a function to get wx access_token which will nerver expires

forever_wxtoken.js

const router = require('express').Router(),
      Promise = require("bluebird"),
      readFileAsync  = Promise.promisify(require("fs").readFile),
      writeFileAsync  = Promise.promisify(require("fs").writeFile),
      request_get = Promise.promisify(require('request').get);

router.route('/accesstoken')
    .get((req, res) => {
        readFileAsync("accesstoken.txt","utf8")
            .then(data =>{
              const token = JSON.parse(data);
              const token_url = global.token_url;//wechat token url
              const iao = token.iao;
              const now_time = (new Date()).getTime();
              if (now_time >= iao) {
                return request_get(token_url)
              }else{
                res.send(data);
              }
            })
            .then(result =>{
              if(result){
                let body = result[0].body;
                res.send(body);
                body = JSON.parse(body);
                let token_iao = new Date();
                body.iao =  token_iao.setSeconds(token_iao.getSeconds() + body.expires_in, 0);
                body.data = body.access_token;
                return writeFileAsync('accesstoken.txt', JSON.stringify(body), 'utf-8');
              }else{

              }
            })
            .catch(err => {
               console.error(err);
            });
    })

module.exports = function(app) {
    app.use('/', router)
};