React-Native + Enzyme + Jest: How to test platform specific behaviour?

TLDR: How can I tell my Enzyme / Jest test it should run the tests as if it was running on iOS? I want to test platform specific behaviour.

I'm building a custom status bar component that adds 20 pixels of height, if it runs on iOS to prevent my content from overlapping with the status bar. (Yes, I know React-Navigation has a SafeAreaView, but this only works for iPhone X, not for e.g. iPad.)

Here is my component:

import React from "react";
import { StatusBar as ReactNativeStatusBar, View } from "react-native";

import styles from "./styles";

const StatusBar = ({ props }) => (
  <View style={styles.container}>
    <ReactNativeStatusBar {...props} />
  </View>
);

export default StatusBar;

Here is the styles.js file:

import { StyleSheet, Platform } from "react-native";

const height = Platform.OS === "ios" ? 20 : 0;

const styles = StyleSheet.create({
  container: {
    height: height
  }
});

export default styles;

And here are the tests so far:

import React from "react";
import { shallow } from "enzyme";
import { View } from "react-native";

import StatusBar from "./StatusBar";

const createTestProps = props => ({
  ...props
});

describe("StatusBar", () => {
  describe("rendering", () => {
    let wrapper;
    let props;
    beforeEach(() => {
      props = createTestProps();
      wrapper = shallow(<StatusBar {...props} />);
    });

    it("should render a <View />", () => {
      expect(wrapper.find(View)).toHaveLength(1);
    });

    it("should give the <View /> the container style", () => {
      expect(wrapper.find(View)).toHaveLength(1);
    });

    it("should render a <StatusBar />", () => {
      expect(wrapper.find("StatusBar")).toHaveLength(1);
    });
  });
});

Now what I would like to do is add two more describe areas that explicitly test for the height to be either 20 on iOS or 0 or Android. The problem is I couldn't find how to emulate the platform with Enzyme / Jest tests.

So how do I tell my test suite that it should run the code for the respective platform?

You can override the RN Platform object and perform different tests for each platform. Here's an example for how a test file would like like:

describe('tests', () => {

    let Platform;
    beforeEach(() => {
        Platform = require('react-native').Platform;
    });

    describe('ios tests', () => {
        beforeEach(() => {
            Platform.OS = 'ios';
        });

        it('should test something on iOS', () => {

        });
    });

    describe('android tests', () => {
        beforeEach(() => {
            Platform.OS = 'android';
        });

        it('should test something on Android', () => {

        });
    });

});

By the way, regardless of the question about testing, setting the status-bar height to 20 on iOS is wrong since it can have different sizes on different devices (iPhone X for example)