express如何解决ajax跨域访问session失效问题详解

 $('#login').click(function () {
  $.ajax({
   url: 'http://localhost:3000/users/yzm',//服务端路由地址
   type: 'get',
   xhrFields:{withCredentials:true},
   dataType: 'json',
   success:function(data){
    $('#yzm_img').html(data)
   },
   error:function(){
    alert('error');
   }
  });
 });
 $('#form_login').submit(function (e) {/!*登录*!/
  e.preventDefault();/!*阻止表单默认事件，页面全局刷新*!/
  var data=$('#form_login').serialize();/!*将表单里的数据包装起来*!/
  $.ajax({
   url : 'http://localhost:3000/users/login',
   type : "post",
   data : data,
   xhrFields:{withCredentials:true},
   dataType:'json',
   success:function(msg) {
    alert("这是返回的数据"+msg);
   },
   error:function(err){
    alert("这是失败的信息"+err);
   }
  });
 });

var express = require('express');
var session = require('express-session');/*引入会话变量*/

var app = express();
app.all('*', function(req, res, next) {
 res.header("Access-Control-Allow-Origin", "http://localhost:63342");//前端域名
 res.header("Access-Control-Allow-Credentials",'true');
 res.header("Access-Control-Allow-Methods","PUT,POST,GET,DELETE,OPTIONS");
 next();
});

router.get('/yzm', function(req, res, next) {
 req.session.yzm='abcd';
}
router.post('/login', function(req, res, next) {
 console.log(req.session.yzm);
}