方法1、如果只需要截取多少个字符的内容，则可以:
代码如下:


或
代码如下:

方法2、
代码如下:

function ismaxlength(obj){
var mlength=obj.getAttribute? parseInt(obj.getAttribute("maxlength")) : ""
if (obj.getAttribute && obj.value.length>mlength)
obj.value=obj.value.substring(0,mlength)
}



这个方法采用截断法，输入到最后一个字符的时候如果再输入则会显示光标闪烁。但可以解决使用CTRL+C复制过来的长度问题,但如果用鼠标复制过来的不还是不行。

方法3、这个方法直接判断输入的长度
代码如下:

function imposeMaxLength(Object, MaxLen)
{
return (Object.value.length }
-->



当输入内容大于15时因为返回为false所以这个实现不会显示光标闪烁的问题，但没有解决复制过来的长度问题即复制过来的内容可以超过最大长度
return (Object.value.length <=MaxLen);但我测试发现当输入字节数=maxlen时还可以输入一个字符，所以我改成 return (Object.value.length 方法4、其实方法4是方法2与方法3的基础上进一步优化。客观的说方法2与方法3都只做了一部分工作
代码如下:
function textlen(x,y){
var thelength = x.value.length;
window.status=thelength+' of '+y+' maximum characters.';
}
function maxtext(x,y){
tempstr = x.value
if(tempstr.length>y){
x.value = tempstr.substring(0,y);
}
textlen(x,y);
}
// -->

Javascript代码
---------------------------------------------------------------------------------------------
代码如下:
function SetTextAreaMaxLength(controlId,length)
{
// JScript File for TextArea
// Keep user from entering more than maxLength characters
function doKeypress(control,length){
maxLength = length;
value = control.value;
if(maxLength && value.length > maxLength-1){
event.returnValue = false;
maxLength = parseInt(maxLength);
}
}
// Cancel default behavior
function doBeforePaste(control,length){
maxLength = length;
if(maxLength)
{
event.returnValue = false;
}
}
// Cancel default behavior and create a new paste routine
function doPaste(control,length){
maxLength = length;
value = control.value;
if(maxLength){
event.returnValue = false;
maxLength = parseInt(maxLength);
var oTR = control.document.selection.createRange();
var iInsertLength = maxLength - value.length + oTR.text.length;
var sData = window.clipboardData.getData("Text").substr(0,iInsertLength);
oTR.text = sData;
}
}
function doDragenter(control,length)
{
maxLength = length;
value = control.value;
if(maxLength){
event.returnValue = false;
}
}
function addEvent(elm, evType, fn, useCapture)
{
if (elm.addEventListener)
{
elm.addEventListener(evType, fn, useCapture);
return true;
}
else if (elm.attachEvent)
{
var r = elm.attachEvent('on' + evType, fn);
return r;
}
else {
elm['on' + evType] = fn;
}
}
function AttacheventTextAreaBeforePaste(obj,length)
{
return function()
{
doBeforePaste(obj,length)
}
}
function AttacheventTextAreaPaste(obj,length)
{
return function()
{
doPaste(obj,length)
}
}
function AttacheventTextAreaKeyPress(obj,length)
{
return function()
{
doKeypress(obj,length)
}
}
function AttacheventTextAreaDragEnter(obj,length)
{
return function()
{
doDragenter(obj,length);
}
}
var obj = document.getElementById(controlId);
addEvent(obj,'keypress',AttacheventTextAreaKeyPress(obj,length),null);
addEvent(obj,'beforepaste',AttacheventTextAreaBeforePaste(obj,length),null);
addEvent(obj,'paste',AttacheventTextAreaPaste(obj,length),null);
addEvent(obj,'dragenter',AttacheventTextAreaDragEnter(obj,length),null);
}
function SetTextAreaMaxLength(controlId,length)
{
// JScript File for TextArea
// Keep user from entering more than maxLength characters
function doKeypress(control,length){
maxLength = length;
value = control.value;
if(maxLength && value.length > maxLength-1){
event.returnValue = false;
maxLength = parseInt(maxLength);
}
}
// Cancel default behavior
function doBeforePaste(control,length){
maxLength = length;
if(maxLength)
{
event.returnValue = false;
}
}
// Cancel default behavior and create a new paste routine
function doPaste(control,length){
maxLength = length;
value = control.value;
if(maxLength){
event.returnValue = false;
maxLength = parseInt(maxLength);
var oTR = control.document.selection.createRange();
var iInsertLength = maxLength - value.length + oTR.text.length;
var sData = window.clipboardData.getData("Text").substr(0,iInsertLength);
oTR.text = sData;
}
}
function doDragenter(control,length)
{
maxLength = length;
value = control.value;
if(maxLength){
event.returnValue = false;
}
}
function addEvent(elm, evType, fn, useCapture)
{
if (elm.addEventListener)
{
elm.addEventListener(evType, fn, useCapture);
return true;
}
else if (elm.attachEvent)
{
var r = elm.attachEvent('on' + evType, fn);
return r;
}
else {
elm['on' + evType] = fn;
}
}
function AttacheventTextAreaBeforePaste(obj,length)
{
return function()
{
doBeforePaste(obj,length)
}
}
function AttacheventTextAreaPaste(obj,length)
{
return function()
{
doPaste(obj,length)
}
}
function AttacheventTextAreaKeyPress(obj,length)
{
return function()
{
doKeypress(obj,length)
}
}
function AttacheventTextAreaDragEnter(obj,length)
{
return function()
{
doDragenter(obj,length);
}
}
var obj = document.getElementById(controlId);
addEvent(obj,'keypress',AttacheventTextAreaKeyPress(obj,length),null);
addEvent(obj,'beforepaste',AttacheventTextAreaBeforePaste(obj,length),null);
addEvent(obj,'paste',AttacheventTextAreaPaste(obj,length),null);
addEvent(obj,'dragenter',AttacheventTextAreaDragEnter(obj,length),null);
}

-----------------------------------------------------------------------------------------------
HTML代码
代码如下:
TextMode="MultiLine" Height="113px" MaxLength="10">

SetTextAreaMaxLength('<%=TextBoxAddress.ClientID %>',10);