前台代码片段

onLoad: function () {
    wx.login({
      success(res) {
        console.log('code: '+res.code)
        if (res.code) {
          wx.request({
            url: 'http://localhost:1996/pay',
            data:{
              code: res.code
            },
            success(res) {
              console.log(res.data)
            }
          });
        } else {
          console.log('登录失败！' + res.errMsg)
        }
      }
    });
  },

后台代码片段

const Router = require('koa-router');
const router = new Router();
const request = require('request');

function getOpenId(code) {
  return new Promise((resolve, reject) => {
    const id = 'wxbd53208a40c5d51c'; // AppID(小程序ID)
    const secret = '92fea41a425cc8ff9d9596c434ec67fb';// AppSecret(小程序密钥)
    let url = `https://api.weixin.qq.com/sns/jscode2session?appid=${id}&secret=${secret}&js_code=${code}&grant_type=authorization_code`;
    request(url, (error, response, body) => {
      if (error) reject(error);
      resolve(body);
    });
  })
}

let index = router.get('/pay', async (ctx, next) => {
  await next();
  ctx.set("Access-Control-Allow-Origin", '*');
  ctx.status = 200;
  ctx.type = 'json';
  if (ctx.query) {
    const {
      code
    } = ctx.query;
    const data = await getOpenId(code);
    ctx.body = data;
  }
}).routes();

module.exports = index;