jest-localstorage-mock: A JavaScript repository from snayan

Use this module with Jest to run web tests that rely on localstorage and / or sessionStorage where you want a working localStorage API with mocked functions.

This module has no runtime dependencies so your project won't pull in additional module dependencies by using this.

Install

This should only be installed as a development dependency (devDependencies) as it is only designed for testing. The module is transpiled via babel to support the current active Node LTS version (6.11.3).

yarn:

yarn add --dev jest-localstorage-mock

npm:

npm i --save-dev jest-localstorage-mock

Setup

The simplest setup is to use the module system, you may also choose to create a setup file if needed.

Module

In your package.json under the jest configuration section create a setupFiles array and add jest-localstorage-mock to the array.

{
  "jest": {
    "setupFiles": ["jest-localstorage-mock"]
  }
}

If you already have a setupFiles attribute you can also append jest-localstorage-mock to the array.

{
  "jest": {
    "setupFiles": ["./__setups__/other.js", "jest-localstorage-mock"]
  }
}

Setup file

Alternatively you can create a new setup file which then requires this module or add the require statement to an existing setup file.

__setups__/localstorage.js

import 'jest-localstorage-mock';
// or
require('jest-localstorage-mock');

Add that file to your setupFiles array:

"jest": {
  "setupFiles": [
    "./__setups__/localstorage.js"
  ]
}

In create-react-app

For a create-react-app project you can replace the suggested mock with this at the beginning of the existing src/setupTests.js file:

require('jest-localstorage-mock');

In tests

By including this in your Jest setup you'll allow tests that expect a localStorage and sessionStorage object to continue to run. The module can also allow you to use the mocks provided to check that your localStorage is being used as expected.

The __STORE__ attribute of localStorage.__STORE__ or sessionStorage.__STORE__ is made available for you to directly access the storage object if needed.

Test Examples

Check that your localStorage calls were made when they were supposed to.

test('should save to localStorage', () => {
  const KEY = 'foo',
    VALUE = 'bar';
  dispatch(action.update(KEY, VALUE));
  expect(localStorage.setItem).toHaveBeenLastCalledWith(KEY, VALUE);
  expect(localStorage.__STORE__[KEY]).toBe(VALUE);
  expect(Object.keys(localStorage.__STORE__).length).toBe(1);
});

Check that your sessionStorage is empty, examples work with either localStorage or sessionStorage.

test('should have cleared the sessionStorage', () => {
  dispatch(action.reset());
  expect(sessionStorage.clear).toHaveBeenCalledTimes(1);
  expect(sessionStorage.__STORE__).toEqual({}); // check store values
  expect(sessionStorage.length).toBe(0); // or check length
});

Check that localStorage calls were not made when they shouldn't have been.

test('should not have saved to localStorage', () => {
  const KEY = 'foo',
    VALUE = 'bar';
  dispatch(action.notIdempotent(KEY, VALUE));
  expect(localStorage.setItem).not.toHaveBeenLastCalledWith(KEY, VALUE);
  expect(Object.keys(localStorage.__STORE__).length).toBe(0);
});

Reset your localStorage data and mocks before each test to prevent leaking.

beforeEach(() => {
  // values stored in tests will also be available in other tests unless you run
  localStorage.clear();
  // or directly reset the storage
  localStorage.__STORE__ = {};
  // you could also reset all mocks, but this could impact your other mocks
  jest.resetAllMocks();
  // or individually reset a mock used
  localStorage.setItem.mockClear();
});

test('should not impact the next test', () => {
  const KEY = 'foo',
    VALUE = 'bar';
  dispatch(action.update(KEY, VALUE));
  expect(localStorage.setItem).toHaveBeenLastCalledWith(KEY, VALUE);
  expect(localStorage.__STORE__[KEY]).toBe(VALUE);
  expect(Object.keys(localStorage.__STORE__).length).toBe(1);
});

test('should not be impacted by the previous test', () => {
  const KEY = 'baz',
    VALUE = 'zab';
  dispatch(action.update(KEY, VALUE));
  expect(localStorage.setItem).toHaveBeenLastCalledWith(KEY, VALUE);
  expect(localStorage.__STORE__[KEY]).toBe(VALUE);
  expect(Object.keys(localStorage.__STORE__).length).toBe(1);
});

See the contributing guide for details on how you can contribute.