Recently, I was revisiting the JavaScript used to limit the number of characters in a TextBox control. This client-side script utilizeddocument.getElementById method and the control ID for HTML markup that was generated by ASP.NET. The problem with this script was that it did not work correctly with multiple TextBox controls on the web page and not cross-browser compatible. So I decided to rewrite it to ease the mentioned problems. Shown in Listing 1 is the new content of the JavaScript. There are two functions resided in it namely validateLimit and get_object. The former function accepts three parameters.
1. TextBox object
2. HTML Div id (to hold the text)
3. Maximum number of characters the TextBox control can hold
The purpose of the later function is to ensure that modern and older browsers are able to access the form elements. Listing 1 function validateLimit(obj, divID, maxchar) {
objDiv = get_object(divID);
if (this.id) obj = this;
var remaningChar = maxchar - obj.value.length;
if (objDiv) {
objDiv.innerHTML = remaningChar + " characters left";
}
if (remaningChar <= 0) {
obj.value = obj.value.substring(maxchar, 0);
if (objDiv) {
objDiv.innerHTML = "0 characters left";
}
return false;
}
else
{ return true; }
}
// Checking for getElementById support
function get_object(id) {
var object = null;
if (document.layers) {
object = document.layers[id];
} else if (document.all) {
object = document.all[id];
} else if (document.getElementById) {
object = document.getElementById(id);
}
return object;
}
Putting everything together. Listing 2 <html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title>Without master page</title>
<script type="text/javascript" src="js/JScript.js" ></script>
</head>
<body>
<form id="form1" runat="server">
<div> <br />
<div id="lblMsg1">240 characters left</div>
<asp:TextBox ID="TextBox1" runat="server" Height="50px" MaxLength="240" TextMode="MultiLine"
Width="600px" ToolTip="Summary:(240 characters)"
onkeyup="return validateLimit(this, 'lblMsg1', 240)"></asp:TextBox>
<asp:RequiredFieldValidator ID="RequiredFieldValidator1" runat="server"
ControlToValidate="TextBox1" Display="Dynamic"
SetFocusOnError="True">*</asp:RequiredFieldValidator>
<br /><br /><br />
<div id="lblMsg2">300 characters left</div>
<asp:TextBox ID="TextBox2" runat="server" Height="50px" MaxLength="300" TextMode="MultiLine"
Width="600px" ToolTip="Summary:(300 characters)"
onkeyup="return validateLimit(this, 'lblMsg2', 300)"></asp:TextBox>
<asp:RequiredFieldValidator ID="RequiredFieldValidator2" runat="server"
ControlToValidate="TextBox2" Display="Dynamic"
SetFocusOnError="True">*</asp:RequiredFieldValidator>
<br /> <br />
<asp:Button ID="Button1" runat="server" onclick="Button1_Click" Text="Button" />
<br />
</div>
</form>
</body>
</html>
Here is the output: Figure 1
 If you find any bugs or disagree with the contents, please drop me a line and I'll work with you to correct it. I would suggest downloading the demo and explore it in order to grasp the full concept of it because I might left out some useful information. 
Tested on IE 6.0/7.0/8.0, Google Chrome, Firefox, Safari document.getElementById On All Browsers – Cross browser getElementById Demo Download |