get the selected gridview row value using javascript

function Read_Data ()
    {
        var str='';
        var Grid_Table = document.getElementById('<%= GridView1.ClientID %>');
        for(var row=1; row<Grid_Table.rows.length; row++)
        {
            for(var col=0; col<Grid_Table.rows[row].cells.length; col++)
            {
                if(col==0)
                    if(document.all)
                        str=str+Grid_Table.rows[row].cells[col].innerText;
                    else
                        str=str+Grid_Table.rows[row].cells[col].textContent;
                else
                    if(document.all)
                        str=str+'--'+Grid_Table.rows[row].cells[col].innerText;
                    else
                        str=str+'--'+Grid_Table.rows[row].cells[col].textContent;
            }
            str=str+'\n';   
        }
        alert(str);
        return false;
    }