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):

.storybook/main.ts

// Replace your-framework with the framework you are using (e.g., react-webpack5, vue3-vite)
import type { StorybookConfig } from '@storybook/your-framework';
 
const config: StorybookConfig = {
  framework: '@storybook/your-framework',
  stories: ['../src/**/*.mdx', '../src/**/*.stories.@(js|jsx|mjs|ts|tsx)'],
  staticDirs: ['../public', '../static'],
};
 
export default config;

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

.storybook/preview.ts

// Replace your-renderer with the renderer you are using (e.g., react, vue, etc.)
import { Preview } from '@storybook/your-renderer';
 
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();
 
const preview: Preview = {
  // ... rest of preview configuration
  loaders: [mswLoader], // 👈 Add the MSW loader to all stories
};
 
export default preview;

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:

YourPage.tsx

import React, { useState, useEffect } from 'react';
 
import { PageLayout } from './PageLayout';
import { DocumentHeader } from './DocumentHeader';
import { DocumentList } from './DocumentList';
 
// Example hook to retrieve data from an external endpoint
function useFetchData() {
  const [status, setStatus] = useState<string>('idle');
  const [data, setData] = useState<any[]>([]);
  useEffect(() => {
    setStatus('loading');
    fetch('https://your-restful-endpoint')
      .then((res) => {
        if (!res.ok) {
          throw new Error(res.statusText);
        }
        return res;
      })
      .then((res) => res.json())
      .then((data) => {
        setStatus('success');
        setData(data);
      })
      .catch(() => {
        setStatus('error');
      });
  }, []);
 
  return {
    status,
    data,
  };
}
 
export function DocumentScreen() {
  const { status, data } = useFetchData();
 
  const { user, document, subdocuments } = data;
 
  if (status === 'loading') {
    return <p>Loading...</p>;
  }
  if (status === 'error') {
    return <p>There was an error fetching the data!</p>;
  }
  return (
    <PageLayout user={user}>
      <DocumentHeader document={document} />
      <DocumentList documents={subdocuments} />
    </PageLayout>
  );
}

This example uses the fetch API to make network requests. If you're using a different library (e.g. axios), you can apply the same principles to mock network requests in Storybook.

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.

YourPage.stories.ts|tsx

// Replace your-framework with the name of your framework (e.g. nextjs, vue3-vite)
import type { Meta, StoryObj } from '@storybook/your-framework';
 
import { http, HttpResponse, delay } from 'msw';
 
import { MyComponent } from './MyComponent';
 
const meta: Meta<typeof MyComponent> = {
  component: DocumentScreen,
};
 
export default meta;
type Story = StoryObj<typeof MyComponent>;
 
// 👇 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: Story = {
  parameters: {
    msw: {
      handlers: [
        http.get('https://your-restful-endpoint/', () => {
          return HttpResponse.json(TestData);
        }),
      ],
    },
  },
};
 
export const MockedError: Story = {
  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:

YourPage.ts|tsx

import { useQuery, gql } from '@apollo/client';
 
import { PageLayout } from './PageLayout';
import { DocumentHeader } from './DocumentHeader';
import { DocumentList } from './DocumentList';
 
const AllInfoQuery = gql`
  query AllInfo {
    user {
      userID
      name
    }
    document {
      id
      userID
      title
      brief
      status
    }
    subdocuments {
      id
      userID
      title
      content
      status
    }
  }
`;
 
interface Data {
  allInfo: {
    user: {
      userID: number;
      name: string;
      opening_crawl: boolean;
    };
    document: {
      id: number;
      userID: number;
      title: string;
      brief: string;
      status: string;
    };
    subdocuments: {
      id: number;
      userID: number;
      title: string;
      content: string;
      status: string;
    };
  };
}
 
function useFetchInfo() {
  const { loading, error, data } = useQuery<Data>(AllInfoQuery);
 
  return { loading, error, data };
}
 
export function DocumentScreen() {
  const { loading, error, data } = useFetchInfo();
 
  if (loading) {
    return <p>Loading...</p>;
  }
 
  if (error) {
    return <p>There was an error fetching the data!</p>;
  }
 
  return (
    <PageLayout user={data.user}>
      <DocumentHeader document={data.document} />
      <DocumentList documents={data.subdocuments} />
    </PageLayout>
  );
}

This example uses GraphQL with Apollo Client to make network requests. If you're using a different library (e.g. URQL or React Query), you can apply the same principles to mock network requests in Storybook.

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.

YourPage.stories.ts|tsx

import type { Meta, StoryObj } from '@storybook/react';
 
import { ApolloClient, ApolloProvider, InMemoryCache } from '@apollo/client';
import { graphql, HttpResponse, delay } from 'msw';
 
import { DocumentScreen } from './YourPage';
 
const mockedClient = new ApolloClient({
  uri: 'https://your-graphql-endpoint',
  cache: new InMemoryCache(),
  defaultOptions: {
    watchQuery: {
      fetchPolicy: 'no-cache',
      errorPolicy: 'all',
    },
    query: {
      fetchPolicy: 'no-cache',
      errorPolicy: 'all',
    },
  },
});
 
//👇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',
    },
  ],
};
const meta: Meta<typeof DocumentScreen> = {
  component: DocumentScreen,
  decorators: [
    (Story) => (
      <ApolloProvider client={mockedClient}>
        <Story />
      </ApolloProvider>
    ),
  ],
};
 
export default meta;
type Story = StoryObj<typeof SampleComponent>;
 
export const MockedSuccess: Story = {
  parameters: {
    msw: {
      handlers: [
        graphql.query('AllInfoQuery', () => {
          return HttpResponse.json({
            data: {
              allInfo: {
                ...TestData,
              },
            }
          });
        }),
      ],
    },
  },
};
 
export const MockedError: Story = {
  parameters: {
    msw: {
      handlers: [
        graphql.query('AllInfoQuery', async () => {
          await delay(800);
          return HttpResponse.json({
            errors: [
              {
                message: 'Access denied',
              },
            ],
          });
        }),
      ],
    },
  },
};

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.