Untitled

<==================separate .js file=================>
function TextLimit() {
    var maxLength = 250;
    var text = document.getElementById("Textbox").value;
    var length = text.length;

    document.getElementById("lblid").innerText = (((maxLength - length) < 0) ? 0 : (maxLength - length)) + " characters left";

    if (length > maxLength) {
        text = text.substr(0, 250);
        alert("Limit is 250 chars");
    }
}

<===========.aspx file below===============================>
<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="TextLimit.aspx.cs" Inherits="limittext250.TextLimit" %>

<!DOCTYPE html>

<html>
<head runat="server">
    <title>Limit 250</title>
    <script src="Scripts/JavaScript.js" type="text/javascript"></script>
   <script>
       window.onload=function() {
            document.getElementById("Textbox").oninput = function () {
                TextLimit();
            }
        };
    </script>

</head>
<body>
    <form id="form1" runat="server" >
        <div>
            Max characters allowed is 250:
            <asp:TextBox ID="Textbox" runat="server" TextMode="MultiLine" Columns="50" Rows="6"  ></asp:TextBox>
            <label id="lblid" runat="server" ></label>

            <%--Following is the commented javascript code that works fine when using in this same file--%>
            <%--<script  type="text/javascript" >
                var maxLength = 250;

                document.getElementById("Textbox").oninput = function () {
                    var length = document.getElementById("<%=Textbox.ClientID%>").value.length;

                    document.getElementById("lblid").innerText = (((maxLength - length)<0)?0:(maxLength-length)) + " characters left";

                    if (length > maxLength) {
                        document.getElementById("<%=Textbox.ClientID%>").value = document.getElementById("<%=Textbox.ClientID%>").value.substr(0,250);
                        alert("Limit is 250 chars");
                    }
                }
            </script>--%>


        </div>
    </form>
</body>
</html>