<!DOCTYPE html>
<html>
<head lang="en">
<meta charset="UTF-8">
<title>限制文件字数字</title>
</head>
<body>
<span class="span">备注信息</br><span id="stay" style="display: none"> 您还可以输入:<span id="txtCount"></span>个字符的描述信息</span></span>
</br><input id="Remark_information" name="Remark_information" type="text">
<script>
var lim=new limit();
lim.txtNote=document.getElementById("Remark_information");
lim.txtLimit=document.getElementById("txtCount");
lim.limitCount=80;
lim.init();
function limit(){
var txtNote;//文本框
var txtLimit;//提示字数的input
var limitCount;//限制的字数
var txtlength;//到达限制时，字符串的长度
this.init=function(){
txtNote=this.txtNote;
txtLimit=this.txtLimit;
limitCount=this.limitCount;
txtNote.oninput=function(){wordsLimit()};txtNote.oninput=function(){wordsLimit()};
txtLimit.innerText=limitCount;
};
function wordsLimit(){
var noteCount=txtNote.value.length;
var InPut=document.getElementById("Remark_information").value.length;
if(InPut<1){
document.getElementById("stay").style.display="none";
return
}
if(InPut>=1){
document.getElementById("stay").style.display="inline";
document.getElementById("stay").style.color="green";
}
if(InPut>70){
document.getElementById("stay").style.color="red";
}
if(noteCount>limitCount){
txtNote.value=txtNote.value.substring(0,limitCount);
txtLimit.innerText=0;
}else{
txtLimit.innerText=limitCount-noteCount;
}
txtlength=txtNote.value.length;//记录每次输入后的长度
}
}
</script>
</body>
来自：http://www.cnblogs.com/zhujiasheng/p/6113983.html