https://stackoverflow.com/questions/38190712/how-to-unit-test-a-node-api-using-sinon-express-with-mongo-db

controller.test.js

const proxyquire = require('proxyquire')
const sinon = require('sinon')
const faker = require('faker')
const assert = require('chai').assert

describe('todo/controller', () => {
  describe('controller', () => {

    let mdl
    let modelStub, serializerStub, populateMethodStub, fakeData
    let fakeSerializedData, fakeError
    let mongoResponse

    before(() => {
      fakeData = faker.helpers.createTransaction()
      fakeError = faker.lorem.word()
      populateMethodStub = {
        populate: sinon.stub().callsFake(() => mongoResponse)
      }
      modelStub = {
        find: sinon.stub().callsFake(() => {
          return populateMethodStub
        }),
        findOne: sinon.stub().callsFake(() => {
          return populateMethodStub
        })
      }

      fakeSerializedData = faker.helpers.createTransaction()
      serializerStub = {
        serialize: sinon.stub().callsFake(() => {
          return fakeSerializedData
        })
      }

      mdl = proxyquire('../todo/controller.js',
        {
          './model': modelStub,
          './serializer': serializerStub
        }
      )
    })

    beforeEach(() => {
      modelStub.find.resetHistory()
      modelStub.findOne.resetHistory()
      populateMethodStub.populate.resetHistory()
      serializerStub.serialize.resetHistory()
    })

    describe('getAll', () => {
      it('should return serialized search result from mongodb', (done) => {
        let resolveFn
        let fakeCallback = new Promise((res, rej) => {
          resolveFn = res
        })
        mongoResponse = Promise.resolve(fakeData)
        let fakeRes = {
          json: sinon.stub().callsFake(() => {
            resolveFn()
          })
        }
        mdl.getAll(null, fakeRes, null)

        fakeCallback.then(() => {
          sinon.assert.calledOnce(modelStub.find)
          sinon.assert.calledWith(modelStub.find, {})

          sinon.assert.calledOnce(populateMethodStub.populate)
          sinon.assert.calledWith(populateMethodStub.populate, '_user', '-password')

          sinon.assert.calledOnce(serializerStub.serialize)
          sinon.assert.calledWith(serializerStub.serialize, fakeData)

          sinon.assert.calledOnce(fakeRes.json)
          sinon.assert.calledWith(fakeRes.json, fakeSerializedData)
          done()
        }).catch(done)
      })

      it('should call next callback if mongo db return exception', (done) => {
        let fakeCallback = (err) => {
          assert.equal(fakeError, err)
          done()
        }
        mongoResponse = Promise.reject(fakeError)
        let fakeRes = sinon.mock()
        mdl.getAll(null, fakeRes, fakeCallback)
      })

    })

    describe('getOne', () => {

      it('should return serialized search result from mongodb', (done) => {
        let resolveFn
        let fakeCallback = new Promise((res, rej) => {
          resolveFn = res
        })
        mongoResponse = Promise.resolve(fakeData)
        let fakeRes = {
          json: sinon.stub().callsFake(() => {
            resolveFn()
          })
        }

        let fakeReq = {
          params: {
            id: faker.random.number()
          },
          user: {
            _id: faker.random.number()
          }
        }
        let findParams = {
          '_id': fakeReq.params.id,
          '_user': fakeReq.user._id
        }
        mdl.getOne(fakeReq, fakeRes, null)

        fakeCallback.then(() => {
          sinon.assert.calledOnce(modelStub.findOne)
          sinon.assert.calledWith(modelStub.findOne, findParams)

          sinon.assert.calledOnce(populateMethodStub.populate)
          sinon.assert.calledWith(populateMethodStub.populate, '_user', '-password')

          sinon.assert.calledOnce(serializerStub.serialize)
          sinon.assert.calledWith(serializerStub.serialize, fakeData)

          sinon.assert.calledOnce(fakeRes.json)
          sinon.assert.calledWith(fakeRes.json, fakeSerializedData)
          done()
        }).catch(done)
      })

      it('should call next callback if mongodb return exception', (done) => {
        let fakeReq = {
          params: {
            id: faker.random.number()
          },
          user: {
            _id: faker.random.number()
          }
        }
        let fakeCallback = (err) => {
          assert.equal(fakeError, err)
          done()
        }
        mongoResponse = Promise.reject(fakeError)
        let fakeRes = sinon.mock()
        mdl.getOne(fakeReq, fakeRes, fakeCallback)
      })

      it('should call next callback with error if mongodb return empty result', (done) => {
        let fakeReq = {
          params: {
            id: faker.random.number()
          },
          user: {
            _id: faker.random.number()
          }
        }
        let expectedError = new Error('No todo item found.')

        let fakeCallback = (err) => {
          assert.equal(expectedError.message, err.message)
          done()
        }

        mongoResponse = Promise.resolve(null)
        let fakeRes = sinon.mock()
        mdl.getOne(fakeReq, fakeRes, fakeCallback)
      })

