do-not-forget-to-return-promises.js

const sinon = require('sinon')
const chai = require('chai')
const expect = chai.expect

const foo = callback => {
  return Promise.resolve().then(() => {
    setTimeout(() => {
      callback()
    }, 5000)
  })
}

describe('foo', () => {
  describe('when promise is not returned', () => {
    const spy = sinon.spy()
    before(() => {
      foo(spy) // not returned
    })
    it('should not call the callback', () => {
      expect(spy.called).to.be.false
    })
  })
})