js动态计算table某列值的总和

/**
 *
 * @tableId 表格id
 * @numId 显示张数id
 * @index 列数
 * @first 首次标识
 * @obj checkbox对象
 */
function countNum(tableId, numId, index, first,obj) {
    var sumStr = document.getElementById(numId);
    //当前行进行加减
    if (null != obj) {
        var num = new Number(obj.parentElement.parentElement.cells[index].innerText);
        if (obj.checked) {
            sumStr.value = new Number(sumStr.value) + num;
        }
        else {
            sumStr.value = new Number(sumStr.value) - num;
        }
    }
    //遍历所有进行加减
    else {
        var table = document.getElementById(tableId);
        var sum = 0;
        var rows = table.rows;
        for (i = 1; i < rows.length; i++) {
                                                //判断是否有复选框
            var checkObj = rows[i].cells[0].all[0];
            var flag = false;
            if (null != checkObj
                && typeof checkObj.type != 'undefined'
                &&  checkObj.type == 'checkbox') {
                flag = checkObj.checked;
            }
                                                //首次默认计算全部或增加选中的值
            if (first || flag) {
                var num = new Number(rows[i].cells[index].innerText);
                sum = sum + num;
            }
        }
        sumStr.value = sum;
    }
}