1.将vue项目打包放到node中发布出来，session获取没有问题。
fetch写法：

let requestConfig = {

  credentials: 'include',
  method: type,
  headers: {
    'Accept': 'application/json',
    'Content-Type': 'application/json',
    "Access-Control-Allow-Credentials": "true"
   },
        mode: "cors",
        cache: "no-cache"
    }

2.前后台分离开发时，session获取不到，sessionid一直在变

let requestConfig = {

  //credentials: 'include',
  method: type,
  headers: {
    'Accept': 'application/json',
    'Content-Type': 'application/json',
    "Access-Control-Allow-Credentials": "true"
   },
        mode: "cors",
        cache: "no-cache"
    }

credentials: 'include',不注释的话会报错：
Response to preflight request doesn't pass access control check: The value of the 'Access-Control-Allow-Origin' header in the response must not be the wildcard '*' when the request's credentials mode is 'include'. Origin 'http://localhost:8001' is therefore not allowed access.

后台跨域用的
//解决前台请求跨域
app.use(cors());