supertest.example.js

const supertest = require('supertest')
const express = require('express')
const router = require('lib/routes/some-router')

describe('#some-router tests', () => {
  // temp express app. allows to tests routes in isolation from broader application
  const app = express()
  
  // app.use(router()) or whatever way works for your structure
  app.use('/stuff', router)
  
  it('should return a 400 error', (done) => {
    const request = supertest(app)
    const itemId = 10
    
    request.get(`/stuff/items/${itemId}`)
      .expect('Content-Type', /json/)
      .expect(400)
      .end(function(err, res) {
        if (err) {
          done(err)
        }
        
        // more assertions?
      
        done()
      });
  })
})