Mocking React Native 0.61 modules with Jest

A few months ago, Facebook announced the release of React Native 0.61.

Version 0.61 seems to be solving several issues that stopped my team from upgrading to 0.60 earlier. All seemed to be going well until I tried to run the tests...

Prior to React Native 0.61, haste was used to map the individual module files. In haste the mapping is done by filename so it's easy enough to find the corresponding file in the react-native repo. So, to mock a module, we could simply do something like:

jest.mock("Button", () => {});

React Native 0.61 dropped support for haste, which means that all these mocks don't work anymore and we started getting a bunch of errors such as:

Cannot find module 'Button'

The React team recommends two ways for mocking react-native modules:

  1. Specify the full path to the module e.g.
    jest.mock("react-native/Libraries/Components/Button", () => {});
  2. Mock out the react-native interface

The first approach seems to be the most popular, especially when mocking individual modules. However, I am not a fan of it, because it is a brittle approach that creates coupling between the test and the internal path. Besides, I wasn't really able to make it work properly.

The second approach seems to be the correct one. The suggested implementation is to mock react-native inside setup.js, such as:

import * as ReactNative from "react-native";

jest.doMock("react-native", () => {
  // Extend ReactNative
  return Object.setPrototypeOf(
    {
      // Redefine an export, like a component
      Button: "Button",
      // Mock out properties of an already mocked export
      LayoutAnimation: {
        ...ReactNative.LayoutAnimation,
        configureNext: jest.fn(),
      },
      Platform: {
        ...ReactNative.Platform,
        OS: "ios",
        Version: 123,
        isTesting: true,
        select: (objs) => objs["ios"],
      },
      // Mock a native module
      NativeModules: {
        ...ReactNative.NativeModules,
        Override: { great: "success" },
      },
    },
    ReactNative
  );
});

I had issues when I followed this approach and my tests didn't seem to get mocked properly. For example, when I tried to access the value of Platform.OS I would get this error:

'Couldn't read OS of undefined'

What worked for me instead was to mock react-native manually inside the tests/__mocks__ folder. So I created a react-native.js file with this content:

import * as ReactNative from "react-native";

export const alert = jest.fn();
export const Alert = { alert };

export const dimensionWidth = 100;
export const Dimensions = {
  get: jest.fn().mockReturnValue({ width: dimensionWidth, height: 100 }),
};

export const Image = "Image";

export const keyboardDismiss = jest.fn();
export const Keyboard = {
  dismiss: keyboardDismiss,
};

export const Platform = {
  ...ReactNative.Platform,
  OS: "ios",
  Version: 123,
  isTesting: true,
  select: (objs) => objs["ios"],
};

export default Object.setPrototypeOf(
  {
    Alert,
    Dimensions,
    Image,
    Keyboard,
    Platform,
  },
  ReactNative
);

I added exports for all the react-native modules that were imported in tested files, one by one, and was able to get rid off the errors.

Also, I exported mocks of any methods as needed to help me test when they were called.

For example, I can now do:

import { alert } from "react-native";

it("showAlert() calls Alert.alert", () => {
  showAlert();
  expect(alert).toHaveBeenCalled();
});

And it works like a charm!

However, soon I came across another obstacle. I need to be able to mock platform detection. By default, we mock Platform.OS to be ios, but not only there were cases that we needed to test scenarios for Android, we are also using react-native for the web. It was really important to be able to test our web app as well.

Before upgrading, we had been using a method that overwrites the OS property of Platform like this:

export const mockPlatform = (OS) => {
  jest.resetModules();
  jest.doMock("Platform", () => ({ OS, select: (objs) => objs[OS] }));
};

And we would call

mockPlatform("android");

or

mockPlatform("web");

before the test.

Unfortunately, after upgrading to 0.61, this also stopped working. The solution turned out to be very simple, even simpler than our previous approach.

To change the OS in a test, first we import the module like:

import { Platform } from "react-native";

and then simply overwrite the platform inside the test

it('renders Element if Android', () => {
  Platform.OS = 'android'
  renderIfAndroid()
  expect(wrapper.find(Element)).exists()).toBe(true)
})

Using the above setup, we have finally been able to mock our modules properly and are one step closer to the react-native 0.61 upgrade.