feat: do not interfere with shared workers

src/mockServiceWorker.js

@@ -174,7 +174,7 @@ async function handleRequest(event, requestId) {
 async function resolveMainClient(event) {
   const client = await self.clients.get(event.clientId)
 
-  if (client.frameType === 'top-level') {
+  if (client?.frameType === 'top-level') {
     return client
   }
 

test/msw-api/setup-worker/scenarios/shared-worker/shared-worker.mocks.ts

@@ -0,0 +1,5 @@
+import { setupWorker } from 'msw'
+
+const worker = setupWorker()
+
+worker.start()

test/msw-api/setup-worker/scenarios/shared-worker/shared-worker.test.ts

@@ -0,0 +1,33 @@
+import * as path from 'path'
+import { pageWith } from 'page-with'
+import { waitFor } from '../../../../support/waitFor'
+
+function createRuntime() {
+  return pageWith({
+    example: path.resolve(__dirname, 'shared-worker.mocks.ts'),
+    contentBase: path.resolve(__dirname),
+  })
+}
+
+test('does not interfere with a shared worker', async () => {
+  const { page, consoleSpy } = await createRuntime()
+
+  await page.evaluate(() => {
+    const worker = new SharedWorker('/worker.js')
+
+    worker.addEventListener('error', () =>
+      console.error('There is an error with worker'),
+    )
+
+    worker.port.onmessage = (event) => {
+      console.log(event.data)
+    }
+
+    worker.port.postMessage('john')
+  })
+
+  await waitFor(() => {
+    expect(consoleSpy.get('error')).toBeUndefined()
+    expect(consoleSpy.get('log')).toContain('hello, john')
+  })
+})

test/msw-api/setup-worker/scenarios/shared-worker/worker.js

@@ -0,0 +1,7 @@
+onconnect = (event) => {
+  const port = event.ports[0]
+
+  port.onmessage = (event) => {
+    port.postMessage(`hello, ${event.data}`)
+  }
+}