How to test Async Storage with Jest?

May be you can try something like this:

mockStorage.js

export default class MockStorage {
  constructor(cache = {}) {
    this.storageCache = cache;
  }

  setItem = jest.fn((key, value) => {
    return new Promise((resolve, reject) => {
      return (typeof key !== 'string' || typeof value !== 'string')
        ? reject(new Error('key and value must be string'))
        : resolve(this.storageCache[key] = value);
    });
  });

  getItem = jest.fn((key) => {
    return new Promise((resolve) => {
      return this.storageCache.hasOwnProperty(key)
        ? resolve(this.storageCache[key])
        : resolve(null);
    });
  });

  removeItem = jest.fn((key) => {
    return new Promise((resolve, reject) => {
      return this.storageCache.hasOwnProperty(key)
        ? resolve(delete this.storageCache[key])
        : reject('No such key!');
    });
  });

  clear = jest.fn((key) => {
    return new Promise((resolve, reject) =>  resolve(this.storageCache = {}));
  });

  getAllKeys = jest.fn((key) => {
    return new Promise((resolve, reject) => resolve(Object.keys(this.storageCache)));
  });
}

and inside your test file:

import MockStorage from './MockStorage';

const storageCache = {};
const AsyncStorage = new MockStorage(storageCache);

jest.setMock('AsyncStorage', AsyncStorage)

// ... do things

For everyone who sees this question in > 2019:

Since Nov 2020, AsyncStorage was renamed back to @react-native-async-storage/async-storage", which causes this warning to appear if you're importing it from react-native:

Warning: Async Storage has been extracted from react-native core and will be removed in a future release.

The new module includes its own mock, so you don't have to worry about writing your own anymore.

Per the project's documentation, you can set it up in 2 different ways:

##With mocks directory

• In your project root directory, create a __mocks__/@react-native-community directory.
• Inside that folder, create async-storage.js file.
• Inside that file, export Async Storage mock.

  export default from '@react-native-async-storage/async-storage/jest/async-storage-mock'
  

Jest should then mock AsyncStorage by default in all your tests. If it doesn't, try calling jest.mock(@react-native-async-storage/async-storage) at the top of your test file.

With Jest setup file

• In your Jest config (probably in package.json or jest.config.js) add the setup file's location:

  "jest": {
    "setupFiles": ["./path/to/jestSetupFile.js"]
  }
  

• Inside your setup file, set up the AsyncStorage mock:

  import mockAsyncStorage from '@react-native-async-storage/async-storage/jest/async-storage-mock';
  
  jest.mock('@react-native-community/async-storage', () => mockAsyncStorage);
  

If you're using TypeScript, using the 2nd option (Jest setup file) is way easier, since with the 1st one (mocks directory) it won't associate @types/react-native-community__async-storage with the mock automatically.