导出节点js中的db对象

let db;
app.use(bodyParser.json());
app.use('/api/todos', todosRoutes); // routes in separate file

MongoClient.connect(
  uri, 
  {useNewUrlParser: true}, 
  (err, client) => {
    if(err) {
      return console.log(err)
    }
    db = client.db('todosApp');
    module.exports = db; // how can I export this db so my routes can use it?
    app.listen(PORT, () => {
      console.log(`app is listening on port ${PORT}`);
    })
  }
)

在我的路线上

const db = require('../app').db;
const collectionName = 'todos';

router.get('/', (req, res) => {
  db.collection(collectionName)
    .find()
    .toArray()
    .then(collection => res.json(collection))
    .catch(err => res.status(500).json(err))
})

我总是得到一个错误,db是未定义的,我可以理解,但不知道如何处理它?