不知道干什么的Js代码

function carltbl_getBandById(tagId) 
{
    if(!tagId)
        return null;
    var parts = tagId.split("_");
    var bandIndex = parts.length - 2;
    var gridId = parts[0];
    var el=carltbl_getElementById(tagId);
    if((gridId.charAt(gridId.length-3)=="g" && 
            gridId.charAt(gridId.length-2)=="r" && 
            gridId.charAt(gridId.length-1)=="c" || gridId.charAt(gridId.length-3)=="s" && 
            gridId.charAt(gridId.length-2)=="g" && 
            gridId.charAt(gridId.length-1)=="r") && 
            el && el.getAttribute("groupRow"))
    {
        gridId=gridId.substr(0,gridId.length-3);
        bandIndex--;
    }
    else if(el && (gridId.charAt(gridId.length-2)=="r" && 
                    gridId.charAt(gridId.length-1)=="c" && 
                    el.tagName=="TD" || gridId.charAt(gridId.length-2)=="g" && 
                    gridId.charAt(gridId.length-1)=="r" && 
                    el.getAttribute("groupRow") || gridId.charAt(gridId.length-2)=="r" && 
                    gridId.charAt(gridId.length-1)=="h" && 
                    el.getAttribute("hiddenRow")))
    {
        gridId=gridId.substr(0,gridId.length-2);
        bandIndex--;
    }
    else if(gridId.charAt(gridId.length-1)=="r" && el && el.tagName=="TR")
        gridId=gridId.substr(0,gridId.length-1);
    else if(gridId.charAt(gridId.length-1)=="c" && el && el.tagName=="TH")
    {
        gridId=gridId.substr(0,gridId.length-1);
        bandIndex=parts[1];
    }
    else
        return null;
    if(!carltbl_getGridById(gridId))
        return null;
    var grid = carltbl_getGridById(gridId);
    return grid.Bands[bandIndex];
}