Mocke Service Worker で React 開発用の REST API をモックする

# はじめに
この記事では、Mock Service Worker を利用して、React 環境で REST API をモックする手順について記載します。

https://mswjs.io/

# 開発環境
開発環境は以下の通りです。

- msw 2.3.1
- Windows11
- VSCode
- React 19.0.0
- Vite 5.2.0
- Node.js 20.14.0
- npm 10.8.1

# Mocke Service Worker のインストール
以下のコマンドで Mocke Service Worker をインストールします。

```powershell
npm install msw@latest --save-dev
```

# 設定ファイルの作成
以下のコマンドで、ブラウザで Mock Service Worker を実行するための設定ファイルを作成します。

```powershell
npx msw init public/ --save
```

コマンドを実行すると、以下のファイルが作成されます。

```public/mockServiceWorker.js
/* eslint-disable */
/* tslint:disable */

/**
 * Mock Service Worker.
 * @see https://github.com/mswjs/msw
 * - Please do NOT modify this file.
 * - Please do NOT serve this file on production.
 */

const PACKAGE_VERSION = '2.3.1'
const INTEGRITY_CHECKSUM = '26357c79639bfa20d64c0efca2a87423'
const IS_MOCKED_RESPONSE = Symbol('isMockedResponse')
const activeClientIds = new Set()

self.addEventListener('install', function () {
  self.skipWaiting()
})

self.addEventListener('activate', function (event) {
  event.waitUntil(self.clients.claim())
})

self.addEventListener('message', async function (event) {
  const clientId = event.source.id

  if (!clientId || !self.clients) {
    return
  }

  const client = await self.clients.get(clientId)

  if (!client) {
    return
  }

  const allClients = await self.clients.matchAll({
    type: 'window',
  })

  switch (event.data) {
    case 'KEEPALIVE_REQUEST': {
      sendToClient(client, {
        type: 'KEEPALIVE_RESPONSE',
      })
      break
    }

    case 'INTEGRITY_CHECK_REQUEST': {
      sendToClient(client, {
        type: 'INTEGRITY_CHECK_RESPONSE',
        payload: {
          packageVersion: PACKAGE_VERSION,
          checksum: INTEGRITY_CHECKSUM,
        },
      })
      break
    }

    case 'MOCK_ACTIVATE': {
      activeClientIds.add(clientId)

      sendToClient(client, {
        type: 'MOCKING_ENABLED',
        payload: true,
      })
      break
    }

    case 'MOCK_DEACTIVATE': {
      activeClientIds.delete(clientId)
      break
    }

    case 'CLIENT_CLOSED': {
      activeClientIds.delete(clientId)

      const remainingClients = allClients.filter((client) => {
        return client.id !== clientId
      })

      // Unregister itself when there are no more clients
      if (remainingClients.length === 0) {
        self.registration.unregister()
      }

      break
    }
  }
})

self.addEventListener('fetch', function (event) {
  const { request } = event

  // Bypass navigation requests.
  if (request.mode === 'navigate') {
    return
  }

  // Opening the DevTools triggers the "only-if-cached" request
  // that cannot be handled by the worker. Bypass such requests.
  if (request.cache === 'only-if-cached' && request.mode !== 'same-origin') {
    return
  }

  // Bypass all requests when there are no active clients.
  // Prevents the self-unregistered worked from handling requests
  // after it's been deleted (still remains active until the next reload).
  if (activeClientIds.size === 0) {
    return
  }

  // Generate unique request ID.
  const requestId = crypto.randomUUID()
  event.respondWith(handleRequest(event, requestId))
})

async function handleRequest(event, requestId) {
  const client = await resolveMainClient(event)
  const response = await getResponse(event, client, requestId)

  // Send back the response clone for the "response:*" life-cycle events.
  // Ensure MSW is active and ready to handle the message, otherwise
  // this message will pend indefinitely.
  if (client && activeClientIds.has(client.id)) {
    ;(async function () {
      const responseClone = response.clone()

      sendToClient(
        client,
        {
          type: 'RESPONSE',
          payload: {
            requestId,
            isMockedResponse: IS_MOCKED_RESPONSE in response,
            type: responseClone.type,
            status: responseClone.status,
            statusText: responseClone.statusText,
            body: responseClone.body,
            headers: Object.fromEntries(responseClone.headers.entries()),
          },
        },
        [responseClone.body],
      )
    })()
  }

  return response
}

// Resolve the main client for the given event.
// Client that issues a request doesn't necessarily equal the client
// that registered the worker. It's with the latter the worker should
// communicate with during the response resolving phase.
async function resolveMainClient(event) {
  const client = await self.clients.get(event.clientId)

  if (client?.frameType === 'top-level') {
    return client
  }

  const allClients = await self.clients.matchAll({
    type: 'window',
  })

  return allClients
    .filter((client) => {
      // Get only those clients that are currently visible.
      return client.visibilityState === 'visible'
    })
    .find((client) => {
      // Find the client ID that's recorded in the
      // set of clients that have registered the worker.
      return activeClientIds.has(client.id)
    })
}

async function getResponse(event, client, requestId) {
  const { request } = event

  // Clone the request because it might've been already used
  // (i.e. its body has been read and sent to the client).
  const requestClone = request.clone()

  function passthrough() {
    const headers = Object.fromEntries(requestClone.headers.entries())

    // Remove internal MSW request header so the passthrough request
    // complies with any potential CORS preflight checks on the server.
    // Some servers forbid unknown request headers.
    delete headers['x-msw-intention']

    return fetch(requestClone, { headers })
  }

  // Bypass mocking when the client is not active.
  if (!client) {
    return passthrough()
  }

  // Bypass initial page load requests (i.e. static assets).
  // The absence of the immediate/parent client in the map of the active clients
  // means that MSW hasn't dispatched the "MOCK_ACTIVATE" event yet
  // and is not ready to handle requests.
  if (!activeClientIds.has(client.id)) {
    return passthrough()
  }

  // Notify the client that a request has been intercepted.
  const requestBuffer = await request.arrayBuffer()
  const clientMessage = await sendToClient(
    client,
    {
      type: 'REQUEST',
      payload: {
        id: requestId,
        url: request.url,
        mode: request.mode,
        method: request.method,
        headers: Object.fromEntries(request.headers.entries()),
        cache: request.cache,
        credentials: request.credentials,
        destination: request.destination,
        integrity: request.integrity,
        redirect: request.redirect,
        referrer: request.referrer,
        referrerPolicy: request.referrerPolicy,
        body: requestBuffer,
        keepalive: request.keepalive,
      },
    },
    [requestBuffer],
  )

  switch (clientMessage.type) {
    case 'MOCK_RESPONSE': {
      return respondWithMock(clientMessage.data)
    }

    case 'PASSTHROUGH': {
      return passthrough()
    }
  }

  return passthrough()
}

function sendToClient(client, message, transferrables = []) {
  return new Promise((resolve, reject) => {
    const channel = new MessageChannel()

    channel.port1.onmessage = (event) => {
      if (event.data && event.data.error) {
        return reject(event.data.error)
      }

      resolve(event.data)
    }

    client.postMessage(
      message,
      [channel.port2].concat(transferrables.filter(Boolean)),
    )
  })
}

async function respondWithMock(response) {
  // Setting response status code to 0 is a no-op.
  // However, when responding with a "Response.error()", the produced Response
  // instance will have status code set to 0. Since it's not possible to create
  // a Response instance with status code 0, handle that use-case separately.
  if (response.status === 0) {
    return Response.error()
  }

  const mockedResponse = new Response(response.body, response)

  Reflect.defineProperty(mockedResponse, IS_MOCKED_RESPONSE, {
    value: true,
    enumerable: true,
  })

  return mockedResponse
}

