This simple program will guide how to do client side validation of Form in JavaScript.

In this just make a form as follows:

1. Name : <asp:TextBox ID="txtName" />
2. Email : <asp:TextBox ID="txtEmail" />
3. Web URL : <asp:TextBox ID="txtWebUrl" />
4. Zip : <asp:TextBox ID="txtZip" />

5. <asp:Button ID="btnSubmit" OnClientClick=" return validate()" runat="server" Text="Submit" />

Now on the source code of this form in script tag write the following code:

<script language="javascript" type="text/javascript">
function validate()
{
      if (document.getElementById("<%=txtName.ClientID%>").value=="")
      {
                 alert("Name Feild can not be blank");
                 document.getElementById("<%=txtName.ClientID%>").focus();
                 return false;
      }
      if(document.getElementById("<%=txtEmail.ClientID %>").value=="")
      {
                 alert("Email id can not be blank");
                document.getElementById("<%=txtEmail.ClientID %>").focus();
                return false;
      }
     var emailPat = /^(\".*\"|[A-Za-z]\w*)@(\[\d{1,3}(\.\d{1,3}){3}]|[A-Za-z]\w*(\.[A-Za-z]\w*)+)$/;
     var emailid=document.getElementById("<%=txtEmail.ClientID %>").value;
     var matchArray = emailid.match(emailPat);
     if (matchArray == null)
    {
               alert("Your email address seems incorrect. Please try again.");
               document.getElementById("<%=txtEmail.ClientID %>").focus();
               return false;
    }
    if(document.getElementById("<%=txtWebURL.ClientID %>").value=="")
    {
               alert("Web URL can not be blank");
               document.getElementById("<%=txtWebURL.ClientID %>").value="http://"
               document.getElementById("<%=txtWebURL.ClientID %>").focus();
               return false;
    }
    var Url="^[A-Za-z]+://[A-Za-z0-9-_]+\\.[A-Za-z0-9-_%&\?\/.=]+$"
    var tempURL=document.getElementById("<%=txtWebURL.ClientID%>").value;
    var matchURL=tempURL.match(Url);
     if(matchURL==null)
     {
               alert("Web URL does not look valid");
               document.getElementById("<%=txtWebURL.ClientID %>").focus();
               return false;
     }
     if (document.getElementById("<%=txtZIP.ClientID%>").value=="")
     {
               alert("Zip Code is not valid");
               document.getElementById("<%=txtZIP.ClientID%>").focus();
               return false;
     }
     var digits="0123456789";
     var temp;
     for (var i=0;i<document.getElementById("<%=txtZIP.ClientID %>").value.length;i++)
     {
               temp=document.getElementById("<%=txtZIP.ClientID%>").value.substring(i,i+1);
               if (digits.indexOf(temp)==-1)
               {
                        alert("Please enter correct zip code");
                        document.getElementById("<%=txtZIP.ClientID%>").focus();
                        return false;
               }
     }
     return true;
}
</script>

And in code behind file just write the below code.

Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
               btnSubmit.Attributes.Add("onclick", "return validate()")
End Sub

Now you will get a form with proper validation.

I hope this is going to help you.