[Unit Testing + Jest + Node server] Testing a Node middleware with Jest

Code:

import {UnauthorizedError} from 'express-jwt'

function errorMiddleware(error, req, res, next) {
  if (res.headersSent) {
    next(error)
  } else if (error instanceof UnauthorizedError) {
    res.status(401)
    res.json({code: error.code, message: error.message})
  } else {
    res.status(500)
    res.json({
      message: error.message,
      // we only add a `stack` property in non-production environments
      ...(process.env.NODE_ENV === 'production' ? null : {stack: error.stack}),
    })
  }
}

export default errorMiddleware

 

Test:

• Function have been called with
• Function have been called Times
• Function have not been called

import {UnauthorizedError} from 'express-jwt'
import errorMiddleware from '../error-middleware'

// ???? Write a test for the UnauthorizedError case
// ???? const error = new UnauthorizedError('some_error_code', {message: 'Some message'})
// ???? const res = {json: jest.fn(() => res), status: jest.fn(() => res)}

// ???? Write a test for the headersSent case
describe('errorMiddleware', () => {
  test('if res.headersSent, then pass down error', () => {
    const res = {
      headersSent: true,
      json: jest.fn(() => res),
      status: jest.fn(() => res),
    }
    const next = jest.fn()
    const error = new Error('error')
    errorMiddleware(error, null, res, next)
    expect(next).toHaveBeenCalledWith(error)
    expect(next).toHaveBeenCalledTimes(1)
    expect(res.json).not.toHaveBeenCalled()
    expect(res.status).not.toHaveBeenCalled()
  })

  test('error is UnauthorizedError', () => {
    const code = 'some_code'
    const message = 'Unauthorized'
    const error = new UnauthorizedError(code, {
      message,
    })
    // return res itself so that response are chainable
    // res.status().json()
    const res = {json: jest.fn(() => res), status: jest.fn(() => res)}
    const next = jest.fn(() => next)
    errorMiddleware(error, null, res, next)
    expect(next).not.toHaveBeenCalled()
    expect(res.status).toHaveBeenCalledWith(401)
    expect(res.status).toHaveBeenCalledTimes(1)
    expect(res.json).toHaveBeenCalledWith({
      code: error.code,
      message: error.message,
    })
    expect(res.json).toHaveBeenCalledTimes(1)
  })

  test('return 500 error', () => {
    const res = {json: jest.fn(() => res), status: jest.fn(() => res)}
    const error = new Error('Server error')
    const next = jest.fn()
    errorMiddleware(error, null, res, next)
    expect(next).not.toHaveBeenCalled()
    expect(res.status).toHaveBeenCalledWith(500)
    expect(res.status).toHaveBeenCalledTimes(1)
    expect(res.json).toHaveBeenCalledWith({
      message: 'Server error',
      stack: error.stack,
    })
    expect(res.json).toHaveBeenCalledTimes(1)
  })
})