maxlength这个属性可以限制长度 输入如果有负的不合理
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>输入框</title>
<!-- <link rel="stylesheet" type="text/css" href="font-awesome-4.7.0/css/font-awesome.min.css"> -->
<style type="text/css">
.box{width: 600px;height:160px;border:10px solid gray;margin:0px auto;padding: 10px;}
img{float: left;}
.box1{float: left;margin-left: 235px;width: 150px;height: 24px;text-align: right;font-size: 14px;color: #888;}
.box1 span{font-size: 16px;font-weight: bold;}
#text{width: 600px;height: 100px;border: 1px solid #888;margin-top: 5px;}
.box #sp1,#sp2,#sp3,#sp4,#sp5,#sp6{/*float: left;*/width: 20px;height: 30px;line-height: 25px;padding: 20px}
#sp1{background:url(./22.png) no-repeat left center;}
#sp2{background:url(./33.png) no-repeat left center;}
#sp3{background:url(./44.png) no-repeat left center;}
#sp4{background:url(./55.png) no-repeat left center;}
#sp5{background:url(./66.png) no-repeat left center;width: 40px;}
#sp6{margin-left: 10px;margin-right: 15px;color: #888}
#bt{float: right;width: 80px;height: 25px;border: none;background: #ffc09f;color:#fff;border-radius: 5px;}
</style>
<script type="text/javascript">
var text,number,m
window.onload=function () {
text=document.getElementById('text')
number=document.getElementById('number')
bt=document.getElementById('bt')
text.onkeyup=function aa(){
m=140-text.value.length//
if (m<0) {
number.style.color="red"
}else{
number.style.color="#888"
}
number.innerHTML=m
}
bt.onclick=function(){
if (m==140) {
alert("您还没有输入")
text.focus()
}else if(m<0){
alert("字数天多，不可以发布")
text.focus()
}else{
alert("发布成功")
}
}
aa()
}
</script>
</head>
<body>
<div class="box">
<img src="./11.png">
<div class="box1">还可以输入 <span id="number"></span>字</div>
<textarea id="text" maxlength="140"></textarea>
<span id="sp1">表情</span>
<span id="sp2">图片</span>
<span id="sp3">视频</span>
<span id="sp4">话题</span>
<span id="sp5">长微博</span>
<span id="sp6">公开</span>
<input type="button" value="发布" id="bt">
</div>
</body>
</html>