Mocking globals in Jest

Is there any way in Jest to mock global objects, such as navigator, or Image*? I've pretty much given up on this, and left it up to a series of mockable utility methods. For example:

// Utils.js
export isOnline() {
    return navigator.onLine;
}

Testing this tiny function is simple, but crufty and not deterministic at all. I can get 75% of the way there, but this is about as far as I can go:

// Utils.test.js
it('knows if it is online', () => {
    const { isOnline } = require('path/to/Utils');

    expect(() => isOnline()).not.toThrow();
    expect(typeof isOnline()).toBe('boolean');
});

On the other hand, if I am okay with this indirection, I can now access navigator via these utilities:

// Foo.js
import { isOnline } from './Utils';

export default class Foo {
    doSomethingOnline() {
		if (!isOnline()) throw new Error('Not online');

        /* More implementation */            
    }
}

...and deterministically test like this...

// Foo.test.js
it('throws when offline', () => {
    const Utils = require('../services/Utils');
    Utils.isOnline = jest.fn(() => isOnline);

    const Foo = require('../path/to/Foo').default;
    let foo = new Foo();

    // User is offline -- should fail
    let isOnline = false;
    expect(() => foo.doSomethingOnline()).toThrow();

    // User is online -- should be okay
    isOnline = true;
    expect(() => foo.doSomethingOnline()).not.toThrow();
});

Out of all the testing frameworks I've used, Jest feels like the most complete solution, but any time I write awkward code just to make it testable, I feel like my testing tools are letting me down.

Is this the only solution or do I need to add Rewire?

*Don't smirk. Image is fantastic for pinging a remote network resource.

As every test suite run its own environment, you can mock globals by just overwriting them. All global variables can be accessed by the global namespace:

global.navigator = {
  onLine: true
}

The overwrite has only effects in your current test and will not effect others. This also a good way to handle Math.random or Date.now.

Note, that through some changes in jsdom it could be possible that you have to mock globals like this:

Object.defineProperty(globalObject, key, { value, writable: true });

The correct way of doing this is to use spyOn. The other answers here, even though they work, don't consider cleanup and pollute the global scope.

// beforeAll
jest
  .spyOn(window, 'navigator', 'get')
  .mockImplementation(() => { ... })

// afterAll
jest.restoreAllMocks();

Jest may have changed since the accepted answer was written, but Jest does not appear to reset your global after testing. Please see the testcases attached.

https://repl.it/repls/DecentPlushDeals

As far as I know, the only way around this is with an afterEach() or afterAll() to clean up your assignments to global.

let originalGlobal = global;
afterEach(() => {
  delete global.x;
})

describe('Scope 1', () => {
  it('should assign globals locally', () => {
    global.x = "tomato";
    expect(global.x).toBeTruthy()
  });  
});

describe('Scope 2', () => {
  it('should not remember globals in subsequent test cases', () => {
    expect(global.x).toBeFalsy();
  })
});

If someone needs to mock a global with static properties then my example should help:

  beforeAll(() => {
    global.EventSource = jest.fn(() => ({
      readyState: 0,
      close: jest.fn()
    }))

    global.EventSource.CONNECTING = 0
    global.EventSource.OPEN = 1
    global.EventSource.CLOSED = 2
  })

If you are using react-testing-library and you use the cleanup method provided by the library, it will remove all global declarations made in that file once all tests in the file have run. This will then not carry over to any other tests run.

Example:

import { cleanup } from 'react-testing-library'

afterEach(cleanup)

global.getSelection = () => {

}

describe('test', () => {
  expect(true).toBeTruthy()
})

If you need to assign and reassign the value of a property on window.navigator then you'll need to:

1. Declare a non-constant variable
2. Return it from the global/window object
3. Change the value of that original variable (by reference)

This will prevent errors when trying to reassign the value on window.navigator because these are mostly read-only.

let mockUserAgent = "";

beforeAll(() => {
  Object.defineProperty(global.navigator, "userAgent", {
    get() {
      return mockUserAgent;
    },
  });
});

it("returns the newly set attribute", () => {
  mockUserAgent = "secret-agent";
  expect(window.navigator.userAgent).toEqual("secret-agent");
});