How to mock react-native Platform.OS with Jest? - react-native

Question

Jest runner use "ios" as default OS (Plaform.OS) for react-native tests.
How to do test in jest that have platform dependent code. An further more, how to mock static constant imported in a module.
Here is the code:
utils.ts:
import { Platform } from "react-native";
export const isIOS = Platform.OS === "ios"; // note constant, not function
items.ts:
import { isIOS } from "#my/utils";
export const items = [{
id: 1,
name: isIOS ? "Apple" : "Android"
}]
test:
import { items } from "./items";
// not working because Platform.OS is used statically during import statement
it("should be android", () => {
Platform.OS = "android"; // too late...
expect(items[0].name).toBe("Android"); // fail: Apple
}
it("should be ios", () => {
Platform.OS = "ios";
expect(items[0].name).toBe("Apple"); // works due to default platform value in Jest
}
I saw some workaround using jest.resetModules() & require inside test/it block (and change Plaform.OS before require) but does exist a more simple way to achieve that ?

Answer 1

In your test file
import { Platform } from 'react-native'
jest.doMock('react-native/Libraries/Utilities/Platform.android.js', () => ({
OS: 'android',
select: jest.fn(),
}))
jest.doMock('react-native/Libraries/Utilities/Platform.ios.js', () => ({
OS: 'android',
select: jest.fn(),
}))
describe('Platform', () => {
it('Should be android', () => {
expect(Platform.OS).toEqual('android')
})
it('Should be ios', () => {
Platform.OS = 'ios'
expect(Platform.OS).toEqual('ios')
})
it('Should be android', () => {
Platform.OS = 'android'
expect(Platform.OS).toEqual('android')
})
})