How can I mock the JavaScript window object using Jest?

I need to test a function which opens a new tab in the browser

openStatementsReport(contactIds) {
  window.open(`a_url_${contactIds}`);
}

I would like to mock the window's open function so I can verify the correct URL is passed in to the open function.

Using Jest, I don't know how to mock the window. I tried to set window.open with a mock function but this way doesn't work. Below is the test case

it('correct url is called', () => {
  window.open = jest.fn();
  statementService.openStatementsReport(111);
  expect(window.open).toBeCalled();
});

But it gives me the error

expect(jest.fn())[.not].toBeCalled()

jest.fn() value must be a mock function or spy.
    Received:
      function: [Function anonymous]

What should I do to the test case?

Solution 1:

The following method worked for me. This approach allowed me to test some code that should work both in the browser and in Node.js, as it allowed me to set window to undefined.

This was with Jest 24.8 (I believe):

let windowSpy;

beforeEach(() => {
  windowSpy = jest.spyOn(window, "window", "get");
});

afterEach(() => {
  windowSpy.mockRestore();
});

it('should return https://example.com', () => {
  windowSpy.mockImplementation(() => ({
    location: {
      origin: "https://example.com"
    }
  }));

  expect(window.location.origin).toEqual("https://example.com");
});

it('should be undefined.', () => {
  windowSpy.mockImplementation(() => undefined);

  expect(window).toBeUndefined();
});

Solution 2:

Instead of window use global

it('correct url is called', () => {
  global.open = jest.fn();
  statementService.openStatementsReport(111);
  expect(global.open).toBeCalled();
});

you could also try

const open = jest.fn()
Object.defineProperty(window, 'open', open);

Solution 3:

There are a couple of ways to mock globals in Jest:

1. Use the mockImplementation approach (the most Jest-like way), but it will work only for those variables which has some default implementation provided by jsdom. window.open is one of them:

   test('it works', () => {
     // Setup
     const mockedOpen = jest.fn();
     // Without making a copy, you will have a circular dependency problem
     const originalWindow = { ...window };
     const windowSpy = jest.spyOn(global, "window", "get");
     windowSpy.mockImplementation(() => ({
       ...originalWindow, // In case you need other window properties to be in place
       open: mockedOpen
     }));
   
     // Tests
     statementService.openStatementsReport(111)
     expect(mockedOpen).toBeCalled();
   
     // Cleanup
     windowSpy.mockRestore();
   });
   

2. Assign the value directly to the global property. It is the most straightforward, but it may trigger error messages for some window variables, e.g. window.href.

   test('it works', () => {
     // Setup
     const mockedOpen = jest.fn();
     const originalOpen = window.open;
     window.open = mockedOpen;
   
     // Tests
     statementService.openStatementsReport(111)
     expect(mockedOpen).toBeCalled();
   
     // Cleanup
     window.open = originalOpen;
   });
   

3. Don't use globals directly (requires a bit of refactoring)

   Instead of using the global value directly, it might be cleaner to import it from another file, so mocking will became trivial with Jest.

File ./test.js

jest.mock('./fileWithGlobalValueExported.js');
import { windowOpen } from './fileWithGlobalValueExported.js';
import { statementService } from './testedFile.js';

// Tests
test('it works', () => {
  statementService.openStatementsReport(111)
  expect(windowOpen).toBeCalled();
});

File ./fileWithGlobalValueExported.js

export const windowOpen = window.open;

File ./testedFile.js

import { windowOpen } from './fileWithGlobalValueExported.js';
export const statementService = {
  openStatementsReport(contactIds) {
    windowOpen(`a_url_${contactIds}`);
  }
}

Solution 4:

We can also define it using global in setupTests

// setupTests.js
global.open = jest.fn()

And call it using global in the actual test:

// yourtest.test.js
it('correct url is called', () => {
    statementService.openStatementsReport(111);
    expect(global.open).toBeCalled();
});