NaN error in java-script calculation in asp.net in Mozilla

                CssClass="textbox" Width="120px" >

                                   

                                   

javascript code function balanceamountbbooking() { var str = document.getElementById('<%=txtbookingamtbbooking.ClientID %>').value; var strtotaldues = document.getElementById('ContentPlaceHolder1_TabContainer1_tabbooking_lbltotalduesamountbbooking'); if (isNaN(str) || str.indexOf(" ") != -1) { alert("Enter numeric value"); document.getElementById('<%=txtbalanceamtbbooking.ClientID %>').value = document.getElementById('<%=lbltotalduesamountbbooking.ClientID %>').innerText; document.getElementById('<%=hdn_balanceamount.ClientID %>').value = document.getElementById('<%=lbltotalduesamountbbooking.ClientID %>').innerText; //hdn_balanceamount document.getElementById('<%=txtbookingamtbbooking.ClientID %>').value = ""; document.getElementById('<%=txtbookingamtbbooking.ClientID %>').focus(); return false; } if (document.getElementById('<%=txtbookingamtbbooking.ClientID %>').value == "")//txtbalanceamtb { // document.getElementById('<%=lbltotalduesamountbbooking.ClientID %>').INNERHTML = "0"; document.getElementById('<%=txtbalanceamtbbooking.ClientID %>').value = document.getElementById('<%=lbltotalduesamountbbooking.ClientID %>').innerText; document.getElementById('<%=hdn_balanceamountbooking.ClientID %>').value = document.getElementById('<%=lbltotalduesamountbbooking.ClientID %>').innerText; } else { document.getElementById('<%=txtbalanceamtbbooking.ClientID %>').value.toString() = parseFloat(strtotaldues.innerText) - parseFloat(document.getElementById('<%=txtbookingamtbbooking.ClientID %>').value); document.getElementById('<%=hdn_balanceamountbooking.ClientID %>').value = parseFloat(strtotaldues.innerText) - parseFloat(document.getElementById('<%=txtbookingamtbbooking.ClientID %>').value); } }