How to test Express router catch branch in this scenario using Jest?

Question

I have a file with two routes.

routes/index.js

const express = require('express')
const router = express.Router()

router.get('', (req, res, next) => {
  try {
    res.status(200).render('../views/')
  } catch (error) {
    next(error)
  }
})

router.get('*', (req, res, next) => {
  try {
    res.status(404).render('../views/not-found')
  } catch (error) {
    next(error)
  }  
})

module.exports = router

I want to test the catch branches. One of the obvious problems is that I can't fake what the callback of router.get does, because then my test would be pointless, I would modify the very thing that I want to test.

I can send a Supertest request to these specific routes, but then I have no control over what happens. I want to avoid creating a hardcoded route, only so I can check whether the middleware is called by the next function. If I understand correctly, if I would have additional things happening in the try block (like a database query) than I could mock that to throw an error.

But I do not have any additional things happening there. Here is what I can not wrap my head around: if I mock what the server does, then I don't test what I already have but something else, so there is no point in doing that. Maybe I misunderstand how things work, but as far as I can see the only option would be to somehow mock what res.status or res.render does (stub them so they throw an error), so somehow mock what Supertest does, but I have no idea how to do that.

Any help would be greatly appreciated!

Answer 1

unit test solution:

route.js:

const express = require('express');
const router = express.Router();

router.get('', (req, res, next) => {
  try {
    res.status(200).render('../views/');
  } catch (error) {
    next(error);
  }
});

router.get('*', (req, res, next) => {
  try {
    res.status(404).render('../views/not-found');
  } catch (error) {
    next(error);
  }
});

module.exports = router;

route.test.js:

describe('64051580', () => {
  afterEach(() => {
    jest.resetModules();
    jest.restoreAllMocks();
  });
  it('should render views', () => {
    const express = require('express');
    const mRouter = { get: jest.fn() };
    jest.spyOn(express, 'Router').mockImplementationOnce(() => mRouter);
    const mReq = {};
    const mRes = { status: jest.fn().mockReturnThis(), render: jest.fn() };
    const mNext = jest.fn();
    mRouter.get.mockImplementation((path, callback) => {
      if (path === '') {
        callback(mReq, mRes, mNext);
      }
    });
    require('./route');
    expect(mRes.status).toBeCalledWith(200);
    expect(mRes.render).toBeCalledWith('../views/');
  });

  it('should handle error', () => {
    const express = require('express');
    const mRouter = { get: jest.fn() };
    jest.spyOn(express, 'Router').mockImplementationOnce(() => mRouter);
    const mReq = {};
    const mErr = new Error('parse');
    const mRes = {
      status: jest.fn().mockReturnThis(),
      render: jest.fn().mockImplementationOnce(() => {
        throw mErr;
      }),
    };
    const mNext = jest.fn();
    mRouter.get.mockImplementation((path, callback) => {
      if (path === '') {
        callback(mReq, mRes, mNext);
      }
    });
    require('./route');
    expect(mNext).toBeCalledWith(mErr);
  });

  it('should render 404 not found view', () => {
    const express = require('express');
    const mRouter = { get: jest.fn() };
    jest.spyOn(express, 'Router').mockImplementationOnce(() => mRouter);
    const mReq = {};
    const mRes = { status: jest.fn().mockReturnThis(), render: jest.fn() };
    const mNext = jest.fn();
    mRouter.get.mockImplementation((path, callback) => {
      if (path === '*') {
        callback(mReq, mRes, mNext);
      }
    });
    require('./route');
    expect(mRes.status).toBeCalledWith(404);
    expect(mRes.render).toBeCalledWith('../views/not-found');
  });
});

unit test result with coverage report:

 PASS  src/stackoverflow/64051580/route.test.js
  64051580
    ✓ should render views (656ms)
    ✓ should handle error (17ms)
    ✓ should render 404 not found view (16ms)

----------|----------|----------|----------|----------|-------------------|
File      |  % Stmts | % Branch |  % Funcs |  % Lines | Uncovered Line #s |
----------|----------|----------|----------|----------|-------------------|
All files |    90.91 |      100 |      100 |    90.91 |                   |
 route.js |    90.91 |      100 |      100 |    90.91 |                16 |
----------|----------|----------|----------|----------|-------------------|
Test Suites: 1 passed, 1 total
Tests:       3 passed, 3 total
Snapshots:   0 total
Time:        4.28s, estimated 10s