js loop to get the value in table

 <script type="text/javascript">
        function getTdValue() {
            var tableId = document.getElementById("tab");
            var str = "";
            //获取table中的某一列的值
            for (var i = 1; i < tableId.rows.length; i++) {
                 alert(tableId.rows[i].cells[3].innerHTML);
            
            }
            //获取table中所有input的值
             var objInput = document.getElementById("tab").getElementsByTagName("input");
             for (i = 0; i < objInput.length; i++) {
                 alert(objInput[i].value);
             }
        }
       
    </script>