Javascript check on button click

I have a button on a form that is activated when I want to save the form. I have an event that is like this:
private void btnaddheader_Click(object sender, System.EventArgs e)
{
  //Code that is activated when button is clicked                  
}

How do I include a javascript for client side validation in this code?

I had a client side script below for traditional onclick for html button element:

 function addpart(formname)
    {
        var msg = "Error! Need the following entries:";
        var flag=true;
            if (document.nido_new.txtorderfulfilmentprefix.value == "" || document.nido_new.txtorderfulfilmentid.value == "" || document.nido_new.txtcalltimehh.value == "" || document.nido_new.txtcalltimemm.value == "")
        {
            if (document.nido_new.txtorderfulfilmentprefix.value == "")
                msg = msg + String.fromCharCode(13) + "* order fulfilment prefix";

            if (document.nido_new.txtorderfulfilmentid.value == "")
                msg = msg + String.fromCharCode(13) + "* order fulfilement number";

            if (document.nido_new.txtcalltimehh.value == "" || document.nido_new.txtcalltimemm.value == "")
                msg = msg + String.fromCharCode(13) + "* call time";
           
            flag=false;
        }
        var myform = document.nido_new
        ctrlDay = "selcalldatedd"
        ctrlMonth = "selcalldatemm"
        ctrlYear = "selcalldateyy"
        if(!(Checkdateb4submit(myform[ctrlDay],myform[ctrlMonth],myform[ctrlYear])))
            {  
                msg = msg + String.fromCharCode(13) + "* call date wrong";
                //alert("Please key in correct call date");
                document.nido_new.selcalldatedd.focus();
                flag=false;
            }
        ctrlDay1 = "selapptdatedd"
        ctrlMonth1 = "selapptdatemm"
        ctrlYear1 = "selapptdateyy"
        if(!(Checkdateb4submit(myform[ctrlDay1],myform[ctrlMonth1],myform[ctrlYear1])))
            {
                msg = msg + String.fromCharCode(13) + "* appointment date wrong";
                //alert("Please key in correct appointment date");
                document.nido_new.selapptdatedd .focus();
                flag=false;
            }
            ctrlhh = "txtcalltimehh"
            if (!(Checkhh(myform[ctrlhh])))
            {
                msg = msg + String.fromCharCode(13) + "* call time hh between 0 and 23";
                //alert("Please enter an integer between 0 and 23");
                document.nido_new.txtcalltimehh.focus();
                flag=false;
            }
            ctrlmm = "txtcalltimemm"
            if (!(Checkmm(myform[ctrlmm])))
            {
                msg = msg + String.fromCharCode(13) + "* call time mm between 0 and 59";
                //alert("Please enter an integer between 0 and 59");
                document.nido_new.txtcalltimemm.focus();
                flag=false;
            }
            ctrlhh1="txtappttimehh"
            if (!(Checkhh(myform[ctrlhh1])))
            {
                msg = msg + String.fromCharCode(13) + "* appointment time hh between 0 and 23";
                //alert("Please enter an integer between 0 and 23");
                document.nido_new.txtappttimehh.focus();
                flag=false;
            }
            ctrlmm1="txtappttimemm"
            if (!(Checkmm(myform[ctrlmm1])))
            {
                msg = msg + String.fromCharCode(13) + "* appointment time mm between 0 and 59";
                //alert("Please enter an integer between 0 and 23");
                document.nido_new.txtappttimemm.focus();
                flag=false;
            }
            if (flag==false)
            {
                alert(msg);
            }
            else
            {
                    document.nido_new.action = 'nido_new_ins.asp';
                    document.nido_new.submit();
            }
    }