How I can use mongoose and Koa.js

Wrap your database initialization into a Promise:

const Cat = mongoose.model('Cat', { name: String });function getCats() {  return new Promise((resolve, reject) => {     const ourCat = Cat.find({ name: 'Zildjian' });     ourCat.exec((er, cats) => {       if (er) {  reject(er);   }       else { resolve(cats); }     });          });}

So then you can just do:

const connection = connectDB(mongoUri);app.use(async ctx => {   await connection;   ctx.body = await getCats();});

Have a look at this repo:

https://github.com/jsnomad/koa-restful-boilerplate

It is quite updated and you will get your mind around the koa-mongoose stack... I think it will answer most of your questions; otherwise ask in the comments and will be able to help you :)

For Example How to Create Koa.js+Mongodb

const Koa = require('koa')const mongoose = require('koa-mongoose')const User = require('./models/user')const app = new Koa()app.use(mongoose({    user: '',    pass: '',    host: '127.0.0.1',    port: 27017,    database: 'test',    mongodbOptions:{        poolSize: 5,        native_parser: true    }}))app.use(async (ctx, next) => {    let user = new User({        account: 'test',        password: 'test'    })    await user.save()    ctx.body = 'OK'})