fix: do not error when not enabled during suspense (vercel#4156)

Author: huozhi

e2e/site/app/suspense-undefined-key/page.tsx

@@ -0,0 +1,30 @@
+'use client'
+
+import { Suspense, useReducer } from 'react'
+import useSWR from 'swr'
+
+const fetcher = async (key: string) => {
+  // Add a small delay to simulate network request
+  await new Promise(resolve => setTimeout(resolve, 100))
+  return 'SWR'
+}
+
+const Section = ({ trigger }: { trigger: boolean }) => {
+  const { data } = useSWR(trigger ? 'test-key' : undefined, fetcher, {
+    suspense: true
+  })
+  return <div>{data || 'empty'}</div>
+}
+
+export default function Page() {
+  const [trigger, toggle] = useReducer(x => !x, false)
+
+  return (
+    <div>
+      <button onClick={toggle}>toggle</button>
+      <Suspense fallback={<div>fallback</div>}>
+        <Section trigger={trigger} />
+      </Suspense>
+    </div>
+  )
+}

e2e/test/suspense-undefined-key.test.ts

@@ -0,0 +1,20 @@
+/* eslint-disable testing-library/prefer-screen-queries */
+import { test, expect } from '@playwright/test'
+
+test.describe('suspense with undefined key', () => {
+  test('should render correctly when key is undefined', async ({ page }) => {
+    await page.goto('./suspense-undefined-key', { waitUntil: 'commit' })
+
+    // Should show content for undefined key (not suspense)
+    await expect(page.getByText('empty')).toBeVisible()
+
+    // Click toggle to enable key
+    await page.getByRole('button', { name: 'toggle' }).click()
+
+    // Should show loading fallback when key becomes defined
+    await expect(page.getByText('fallback')).toBeVisible()
+
+    // Should eventually show the fetched data
+    await expect(page.getByText('SWR')).toBeVisible()
+  })
+})

src/index/use-swr.ts

@@ -714,14 +714,14 @@ export const useSWRHandler = <Data = any, Error = any>(
   // If there is no `error`, the `revalidation` promise needs to be thrown to
   // the suspense boundary.
   if (suspense) {
+    const hasKeyButNoData = key && isUndefined(data)
     // SWR should throw when trying to use Suspense on the server with React 18,
     // without providing any fallback data. This causes hydration errors. See:
     // https://github.com/vercel/swr/issues/1832
-    if (!IS_REACT_LEGACY && IS_SERVER) {
+    if (!IS_REACT_LEGACY && IS_SERVER && hasKeyButNoData) {
       throw new Error('Fallback data is required when using Suspense in SSR.')
     }
 
-    const hasKeyButNoData = key && isUndefined(data)
     // Always update fetcher and config refs even with the Suspense mode.
     if (hasKeyButNoData) {
       fetcherRef.current = fetcher