<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
<style>
.cont{width:300px;height:400px;border:solid 1px black;margin:40px auto;}
.msg{height: 300px;border-bottom:solid 1px black;box-sizing: border-box;overflow: auto;}
.msg p{margin: 0;max-width: 220px;border-radius: 6px;margin: 6px;padding: 6px;clear: both;color: #fff;}
.msg p:nth-child(2n){float: left;background: #277;}
.msg p:nth-child(2n-1){float: right;background: #772;}
.form{height: 100px;}
.form *{border: none;background: none;padding: 0;margin: 0;float: left;height: 100px;}
#txt{width:240px;resize: none;outline: none;padding: 10px;}
#send{width:40px;border-left: solid 1px black;outline: none;}
</style>
</head>
<body>
<div class="cont">
<div class="msg"></div>
<div class="form">
<textarea id="txt"></textarea>
<input type="button" value="发送" id="send">
</div>
</div>
<script>
var osend = document.getElementById("send")
var otxt = document.getElementById("txt")
var omsg = document.querySelector(".msg")
osend.onclick = function(){
// 如果输入框为空，结束
if(otxt.value === "") return;
// 创建元素
var p = document.createElement("p");
// 设置内容
p.innerHTML = otxt.value;
// 插入元素
omsg.appendChild(p);
// 清空输入框
otxt.value = "";
// 每次发送之后，设置滚动条的滚动距离为最大高度，保证绝对显示最下方信息
omsg.scrollTop = omsg.scrollHeight;
}
原文：https://www.cnblogs.com/dy0302/p/13321683.html