Javascript validation gives me the errror

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Javascript

when ever im going to validate through unhide textbox it is ging to be validate but any 1 of the textbox is hide so the javascript code does not validate it throws error

What I have tried:

function validateAGDBAndSLDB() {
debugger;
var txtVal = (document.getElementById("<%=txtAGDB1.ClientID%>").style.display = 'none') + (document.getElementById("<%=txtAGDB2.ClientID%>").value);
(document.getElementById("<%=txtAGDB3.ClientID%>").value) + (document.getElementById("<%=txtAGDB4.ClientID%>").value);
(document.getElementById("<%=txtAGDB5.ClientID%>").value) + (document.getElementById("<%=txtAGDB6.ClientID%>").value);
(document.getElementById("<%=txtAGDB7.ClientID%>").value) + (document.getElementById("<%=txtAGDB8.ClientID%>").value);
(document.getElementById("<%=txtAGDB9.ClientID%>").value) + (document.getElementById("<%=txtAGDB10.ClientID%>").value);
(document.getElementById("<%=txtSLDB.ClientID%>").value) + (document.getElementById("<%=txtSLDB2.ClientID%>").value);
(document.getElementById("<%=txtSLDB3.ClientID%>").value) + (document.getElementById("<%=txtSLDB4.ClientID%>").value);
(document.getElementById("<%=txtSLDB5.ClientID%>").value) + (document.getElementById("<%=txtSLDB6.ClientID%>").value);
(document.getElementById("<%=txtSLDB7.ClientID%>").value) + (document.getElementById("<%=txtSLDB8.ClientID%>").value);
(document.getElementById("<%=txtSLDB9.ClientID%>").value) + (document.getElementById("<%=txtSLDB10.ClientID%>").value);
if (txtVal >= 1 && txtVal <= 240) {
return true;
}
else
alert('Value must be in between 1 to 240');
return false;
}