How to check if ASP:TextBox control has value

Hi,
i want check some TextBox control if they have value do some addition else do assignment, i used the jquery script below it didn't work for me

if ($('#<%=TextBox797.ClientID%>').val() != "" && $('#<%=TextBox797.ClientID%>').val() >= 0) {
                  document.getElementById('<%=TextBox797.ClientID%>').value = parseInt(document.getElementById('<%=TextBox797.ClientID%>').value) + parseInt(document.getElementById('<%=TextBox651.ClientID%>').value);
              } else {
                  document.getElementById('<%=TextBox797.ClientID%>').value = parseInt(document.getElementById('<%=TextBox651.ClientID%>').value);
              }

Please kindly show me me what am doing wrong
thank you

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