//循环遍历id
var str_id = "";
$("selector").each(function () {
    str_id += $(this).prop("id") + ",";
});

var str_id = "abc,def,ghi,";//假如str_id由上边遍历得到的数值
if (str_id.length > 0) {
    //1
    str_id = str_id.substr(0, str_id.length - 1);  
    //2
    str_id = str_id.slice(0, str_id.length - 1);
    //3
    str_id = str_id.substring(0, str_id.length - 1);  
    //4
    str_id = str_id.substring(0, str_id.lastIndexOf(','));  
}
//保存为数组
var arr = str_id.split(",");