服务端跨域setcookie失败

前端域名 www.a.com 后端域名 list.a.com 后端setcookie domain .a.com 如果失败，前端ajax添加 $.ajaxSetup

$.ajaxSetup({
    xhrFields: {
        withCredentials: true
    },
    crossDomain: true
});

let baseUrl = "xxx.com"
$.ajax({
    type: "post",
    contentType: "application/json;charset=UTF-8",
    url: baseUrl + "/auth/",
    data: JSON.stringify(params),
    cache: false,
    crossDomain: true == !(document.all),
    success: function (res) {
        alert("ok")
    },
    fail: function (err) {
        alert("验证未通过")
    }
})