How to mock window.navigator.language using jest

window.navigator and its properties are read-only, this is the reason why Object.defineProperty is needed to set window.navigator.language. It's supposed to work for changing property value multiple times.

The problem is that the component is already instantiated in beforeEach, window.navigator.language changes don't affect it.

Using Object.defineProperty for mocking properties manually will require to store original descriptor and restore it manually as well. This can be done with jest.spyOn. jest.clearAllMocks() wouldn't help for manual spies/mocks, it may be unneeded for Jest spies.

It likely should be:

let languageGetter;

beforeEach(() => {
  languageGetter = jest.spyOn(window.navigator, 'language', 'get')
})

it('should do thing 1', () => {
  languageGetter.mockReturnValue('de')
  wrapper = shallow(<Component {...props} />)
  expect(wrapper.state('currentLanguage')).toEqual('de')
})
...

Alternatively

jest config file

setupFiles: ['./test/mock-data/globals.js']

globals.js

const navigator = { language: 'Chalcatongo Mixtec', ...anyOtherPropertiesYouNeed };

Object.defineProperty(window, 'navigator', {
   value: navigator,
   writable: true
});

then you can mutate freely in your individual test setup

Adding a little bit to Estus Flask's answer, you can also spy on your setup file:

In jest config file activate the setupFiles feature:

setupFiles: ['./test/mock-data/globals.js']

Then inside globals.js spy on the userAgent or any other property:

global.userAgent = jest.spyOn(navigator, 'userAgent', 'get');

Finally in your test mock the return value:

describe('abc', () => {
  global.userAgent.mockReturnValue(
    'Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_4)\
           AppleWebKit/600.1.2 (KHTML, like Gecko)\
           Version/13.0.0 Safari/600.1.2'
  );
  test('123', async () => {
    const result = await fnThatCallsOrUseTheUserAgent();
    expect(result).toEqual('Something');
  });
});