Unlock the Power of UI Component Development with Storybook and Mock Service Worker
When it comes to building robust UI components for JavaScript applications, Storybook is an unbeatable choice. It allows you to preview components in multiple states, serves as interactive documentation for your code, and provides a live environment for story-first development. However, when dealing with components that make API requests, you need an API mocking solution to take control of responses and remove actual HTTP communication from the story.
Introducing Mock Service Worker (MSW)
MSW is a powerful API mocking library that supports both browser and Node.js environments. Its key feature is request interception on the network level via Service Worker, ensuring zero changes to the component being tested or developed. This means the component remains unaware of any mocking and continues making requests as it would in production.
Setting Up a Storybook and Mock Service Worker Project
To get started, create a new Create React App project. Both Storybook and MSW are framework-agnostic tools, so you can integrate them into any other JavaScript project, including Angular, Vue.js, or Svelte.
Installing Storybook and MSW
First, install Storybook by following the instructions in the Getting started page of the Storybook documentation. Once installed, add the msw package to your project.
Initializing Service Worker
MSW uses a worker script to enable request interception in a browser. Run the npx msw init command and provide it with a relative path to your project’s public directory (e.g., ./public folder).
Creating a React Component
Create a React component that displays a short detail about a GitHub user. The component will fetch user details using a custom useFetch hook, which makes an HTTP request to the GitHub API.
Writing a Story
Add a story for the GitHubUser component that showcases its default behavior. However, the HTTP request it makes doesn’t belong in Storybook. To solve this, we’ll add API mocking to the mix.
Implementing API Mocking
To let MSW know which API calls to mock, declare a set of request handlers – functions that describe request predicates and response resolvers. Create an src/mocks directory and add a handlers.js file to store the request handlers.
Declaring Request Handlers
In the handlers.js file, declare the request handler for the GET /user/:userId request. This handler will be reused for multiple purposes, including Storybook, local development, testing, and debugging.
MSW and API Integration
To enable API mocking globally in Storybook, edit the .storybook/preview.js file to conditionally require the worker and start it. This ensures the Service Worker is activated only in a browser environment.
Per-Story API Responses
To showcase a single component in multiple states, override request handlers on a per-story basis using story decorators. This allows you to illustrate various HTTP communication scenarios, such as loading states and error responses.
Mocking a Loading State
Use the worker.use() method to provision a runtime request handler that delays the response indefinitely, simulating a loading state.
Mocking Error Responses
Create a separate story for the error response and have a runtime request handler that always responds with a specific HTTP server error. Use context utilities like ctx.status to model the precise HTTP response.
Conclusion
In this tutorial, we’ve explored the synergy between Storybook and Mock Service Worker, demonstrating how to integrate the two technologies seamlessly. By leveraging MSW’s granular control over mocked API responses, you can present the same component in multiple states, ensuring a robust and reproducible UI development experience.