SOURCE

var lis = document.getElementsByTagName('li');  // 记录每个标签的遍历位置，不用this，这里的this指的是window
for(var i = 0;i < lis.length;i++) {
  lis[i].onclick = function () {
    for(var j = 0;j < lis.length;j++) {
      lis[j].className ='';
    }
    // 不能用遍历的i,要用this下的属性index
    this.className = 'active';
  }
}
var txt = document.getElementById('txt');
var lab = document.getElementById('lab');
txt.oninput = function () {
  if(this.value == '') {
    lab.style.display = 'block';
  }
  else {
    lab.style.display = 'none';
  }
}

<div class="wrapper">
  <ul>
    <li class="active"><a href="#">宝贝</a></li>
    <li><a href="#">天猫</a></li>
    <li><a href="#">店铺</a></li>
  </ul>
  <div class="search">
    <input type="text" id="txt">
    <label for = "txt" id="lab">请输入要购买的商品</label>
  </div>
</div>

*{
  padding: 0;
  margin: 0;
}
.wrapper {
  width: 800px;
  margin: 30px auto;
}
ul {
  list-style: none;
  overflow: hidden;  /* 清除浮动 */
}

ul li {
  float: left;
  width: 150px;
  height: 50px;
  line-height: 50px;
  text-align: center;
  background-color: #fff;
}
li a {
  color: red;
  text-decoration: none;
}
li.active {
  background-color: red;
}
li.active a {
  color: white;
}
.search {
  margin: 20px 0;
  position: relative;
}
#txt {
  display: block;
  outline: none;
  width: 447px;
  height: 50px;
  position: absolute;
  border: 2px solid orange;
  border-radius: 10px;
}
#lab {
  display: block;
  position: absolute;
  top: 15px;
  lest: 30px;
  font-size: 20px;
  color: rgba(0,0,0,0.5);
}