In a React Native Jest suite, how do you wire the mocks that AsyncStorage, NetInfo and Reanimated ship instead of writing your own?

## Why library-shipped mocks come first A library with a native module cannot run under Jest, because Node has no native binary. You can write a `jest.mock` factory yourself, but a mock the library **ships** has two advantages: it matches the library's current API, and it moves with the library when you upgrade. The first place to look is the library's own testing or Jest guide. The three libraries interviewers most often name each take a different shape. ## AsyncStorage 3: a working in-memory store `@react-native-async-storage/async-storage` exports a mock at the subpath **`/jest`**. It is not a set of empty stubs: it is a **full in-memory implementation** of the AsyncStorage interface (`getItem`, `setItem`, `removeItem`, `getMany`, `setMany`, `removeMany`, `getAllKeys`, `clear`) backed by a `Map`. Two ways to install it: 1. Create `__mocks__/@react-native-async-storage/async-storage.js` next to `node_modules` containing `module.exports = require('@react-native-async-storage/async-storage/jest');`. Jest picks it up automatically for every test. 2. Call `jest.mock('@react-native-async-storage/async-storage', () => require('@react-native-async-storage/async-storage/jest'))` at the top of one test file. Because the store is real, data written in one test is still there in the next test of the same file. The mock also exports `clearAllMockStorages`, which its guide describes as clearing all in-memory storages. In 3.1.1's source it empties the registry that `createAsyncStorage` reads from; an instance a module already holds, including the default export, keeps its own map. So `await AsyncStorage.clear()` (or `clear()` on each instance your code holds) in `beforeEach` is the reliable reset. The library's Jest guide also notes that the package ships as ES module source, so it must be allowed through the transform step. ## NetInfo: preset answers you override `@react-native-community/netinfo` ships **`jest/netinfo-mock.js`**. Its README recommends registering it in a setup file: ```js import mockRNCNetInfo from '@react-native-community/netinfo/jest/netinfo-mock.js'; jest.mock('@react-native-community/netinfo', () => mockRNCNetInfo); ``` Every function is a `jest.fn()`. `fetch` and `refresh` resolve, and `useNetInfo` returns, a default state: `type: 'cellular'`, `isConnected: true`, `isInternetReachable: true`. `addEventListener` returns an unsubscribe function. So an app under test is **online by default**; an offline banner test must override the hook, for example `jest.mocked(useNetInfo).mockReturnValue({ ...state, isConnected: false })`. ## Reanimated 4: a resolver and a setup call Reanimated 4 does not ship a single module mock. Its Jest guide asks for two changes: - in `jest.config.js`, `resolver: 'react-native-reanimated/jest/resolver'`, which makes Jest resolve Reanimated's and React Native Worklets' web implementations instead of the native ones; - in a `setupFilesAfterEnv` file, `require('react-native-reanimated').setUpTests()`, which takes an optional config whose default is `{ fps: 60 }` and registers the matchers `toHaveAnimatedStyle` and `toHaveAnimatedProps`. The guide tells you to follow React Native Worklets' Jest guide first, since Reanimated 4's worklets live in that package. With this in place, `jest.useFakeTimers()` plus `jest.advanceTimersByTime(250)` moves an animation to its 250 ms point, and `toHaveAnimatedStyle` checks the style there. ## Side by side | Library | What you add | What the double does | Reset between tests | |---|---|---|---| | AsyncStorage 3 | `__mocks__` file or `jest.mock` pointing at `/jest` | Real in-memory storage | `await AsyncStorage.clear()` on each held instance | | NetInfo 12 | `jest.mock` with `jest/netinfo-mock.js` | `jest.fn()`s returning a connected cellular state | Re-set return values; clear mocks | | Reanimated 4 | Jest `resolver` plus `setUpTests()` | Web implementation driven by timers | Fake timers per test | ## Where each piece of wiring lives 1. **`__mocks__` directory** next to `node_modules`: automatic for every suite, good for AsyncStorage. 2. **`setupFiles`**: runs before the test framework loads, good for `jest.mock` calls such as NetInfo's that every suite needs. 3. **`setupFilesAfterEnv`**: runs after the framework is installed, required for anything that calls `expect.extend`, such as Reanimated's `setUpTests()`. 4. **Inside one test file**: for a mock that only that suite needs, or a suite that must override a global default. ## When to write your own anyway - The library ships no mock, or its mock lacks a method you call. - You want the test to depend on **your** wrapper, not the library's API, in which case you mock the wrapper instead. - The shipped mock's defaults hide a branch you need, such as NetInfo's online default hiding offline handling. The interview point is judgment: know that the mocks exist, how each is wired, and what default each one quietly assumes.