//app.js
App({
 onLaunch() {
  wx.login({ // 登录
   // 发送 res.code 到后台换取 openId, sessionKey, unionId
   success: res => {
    wx.request({
     url: 'api/login',
     method: 'POST',
     data: {
      code: res.code
     },
     success(res) {
      //必须先清除，否则res.header['Set-Cookie']会报错
      wx.removeStorageSync('sessionid') ;
      //储存res.header['Set-Cookie']
      wx.setStorageSync("sessionid", res.header["Set-Cookie"]) ;
     }
    });
   }
  });
 }
})

//index.js
Page({
 onLoad(options) {
  wx.request({
   url: api + '/list',
   method: 'GET',
   header: { //将sessionid放在cookie中以请求头的方式带回给服务端
    'cookie': wx.getStorageSync("sessionid")
   },
   success(res) {
    console.log(res);
   }
  })
 }
})