const app = require('koa')();

console.log(typeof app);    // object

主要错误就是出现在项目实例是对象，在express下，就不错出现这种问题

const app = require('express')();

const.log(typeof app);        // function 

完整代码：

// app.js
const app = require('koa')();
app.listen(3000);
module.exports = app;

// article.test.js
const expect = require('chai').expect;
const request = require('supertest');
const app = require('../app');

describe('api: /article', () => {
    describe('GET /article', (done) => {
        it('response is success', () => {
            request(app)
                .get('http://localhost/article')
                .expect(200)
                .end(() => {
                    done();
                });
            // console.log(typeof app);
        })
    });
});