Mock 3rd party library constructor with jest

Question

I'm writing unit tests with jest and I have to test a function that is calling a constructor from a 3rd party library (the goal of the test is to check that the call is made with good arguments

The 3rd patry library is Popper.js

I have made a jest.spyOn(Popper.prototype, 'constructor').mockImplementation( () => {}) but it is throwing error that came from the inside of the constructor (thus it is not the mock function that has been called)

Here is the code of my test

  import Popper from 'popper.js';

  it('should call Popper constructor with correct argument', () => {
    // Arrange
    jest.mockImplementation(Popper.prototype, 'constructor', () => {});
    const refElem = document.createElement('div');
    const popElem = document.createElement('div');
    const placement = 'top';
    const container = document.createElement('div');

    // Act
    popup.create(refElem, popElem, placement, container);

    // Assert
    expect(Popper.prototype.constructor).toHaveBeenCalled();

  });

Did you have any luck with your testing? I've mocked a couple of my constructors with just `jest.fn()`, whether or not it's correct (I doubt it) it works. — MattyK14, Jan 11 '18 at 20:10
@MattyK14 The main problem in this question was that it wasn't one of my constructor, but one of a 3rd part library; anyway feel free to post youor solution if it works, and take a look at mine — CharybdeBE, Jan 12 '18 at 07:20

score 6 · Accepted Answer · edited Apr 19 '19 at 02:55

I finally managed to do something about it. I have created a mock module manually (because jest.genmockfromModule does not seem to work)

jest.mock ('popper.js', () =>
{
  class Popper {
    constructor(a,b,c){
      this.spy(a,b,c);
    }
    spy(a,b,c) {}
    destroy() {}
  }
  return Popper;
});

The spy function is the one that you can "spyOn" when you want to know if the constructor has been called with the good parameters

(here you have 3 arguments because of popper.js)

Thus I use it like so in my spec file :

import Popper from 'popper.js';
 ...
jest.spyOn(Popper.prototype, 'spy');

Mock 3rd party library constructor with jest

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