req, res and chaining

README.md

How to test it in an Express app

app.get('/playground', handler);

200

http://localhost:9000/playground

Request URL: http://localhost:9000/playground
Request Method: GET
Status Code: 200 OK

{"message":"OK"}

500

http://localhost:9000/playground?error=KO

Request URL: http://localhost:9000/playground?error=KO
Request Method: GET
Status Code: 500 Internal Server Error

{"message":"KO"}

handler.js

async function handler({ query: { error } }, res) {
  if (error) {
    return res.status(500).send({ message: error });
  }
  return res.status(200).send({ message: 'OK' });
}

module.exports = handler;

handler.test.js

const handler = require('./handler');

describe('handler', () => {
  const scenarios = [
    {
      description: 'w/o error',
      error: null,
      status: 200
    },
    {
      description: 'w/ error',
      error: 'Oh Noes!',
      status: 500
    }
  ];

  scenarios.forEach(({ description, error, status }) => {
    it(description, async () => {
      const req = { query: { error } };
      const res = {
        send: jest.fn(),
        status: jest.fn(() => res)
      };

      await handler(req, res);
      expect(res.status).toBeCalledWith(status);
      expect(res.send.mock.calls[0][0]).toMatchSnapshot();
    });
  });
});

handler.test.js.snap

// Jest Snapshot v1, https://goo.gl/fbAQLP

exports[`handler w/ error 1`] = `
Object {
  "message": "Oh Noes!",
}
`;

exports[`handler w/o error 1`] = `
Object {
  "message": "OK",
}
`;

Very nice example. It saves me a lot of time

Yes, this is a very good example. One question if we don't pass the status mock function to your mock response then what will happen?