Mock node external module default with chained method using jest

问题

In our node CLI we have a simple method:

'use strict';

const ora = require('ora');

module.exports = function startSpinner({ textOnStart, color, spinnerType }) {
  const spinner = ora({
    text: textOnStart,
    color: color || 'cyan',
    spinner: spinnerType || ''
  }).start();
};

We try to use jest to test this method. We have two tests to achieve:

• Testing that ora has been called with proper object argument
• Testing that the method start() was called afterward

That being said we cannot achieve to mock ora module properly.

ora is a third party that is basically constructed as follow:

class Ora {
    constructor(options){}
    start(){ }
}

const oraFactory = function (opts) {
    return new Ora(opts);
};

module.exports = oraFactory;
module.exports.default = oraFactory;

We are looking for a way to mock ora.

We tried to use auto mock:

const ora = require('ora');

jest.mock('ora');

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

describe('startSpinner', () => {
  beforeEach(() => {
    startSpinner({});
  });

  describe('ora', () => {
    it('should call ora', () => {
      expect(ora).toHaveBeenCalled();
    });

    it('should call ora start', () => {
      expect(ora.start).toHaveBeenCalled();
    });
  });
});

But both tests fail with respectively:

Matcher error: received value must be a mock or spy function

Received has type:  function
Received has value: [Function oraFactory]

and

Matcher error: received value must be a mock or spy function

Received has value: undefined

We tried to use a custom mock:

const ora = require('ora');

jest.mock('ora', () => {
  return jest.fn().mockImplementation(() => {
    return { start: jest.fn() };
  });
});

and it ends up with the same exact result.

We even tried to convert our test to typescript and then use:

import * as ora from 'ora';

const startMock = jest.fn();
jest.mock('ora', () => {
  return jest.fn().mockImplementation(() => {
    return { start: startMock };
  });
});

Then we were able to test successfuly that ora was called. But we ended up with an error for expect(ora.start).toHaveBeenCalled(); or even expect((ora as any).start).toHaveBeenCalled();:

error TS2339: Property 'start' does not exist on type 'typeof import("/Users/Dev/cli/node_modules/ora/index")'.

Surely caused by the fact the type definition of imported ora is export default function ora(options?: Options | string): Ora;

How to then mock a third party like ora in jest's node test environnement?

回答1:

You've got a couple of options:

---

You can mock ora like this:

jest.mock('ora', () => {
  const start = jest.fn();
  const result = { start };
  return jest.fn(() => result);
});

...and then call ora to get the object it returns (since it always returns the same object) and use that object to access start:

it('should call ora start', () => {
  const result = ora();
  expect(result.start).toHaveBeenCalled();  // Success!
});

---

Or if you want you can attach the start mock as a property to the ora mock as an easy way to access it during your tests like this:

const ora = require('ora');

jest.mock('ora', () => {
  const start = jest.fn();
  const result = { start };
  const ora = jest.fn(() => result);
  ora.start = start;  // attach the start mock to ora
  return ora;
});

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

describe('startSpinner', () => {
  beforeEach(() => {
    startSpinner({});
  });

  describe('ora', () => {
    it('should call ora', () => {
      expect(ora).toHaveBeenCalled();  // Success!
    });

    it('should call ora start', () => {
      expect(ora.start).toHaveBeenCalled();  // Success!
    });
  });
});

来源：https://stackoverflow.com/questions/55437111/mock-node-external-module-default-with-chained-method-using-jest