Mocking network requests

For components that make network requests (e.g. fetching data from a REST or GraphQL API), you can mock those requests using a tool like Mock Service Worker (MSW). MSW is an API mocking library, which relies on service workers to capture network requests and provides mocked data in response.

The MSW addon brings this functionality into Storybook, allowing you to mock API requests in your stories. Below is an overview of how to set up and use the addon.

Set up the MSW addon

First, if necessary, run this command to install MSW and the MSW addon:

npm install msw msw-storybook-addon --save-dev

If you're not already using MSW, generate the service worker file necessary for MSW to work:

npx msw init public/

Then ensure the staticDirs property in your Storybook configuration will include the generated service worker file (in /public, by default):

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)'],
  staticDirs: ['../public', '../static'],
};

Finally, initialize the addon and apply it to all stories with a project-level loader:

import { initialize, mswLoader } from 'msw-storybook-addon';
 
/*
 * Initializes MSW
 * See https://github.com/mswjs/msw-storybook-addon#configuring-msw
 * to learn how to customize it
 */
initialize();
 
export default {
  // ... rest of preview configuration
  loaders: [mswLoader], // 👈 Add the MSW loader to all stories
};

Mocking REST requests

If your component fetches data from a REST API, you can use MSW to mock those requests in Storybook. As an example, consider this document screen component:

With the MSW addon, we can write stories that use MSW to mock the REST requests. Here's an example of two stories for the document screen component: one that fetches data successfully and another that fails.

import { http, HttpResponse, delay } from 'msw';
 
import { DocumentScreen } from './YourPage';
 
export default {
  component: DocumentScreen,
};
 
// 👇 The mocked data that will be used in the story
const TestData = {
  user: {
    userID: 1,
    name: 'Someone',
  },
  document: {
    id: 1,
    userID: 1,
    title: 'Something',
    brief: 'Lorem ipsum dolor sit amet, consectetur adipiscing elit.',
    status: 'approved',
  },
  subdocuments: [
    {
      id: 1,
      userID: 1,
      title: 'Something',
      content:
        'Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua.',
      status: 'approved',
    },
  ],
};
 
export const MockedSuccess = {
  parameters: {
    msw: {
      handlers: [
        http.get('https://your-restful-endpoint/', () => {
          return new HttpResponse.json(TestData);
        }),
      ],
    },
  },
};
 
export const MockedError = {
  parameters: {
    msw: {
      handlers: [
        http.get('https://your-restful-endpoint', async () => {
          await delay(800);
          return new HttpResponse(null, {
            status: 403,
          });
        }),
      ],
    },
  },
};

Mocking GraphQL requests

GraphQL is another common way to fetch data in components. You can use MSW to mock GraphQL requests in Storybook. Here's an example of a document screen component that fetches data from a GraphQL API:

The MSW addon allows you to write stories that use MSW to mock the GraphQL requests. Here's an example demonstrating two stories for the document screen component. The first story fetches data successfully, while the second story fails.

Configuring MSW for stories

In the examples above, note how each story is configured with parameters.msw to define the request handlers for the mock server. Because it uses parameters in this way, it can also be configured at the component or even project level, allowing you to share the same mock server configuration across multiple stories.