```

# モックファイルの作成
モック用途のファイルを2つ作成します。

まずは、```src/mocks/handler.js``` で REST API をモックします。
以下のファイルでは、`https://example.com/user` の `GET` リクエストが来たら `{
      id: "c7b3d8e0-5e0b-4b0f-8b3a-3b9f4b3d3b3d",
      firstName: "John",
      lastName: "Maverick",
    }` を返すようにしています。


```src/mocks/handler.js
import { http, HttpResponse } from "msw";

export const handlers = [
  // Intercept "GET https://example.com/user" requests...
  http.get("https://example.com/user", () => {
    // ...and respond to them using this JSON response.
    return HttpResponse.json({
      id: "c7b3d8e0-5e0b-4b0f-8b3a-3b9f4b3d3b3d",
      firstName: "John",
      lastName: "Maverick",
    });
  }),
];

```

次に ```src/mocks/browser.js``` で、先ほど作成した ```src/mocks/handler.js``` が Service Worker として利用できるようにします。

```src/mocks/browser.js
import { setupWorker } from "msw/browser";
import { handlers } from "./handlers";

export const worker = setupWorker(...handlers);

```

# モックの有効化
React を render する前に `worker.start()` を実行して、モックを有効化します。

```src/main.jsx
import React from "react";
import ReactDOM from "react-dom/client";
import App from "./App.jsx";
import "./index.css";

async function enableMocking() {
  const { worker } = await import("./mocks/browser");

  // `worker.start()` returns a Promise that resolves
  // once the Service Worker is up and ready to intercept requests.
  return worker.start();
}

enableMocking().then(() => {
  ReactDOM.createRoot(document.getElementById("root")).render(
    <React.StrictMode>
      <App />
    </React.StrictMode>
  );
});

```

# 動作確認
動作確認用として `src/App.jsx` にボタンをクリックしたらモックした REST API を呼び出す関数を作成します。

```src/App.jsx
import "./App.css";

function App() {
  const onClick = async () => {
    const response = await fetch("https://example.com/user");
    const jsonResponse = await response.json();
    console.log("🚀 ~ onClick ~ jsonResponse:", jsonResponse);
  };

  return (
    <>
      <button onClick={onClick}>User</button>
    </>
  );
}

export default App;

```

最後に動作確認をします。

![Vite-React-Google-Chrome-2024-06-07-20-43-23.gif](https://qiita-image-store.s3.ap-northeast-1.amazonaws.com/0/318188/b7b1287e-3f22-c823-ca98-66130407add0.gif)

Mock Service Worker が有効化されていると以下のログが表示されます。

```console
msw_browser.js?v=c5dc5108:230 [MSW] Mocking enabled.
```

また、ボタンクリック後、モックした値が表示されることも確認できました！

# 参考
- [Browser integration](https://mswjs.io/docs/integrations/browser)
- [Getting started](https://mswjs.io/docs/getting-started)