Javascript 带有下拉列表的搜索栏

function f(filter){
  menuItems = document.querySelectorAll('#myMenu > li');
  for(const menuItem of menuItems){    
    const textContent = menuItem.textContent.trim().toUpperCase();
    //console.log(`"${textContent}" "${filter}" "${textContent.indexOf(filter)}"`);
    if (textContent.indexOf(filter.toUpperCase()) > -1) {
      menuItem.classList.remove('hidden');
    }else{
      menuItem.classList.add('hidden');      
    }        
  }
}

.hidden{
  display: none;
}
body {
  font-family: "Lato", sans-serif;
}

/* Fixed sidenav, full height */
.sidenav {
  height: 100%;
  width: 200px;
  position: fixed;
  z-index: 1;
  top: 0;
  left: 0;
  background-color: #111;
  overflow-x: hidden;
  padding-top: 20px;
}

/* Style the sidenav links and the dropdown button */
.sidenav a, .dropdown-btn {
  padding: 6px 8px 6px 16px;
  text-decoration: none;
  font-size: 20px;
  color: #818181;
  display: block;
  border: none;
  background: none;
  width: 100%;
  text-align: left;
  cursor: pointer;
  outline: none;
}

/* On mouse-over */
.sidenav a:hover, .dropdown-btn:hover {
  color: #f1f1f1;
}

/* Main content */
.main {
  margin-left: 200px; /* Same as the width of the sidenav */
  font-size: 20px; /* Increased text to enable scrolling */
  padding: 0px 10px;
}

/* Add an active class to the active dropdown button */
.active {
  background-color: green;
  color: white;
}

/* Dropdown container (hidden by default). Optional: add a lighter background color and some left padding to change the design of the dropdown content */
.dropdown-container {
  display: none;
  background-color: #262626;
  padding-left: 8px;
}

/* Optional: Style the caret down icon */
.fa-caret-down {
  float: right;
  padding-right: 8px;
}

/* Some media queries for responsiveness */
@media screen and (max-height: 450px) {
  .sidenav {padding-top: 15px;}
  .sidenav a {font-size: 18px;}
}

<div class="sidenav">
  
  <input type="text" id="mySearch" onkeyup="f(this.value)" placeholder="Search.." title="Type in a category">
  
  <ul id="myMenu">
    <li><a href="#about">About</a></li>
    <li><a href="#services">Services</a></li>
    <li><a href="#clients">Clients</a></li>
    <li><a href="#contact">Contact</a></li>
    <li>
      <button class="dropdown-btn">
        Dropdown
        <i class="fa fa-caret-down"></i>
      </button>
      <div class="dropdown-container">
        <ol>
          <li><a href="#">Link 1</a></li>
          <li><a href="#">Link 2</a></li>
          <li><a href="#">Link 3</a></li>
        </ol>
      </div>
    </li>    
    <li>
      <a href="#contact">Search</a>
    </li>
  </ul>
  
</div>