js作业微博发言与轮播（有瑕疵）

微博

<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>Document</title>
  <!-- <link rel="stylesheet" href="css/weibo.css"> -->
  <link rel="stylesheet" href="./css/weibo.css">

<body>
  <div class="w">
    <div class="controls">
      <textarea placeholder="说点什么吧..." id="area" cols="30" rows="10" maxlength="200"></textarea>
      <div>
        <span class="Count">0</span>
        <span>/200<</span>
        <button>发布</button>
      </div>
    </div>
    <div class="contentList">
      <ul>

      </ul>
    </div>
  </div>
  <script >
    const txt=document.querySelector("textarea")
    const o=document.querySelector(".Count")
    const b=document.querySelector("button")
    const ul=document.querySelector("ul")
    const del=document.querySelector(".del")
        txt.oninput=()=>{
      o.innerHTML=txt.value.length
    }
    b.addEventListener("click",function(){
      if(txt.value.trim().length >0){
        let lli=document.createElement("li")
        lli.innerHTML=`${txt.value.trim()} <button class="del">删除</button>`
        const del=lli.querySelector(".del")
        del.onclick = function() {
          ul.removeChild(lli)
        }
        ul.insertBefore(lli,ul.children[0])
        txt.value=null
        o.innerHTML="0"
      }else{
        alter("请输入内容！！")
    }
    }) 
  </script>
</body>
</html>

css

* {
  margin: 0;
  padding: 0;
}

ul {
  list-style: none;
}

.w {
  width: 900px;
  margin: 0 auto;
}

.controls textarea {
  width: 878px;
  height: 100px;
  resize: none;
  border-radius: 10px;
  outline: none;
  padding-left: 20px;
  padding-top: 10px;
  font-size: 18px;
}

.controls {
  overflow: hidden;
}

.controls div {
  float: right;
}

.controls div span {
  color: #666;
}

.controls div .useCount {
  color: red;
}

.controls div button {
  width: 100px;
  outline: none;
  border: none;
  background: rgb(0, 132, 255);
  height: 30px;
  cursor: pointer;
  color: #fff;
  font: bold 14px '宋体';
  transition: all 0.5s;
}

.controls div button:hover {
  background: rgb(0, 225, 255);
}

.controls div button:disabled {
  background: rgba(0, 225, 255, 0.5);
}

.contentList {
  margin-top: 50px;
}

.contentList ul {
  height: 800px;
}

.contentList li {
  /* display: none; */
  padding: 20px 0;
  border-bottom: 1px dashed #ccc;
  height: 60px;
}

.contentList li a {
  margin-left: 800px;
}

轮播

<!DOCTYPE html>
<html>
	<head>
		<meta charset="utf-8">
		<title></title>
		<style>
			body{
				margin: 0;
				padding: 0;
			}
			.play{
				margin: 0 auto;
				width:400px;
				height:225px;
				background-image: url(./img/人1.jpg);
				background-size:cover; 
				position: relative;
			}
			.l,.r{
				height: 50px;
				width: 25px;
				background-color: aqua;
				position: absolute;
		        top:35%;
			}
			.l{
				left: 0%;
			}
			.r{
				right: 0%;
			}
		</style>
	</head>
	<body>
		<div class="play">
			<div class="l"></div>
			<div class="r"></div>
		</div>
		<script>
		 
		  let i = 1;
		  let left=document.querySelector(".l")
		  let right=document.querySelector(".r")
			function play(){
				i++ ;
				if(i > 5){
					i = 1;
				}
				if(i < 0){
					i = 5;
				}
				let img = document.getElementsByClassName("play")[0];
				img.style.background = "url(./img/人"+i+".jpg)";
				img.style.backgroundSize="cover";	
				
			}
			left.addEventListener("click",function(){
				i--
				img.style.background = "url(./img/人"+i+".jpg)";
				img.style.backgroundSize="cover";	
				})
			right.addEventListener("click",function(){
					i++
					img.style.background = "url(./img/人"+i+".jpg)";
				    img.style.backgroundSize="cover";
				})
			setInterval(play,3000);
		</script>
	</body>
</html>

我的转换键时而有用，时而没用，不知道为什么