Validate textbox and dropdownlist in asp.net using java script

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ASP.NET

Java

script

    var Pname = document.getElementById('<%=txtdate.ClientID %>').value;
            if (Pname == " " ) {
                alert("Please Enter Date.");
                return false;
            }
  var vechileno = document.getElementById('<%=txtVehicleNo.ClientID %>').value;
            if (vechileno == " " ) {
                alert("Please Enter  Vehicle Number.");
                return false;
            }

           var ddlItemtype = document.getElementById('<%=ddlItemtype.ClientID%>');
            var ddlitem = ddlItemtype.options[ddlItemtype.selectedIndex].value;
            if (ddlitem == 0) {
                alert("Please Select Item.");
                return false;
            }
            var ddltrader = document.getElementById('<%=ddltradername.ClientID%>');
            var ddlproductype1 = ddltrader.options[ddltrader.selectedIndex].value;
            if (ddlproductype1 == 0) {
                alert(" Please Select Trader Name.");
                return false;
            }
}

On button click I am calling this function in javascript
I have manytextboxes and dropdown list like this . I want to use for each loop and display the error.

I am writing the code all the time like this. And the length of coding increasing. Is there any optimize way to reduce the code.