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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();

  }); 

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();

  }); 

• javascript
• typescript
• unit-testing
• jestjs
• popper.js

Share Improve this question Follow edited May 17, 2019 at 15:21 skyboyer 23.8k77 gold badges6262 silver badges7171 bronze badges asked Nov 20, 2017 at 14:04 CharybdeBECharybdeBE 1,85311 gold badge2121 silver badges3737 bronze badges 2

• 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 Commented Jan 11, 2018 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 Commented Jan 12, 2018 at 7:20

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1 Answer 1

Sorted by: Reset to default 6

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');

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