Javascript validation for .NET:

function validate()
{
msg="";
var Keyphrase=document.getElementById("<%=textbox1.ClientID %>").value.length;// Finding length
if (document.getElementById("<%=textbox1.ClientID%>").value=="")
{
msg="Enter the Keyphrase\n";
alert(msg);
document.getElementById("<%=textbox1.ClientID%>").focus();
return false;
}
if (document.getElementById("<%=textbox1.ClientID%>").value==""||Keyphrase>4000)//Comparing Length
{
msg="Keyphrase Maximum charecter is 4000\n";
alert(msg);
document.getElementById("<%=textbox1.ClientID%>").focus();
return false;
}
return true;
}

In page Load Event:
Button_name.Attributes.Add("onclick", "return validate()")