JavaScript实现简易微博留言功能

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>微博留言</title>
</head>
<script>
window.onload=function(){
  var text=document.getElementById('text');
  var btn =document.getElementById('btn');
  var oul =document.getElementsByClassName('content')[0];
  btn.onclick=function(){
    var oli= document.createElement('li')
    oli.innerHTML=text.value;
    var arr= document.getElementsByTagName('li');
    if(arr.length>0){
      oul.insertBefore(oli,arr[0])
    }else{
      oul.appendChild(oli)
    }
  }
}
</script>
<body>
  <input type="text" placeholder="请输入您的留言" id="text" />
  <input type="button" value="留言" id="btn">
  <ul class="content">
  </ul>
</body>
</html>