How do you mock FileSystem.cacheDirectory in jest?

Question

I am trying to test a service that uses expo-file-system. Specifically I just want to mock out FileSystem.cacheDirectory since that's the first instance of my code that uses FileSystem.

I tried __mocks__/expo-file-system.ts

export default {
  cacheDirectory: jest.fn(() => "<cache>/"),
};

And

jest.mock("expo-file-system", () => {
  return jest.fn().mockImplementation(() => {
    return {
      cacheDirectory: jest.fn(() => "<cache>/"),
    };
  });
});

My test I think looks simple enough

import * as FileSystem from "expo-file-system";

// This part I tried commenting out 
jest.mock("expo-file-system", () => {
  return jest.fn().mockImplementation(() => {
    return {
      cacheDirectory: jest.fn(() => "<cache>/"),
    };
  });
});

it("verify mock", () => {
  expect(FileSystem.cacheDirectory).toBe("<cache>/");
});

But I always get FileSystem.cacheDirectory as undefined

I also have the jest-expo dependency and preset.

This is a simple scenario compared to Why doesn't expo-file-system work in Jest?

Answer 1

If you look in node_modules\jest-expo\src\preset\setup.js you can find the jest.mock("expo-file-system", () => ({...})); call.

Notice that cacheDirectory property is not included in the mock, which is why it is undefined when the tests are run.

I don't know how to get the current mock set for a module in jest (ie. as we often see with ...jest.requireActual(...)), so if you want to give cacheDirectory a value, the best solution I see is to copy\paste the entire jest.mock(...) call to your test file and add cacheDirectory to the mocked module prior to running your tests.

ie.

jest.mock('expo-file-system', () => ({
    downloadAsync: jest.fn(() => Promise.resolve({ md5: 'md5', uri: 'uri' })),
    getInfoAsync: jest.fn(() => Promise.resolve({ exists: true, md5: 'md5', uri: 'uri' })),
    readAsStringAsync: jest.fn(() => Promise.resolve()),
    writeAsStringAsync: jest.fn(() => Promise.resolve()),
    deleteAsync: jest.fn(() => Promise.resolve()),
    moveAsync: jest.fn(() => Promise.resolve()),
    copyAsync: jest.fn(() => Promise.resolve()),
    makeDirectoryAsync: jest.fn(() => Promise.resolve()),
    readDirectoryAsync: jest.fn(() => Promise.resolve()),
    createDownloadResumable: jest.fn(() => Promise.resolve()),
    cacheDirectory: "file:///test-directory/",
  }));