javascript+jquery-ajax:有条件的"数据“;要素

我想围绕ajax调用创建一个函数，这样我就可以重用它。但我很难根据条件（通过switch）填充它的data: ({ … })部分。

var d = {};
function lookup (what,etat,compt,vile,zip) {
    switch(what) {
        case "cou_mun":
            d: ({
                lookup: "county",
                state: etat
            })
            break;
        case "city":
            d: ({
                lookup: "city",
                state: etat,
                county: compt
            })
            break;
        case "zipcode":
            d: ({
                lookup: "zipcode",
                state: etat,
                county: compt,
                city: vile
            })
            break;
        default:break;
    }
    $.ajax({
        url: "socioGeo_envnLookup.php",
        type: "POST",
        data: d,
            //lookup: "county", //"county" or "city" or "zipcode"
            //state: etat
            //county: cou_mun,
            //city: city,
        success: function(what,data) {
            $('select[name="'+what+'"]').html(data);
        }
    }); //$.ajax
} //lookup()

然后我会把它称为lookup("city",state,county);

当我尝试进行发送未定义变量的ajax调用时，IE会抱怨，所以我想动态设置data: ({ … })。