Mocking Express Chained Response with JEST

Question

I am pretty new to the jest and typescript, I am trying to create a small unit test for a controller function in jest

import { Request, Response } from 'express';

const healthCheck = (_req: Request, _res: Response) => {
  const value = _req.params.name ?? 'World!';
  return _res.status(200).json({ message: `Hello ${value}` });
};

export default healthCheck;

The unit test i wrote for the above function is

import { Request, Response } from 'express';

import healthCheck from './health.controller';

describe('Health Controller', () => {
  it('healthCheck should send 200 on an optional path param', () => {
    const req = { params: {} } as Request;
    const res = {
      json: jest.fn(),
      status: jest.fn(),
    } as unknown as Response;

    healthCheck(req, res);

    expect(res.status).toHaveBeenCalledWith(200);
  });
});

I am getting an error

TypeError: Cannot read property 'json' of undefined
>  8 |   return _res.status(200).json({ message: `Hello ${value}` });

why i am getting the json of undefined, even if i mock the property?

Answer 1

Your mocks need a bit tweaking:

const res = {} as unknown as Response;
res.json = jest.fn();
res.status = jest.fn(() => res); // chained

As @hellitsjoe pointed out, Express chains the call.

_res.status(200).json({ message: `Hello ${value}` })

So your mocks need to return other mocks for everything to work.

Answer 2

Since you're calling res.status().json(), json should be a function on the return value of status(), rather than on res.

A popular library for this type of mocking is node-mocks-http, which gives you req and res objects to pass into handlers so you don't have to mock them yourself.