Mocking modules

Components can also depend on modules that are imported into the component file. These can be from external packages or internal to your project. When rendering those components in Storybook or testing them, you may want to mock those modules to control their behavior.

There are two primary approaches to mocking modules in Storybook. They both involve creating a mock file to replace the original module. The difference between the two approaches is how you import the mock file into your component.

For either approach, relative imports of the mocked module are not supported.

Mock files

To mock a module, create a file with the same name and in the same directory as the module you want to mock. For example, to mock a module named session, create a file next to it named session.mock.js|ts, with a few characteristics:

• It must import the original module using a relative import.
  • Using a subpath or alias import would result in it importing itself.
• It should re-export all exports from the original module.
• It should use the fn utility to mock any necessary functionality from the original module.
• It should use the mockName method to ensure the name is preserved when minified
• It should not introduce side effects that could affect other tests or components. Mock files should be isolated and only affect the module they are mocking.

Here's an example of a mock file for a module named session:

import { fn } from '@storybook/test';
import * as actual from './session';
 
export * from './session';
export const getUserFromSession = fn(actual.getUserFromSession).mockName('getUserFromSession');

When you use the fn utility to mock a module, you create full Vitest mock functions. See below for examples of how you can use a mocked module in your stories.

Mock files for external modules

You can't directly mock an external module like uuid or node:fs. Instead, you must wrap it in your own module, which you can mock like any other internal one. For example, with uuid, you could do the following:

// lib/uuid.ts
import { v4 } from 'uuid';
 
export const uuidv4 = v4;

And create a mock for the wrapper:

// lib/uuid.mock.ts
import { fn } from '@storybook/test';
 
import * as actual from './uuid';
 
export const uuidv4 = fn(actual.uuidv4).mockName('uuidv4');

Subpath imports

The recommended method for mocking modules is to use subpath imports, a feature of Node packages that is supported by both Vite and Webpack.

To configure subpath imports, you define the imports property in your project's package.json file. This property maps the subpath to the actual file path. The example below configures subpath imports for four internal modules:

{
  "imports": {
    "#api": {
      // storybook condition applies to Storybook
      "storybook": "./api.mock.ts",
      "default": "./api.ts"
    },
    "#app/actions": {
      "storybook": "./app/actions.mock.ts",
      "default": "./app/actions.ts"
    },
    "#lib/session": {
      "storybook": "./lib/session.mock.ts",
      "default": "./lib/session.ts"
    },
    "#lib/db": {
      // test condition applies to test environments *and* Storybook
      "test": "./lib/db.mock.ts",
      "default": "./lib/db.ts"
    },
    "#*": ["./*", "./*.ts", "./*.tsx"]
  }
}

There are two aspects to this configuration worth noting:

First, each subpath must begin with #, to differentiate it from a regular module path. The #* entry is a catch-all that maps all subpaths to the root directory.

Second, note the storybook, test, and default keys in each module's entry. The storybook value is used to import the mock file when loaded in Storybook, while the default value is used to import the original module when loaded in your project. The test condition is also used within Storybook, which allows you to use the same configuration in Storybook and your other tests.

With the package configuration in place, you can then update your component file to use the subpath import:

// AuthButton.ts
// ➖ Remove this line
// import { getUserFromSession } from '../../lib/session';
// ➕ Add this line
import { getUserFromSession } from '#lib/session';
 
// ... rest of the file

Builder aliases

If your project is unable to use subpath imports, you can configure your Storybook builder to alias the module to the mock file. This will instruct the builder to replace the module with the mock file when bundling your Storybook stories.

export default {
  // Replace your-framework with the framework you are using (e.g., react-webpack5, vue3-vite)
  framework: '@storybook/your-framework',
  stories: ['../src/**/*.mdx', '../src/**/*.stories.@(js|jsx|mjs|ts|tsx)'],
  viteFinal: async (config) => {
    if (config.resolve) {
      config.resolve.alias = {
        ...config.resolve?.alias,
        // 👇 External module
        lodash: require.resolve('./lodash.mock'),
        // 👇 Internal modules
        '@/api': path.resolve(__dirname, './api.mock.ts'),
        '@/app/actions': path.resolve(__dirname, './app/actions.mock.ts'),
        '@/lib/session': path.resolve(__dirname, './lib/session.mock.ts'),
        '@/lib/db': path.resolve(__dirname, './lib/db.mock.ts'),
      };
    }
 
    return config;
  },
};

Using mocked modules in stories

When you use the fn utility to mock a module, you create full Vitest mock functions which have many useful methods. For example, you can use the mockReturnValue method to set a return value for the mocked function or mockImplementation to define a custom implementation.

Here, we define beforeEach on a story (which will run before the story is rendered) to set a mocked return value for the getUserFromSession function used by the Page component:

import { getUserFromSession } from '#api/session.mock';
import { Page } from './Page';
 
export default {
  component: Page,
};
 
export const Default = {
  async beforeEach() {
    // 👇 Set the return value for the getUserFromSession function
    getUserFromSession.mockReturnValue({ id: '1', name: 'Alice' });
  },
};

Spying on mocked modules

The fn utility also spies on the original module's functions, which you can use to assert their behavior in your tests. For example, you can use interaction tests to verify that a function was called with specific arguments.

For example, this story checks that the saveNote function was called when the user clicks the save button:

import { expect, userEvent, within } from '@storybook/test';
 
import { saveNote } from '#app/actions.mock';
import { createNotes } from '#mocks/notes';
import NoteUI from './note-ui';
 
export default {
  title: 'Mocked/NoteUI',
  component: NoteUI,
};
 
const notes = createNotes();
 
export const SaveFlow = {
  name: 'Save Flow ▶',
  args: {
    isEditing: true,
    note: notes[0],
  },
  play: async ({ canvasElement, step }) => {
    const canvas = within(canvasElement);
 
    const saveButton = canvas.getByRole('menuitem', { name: /done/i });
    await userEvent.click(saveButton);
    // 👇 This is the mock function, so you can assert its behavior
    await expect(saveNote).toHaveBeenCalled();
  },
};

Setting up and cleaning up

Before the story renders, you can use the asynchronous beforeEach function to perform any setup you need (e.g., configure the mock behavior). This function can be defined at the story, component (which will run for all stories in the file), or project (defined in .storybook/preview.js|ts, which will run for all stories in the project).

You can also return a cleanup function from beforeEach which will be called after your story unmounts. This is useful for tasks like unsubscribing observers, etc.

Here's an example of using the mockdate package to mock the Date and reset it when the story unmounts.

import MockDate from 'mockdate';
 
import { getUserFromSession } from '#api/session.mock';
import { Page } from './Page';
 
export default {
  component: Page,
  // 👇 Set the value of Date for every story in the file
  async beforeEach() {
    MockDate.set('2024-02-14');
 
    // 👇 Reset the Date after each story
    return () => {
      MockDate.reset();
    };
  },
};
 
export const Default = {
  async play({ canvasElement }) {
    // ... This will run with the mocked Date
  },
};