    })
  })
})

model.test.js

const proxyquire = require('proxyquire')
const sinon = require('sinon')
const faker = require('faker')

describe('todo/model', () => {
  describe('todo schema', () => {
    let mongooseStub, SchemaConstructorSpy
    let ObjectIdFake, mongooseModelSpy, SchemaSpy

    before(() => {
      ObjectIdFake = faker.lorem.word()
      SchemaConstructorSpy = sinon.spy()
      SchemaSpy = sinon.spy()

      class SchemaStub {
        constructor(...args) {
          SchemaConstructorSpy(...args)
          return SchemaSpy
        }
      }

      SchemaStub.Types = {
        ObjectId: ObjectIdFake
      }

      mongooseModelSpy = sinon.spy()
      mongooseStub = {
        "Schema": SchemaStub,
        "model": mongooseModelSpy
      }

      proxyquire('../todo/model.js',
        {
          'mongoose': mongooseStub
        }
      )
    })

    it('should return new Todo model by schema', () => {
      let todoSchema = {
        title: {
          type: String
        },

        _user: {
          type: ObjectIdFake,
          ref: 'User'
        }
      }
      sinon.assert.calledOnce(SchemaConstructorSpy)
      sinon.assert.calledWith(SchemaConstructorSpy, todoSchema)

      sinon.assert.calledOnce(mongooseModelSpy)
      sinon.assert.calledWith(mongooseModelSpy, 'Todo', SchemaSpy)
    })
  })
})

routes.test.js

const proxyquire = require('proxyquire')
const sinon = require('sinon')
const faker = require('faker')

describe('todo/routes', () => {
  describe('router', () => {
    let expressStub, controllerStub, RouterStub, rootRouteStub, idRouterStub

    before(() => {
      rootRouteStub = {
        "get": sinon.stub().callsFake(() => rootRouteStub),
        "post": sinon.stub().callsFake(() => rootRouteStub)
      }
      idRouterStub = {
        "get": sinon.stub().callsFake(() => idRouterStub),
        "put": sinon.stub().callsFake(() => idRouterStub),
        "delete": sinon.stub().callsFake(() => idRouterStub)
      }
      RouterStub = {
        route: sinon.stub().callsFake((route) => {
          if (route === '/:id') {
            return idRouterStub
          }
          return rootRouteStub
        })
      }

      expressStub = {
        Router: sinon.stub().returns(RouterStub)
      }

      controllerStub = {
        getAll: sinon.mock(),
        create: sinon.mock(),
        getOne: sinon.mock(),
        update: sinon.mock(),
        delete: sinon.mock()
      }

      proxyquire('../todo/routes.js',
        {
          'express': expressStub,
          './controller': controllerStub
        }
      )
    })

    it('should map root get router with getAll controller', () => {
      sinon.assert.calledWith(RouterStub.route, '/')
      sinon.assert.calledWith(rootRouteStub.get, controllerStub.getAll)
    })

    it('should map root post router with create controller', () => {
      sinon.assert.calledWith(RouterStub.route, '/')
      sinon.assert.calledWith(rootRouteStub.post, controllerStub.create)
    })

    it('should map /:id get router with getOne controller', () => {
      sinon.assert.calledWith(RouterStub.route, '/:id')
      sinon.assert.calledWith(idRouterStub.get, controllerStub.getOne)
    })

    it('should map /:id put router with update controller', () => {
      sinon.assert.calledWith(RouterStub.route, '/:id')
      sinon.assert.calledWith(idRouterStub.put, controllerStub.update)
    })

    it('should map /:id delete router with delete controller', () => {
      sinon.assert.calledWith(RouterStub.route, '/:id')
      sinon.assert.calledWith(idRouterStub.delete, controllerStub.delete)
    })
  })
})

serializer.test.js

const proxyquire = require('proxyquire')
const sinon = require('sinon')

describe('todo/serializer', () => {
  describe('json serializer', () => {
    let JSONAPISerializerStub, SerializerConstructorSpy

    before(() => {
      SerializerConstructorSpy = sinon.spy()

      class SerializerStub {
        constructor(...args) {
          SerializerConstructorSpy(...args)
        }
      }

      JSONAPISerializerStub = {
        Serializer: SerializerStub
      }

      proxyquire('../todo/serializer.js',
        {
          'jsonapi-serializer': JSONAPISerializerStub
        }
      )
    })

    it('should return new instance of Serializer', () => {
      let schema = {
        attributes: ['title', '_user']
        ,
        _user: {
          ref: 'id',
          attributes: ['username']
        }
      }
      sinon.assert.calledOnce(SerializerConstructorSpy)
      sinon.assert.calledWith(SerializerConstructorSpy, 'todos', schema)
    })
  })
})