node.jsunit-testinggoogle-cloud-platformmockingjestjs

Mock new Function() with Jest

I'm having trouble trying to mock a module with a constructor

// code.js
const ServiceClass = require('service-library');
const serviceInstance = new ServiceClass('some param');
exports.myFunction = () => {
  serviceInstance.doSomething();
};

And the test code:

// code.test.js
const ServiceClass = require('service-library');
jest.mock('service-library');
const {myFunction} = require('../path/to/my/code');

test('check that the service does something', () => {
  // ????
});

It's not like the Documentation example Mocking Modules because you need to instantiate the module after importing it. And isn't either like Mocking a Function.

How could I mock this doSomething() function while testing?

For reference, I'm trying to mock @google-cloud/* packages here. And I have a few projects that could take advantage on this.

Solution

  • You need to mock the whole module first so that returns a jest mock. Then import into your test and set the mock to a function that returns an object holding the spy for doSomething. For the test there is no difference between the mocks of a class called with new and a function called with new.

    import ServiceLibrary from 'service-library'

jest.mock('service-library', () => jest.fn())

const doSomething = jest.fn()
ServiceLibrary.mockImplementation(() => ({doSomething}))