//动态合并单元格
function uniteTable(tableId,colLength) {//表格ID，表格列数
    var tb=document.getElementById(tableId);
    tb.style.display='';
    var i = 0;
    var j = 0;
    var rowCount = tb.rows.length; // 行数
    var colCount = tb.rows[0].cells.length; // 列数
    var obj1 = null;
    var obj2 = null;
    //为每个单元格命名
    for (i = 0; i < rowCount; i++) {
        for (j = 0; j < colCount; j++) {
            tb.rows[i].cells[j].id = "tb__" + i.toString() + "_" + j.toString();
        }
    }
    //合并行
    for (i = 0; i < colCount; i++) {
        if (i == colLength) break;
        obj1 = document.getElementById("tb__0_" + i.toString())
        for (j = 1; j < rowCount; j++) {
            obj2 = document.getElementById("tb__" + j.toString() + "_" + i.toString());
            if (obj1.innerText == obj2.innerText && ((obj2.innerText != "" || obj1.innerText != "")&&(obj1.innerText != "-"|| obj2.innerText != "-"))) {
                obj1.rowSpan++;
                obj2.parentNode.removeChild(obj2);
            } else {
                obj1 = document.getElementById("tb__" + j.toString() + "_" + i.toString());
            }
        }
    }
    //合并列
    for (i = 0; i < rowCount; i++) {
        colCount = tb.rows[i].cells.length;
        obj1 = document.getElementById(tb.rows[i].cells[0].id);
        for (j = 1; j < colCount; j++) {
            if (j >= colLength) break;
            if (obj1.colSpan >= colLength) break;
            obj2 = document.getElementById(tb.rows[i].cells[j].id);
            if (obj1.innerText == obj2.innerText && ((obj2.innerText != "" || obj1.innerText != "")&&(obj1.innerText != "-"|| obj2.innerText != "-"))) {
                obj1.colSpan++;
                obj2.parentNode.removeChild(obj2);
                j = j - 1;
            }
            else {
                obj1 = obj2;
                j = j + obj1.rowSpan;
            }
        }
    }
}

原文链接：https://blog.csdn.net/ag_wenbi/article/details/83828025