SpyOn a mocked jest module not spying properly

Question

I have simple service that I need unit tested using jest:

the crux of the code is this:

 domtoimage.toBlob(node, {filter: filter})
    .then(function (blob) {
      FileSaver.saveAs(blob, fileName);
 });

I have wrote my unit test module as such:

import FileSaver from "file-saver";
import domtoimage from "dom-to-image";


jest.mock('dom-to-image', () => {
  return {
    toBlob: (arg)=>{
      let promise = new Promise((resolve, reject) => {
        resolve('myblob')
      });
      return promise;
    }
  }
});
jest.mock('file-saver', ()=>{
  return {
    saveAs: (blob, filename) =>{
      return filename;
    }
  }
});

And in my test, I have the following spy set up

const spy = jest.spyOn(FileSaver, 'saveAs');

and calling my in-test function.

however, the expect statement: expect(spy).toBeCalled() returns false:

expect(jest.fn()).toBeCalled()

However, in webstorm, when I debug the unit test, I can clearly see that my mocked function is being called (the breakpoint is reached inside function).

What am i missing?

Answer 1

Suggestion 1

Maybe spyOn and module mocks don't play well together. You could try using a jest.fn() directly inside the module mock like so

jest.mock('file-saver', ()=>{
  return {
    saveAs: jest.fn((blob, filename) => {
      return filename;
    })
  }
});

and then

expect(FileSaver.saveAs).toBeCalled()

Remember to call jest.clearAllMocks() or similar between tests.

Suggestion 2 I've had issues with jest.mock working in unexpected ways with the jest module cache, especially when working with singleton imports. Maybe you have this issue. if file-saver and dom-to-image don't have any state initialized or side-effects on import time you should be able to swap jest.mock out for overrides of the functions you need to mock.

beforeEach(() => {
  FileSaver.saveAs = jest.fn(...);
  domtoimage.toBlob = jest.fn(...);
})