" id="search-input" placeholder="输入关键词">
  <button id="search-btn">搜索</button>
  <select id="nav-links">
    <option value="link1">导航链接1</option>
    <option value="link2">导航链接2</option>
    <option value="link3">导航链接3</option>
  </select>
</div>

.search-bar {
  display: flex;
  align-items: center;
  justify-content: center;
  width: 300px;
  height: 40px;
  border: 1px solid #ccc;
  border-radius: 20px;
  padding: 5px;
}

#search-input {
  flex: 1;
  border: none;
  outline: none;
}

#search-btn {
  border: none;
  outline: none;
  background-color: #007bff;
  color: #fff;
  padding: 5px 10px;
  border-radius: 20px;
  cursor: pointer;
}

#nav-links {
  width: 100px;
  height: 30px;
  border: 1px solid #ccc;
  border-radius: 5px;
}

document.getElementById("search-btn").addEventListener("click", function() {
  var keyword = document.getElementById("search-input").value;
  var selectedLink = document.getElementById("nav-links").value;
  
  // 执行搜索操作，根据关键词和导航链接进行搜索
  // ...
});