fix: setting assetPrefix to URL format breaks HMR

docs/02-app/02-api-reference/05-next-config-js/assetPrefix.mdx

@@ -23,16 +23,25 @@ description: Learn how to use the assetPrefix config option to configure your CD
 > suited for hosting your application on a sub-path like `/docs`.
 > We do not suggest you use a custom Asset Prefix for this use case.
 
-To set up a [CDN](https://en.wikipedia.org/wiki/Content_delivery_network), you can set up an asset prefix and configure your CDN's origin to resolve to the domain that Next.js is hosted on.
-
-Open `next.config.js` and add the `assetPrefix` config:
+## Set up a CDN
 
-```js filename="next.config.js"
-const isProd = process.env.NODE_ENV === 'production'
+To set up a [CDN](https://en.wikipedia.org/wiki/Content_delivery_network), you can set up an asset prefix and configure your CDN's origin to resolve to the domain that Next.js is hosted on.
 
-module.exports = {
-  // Use the CDN in production and localhost for development.
-  assetPrefix: isProd ? 'https://cdn.mydomain.com' : undefined,
+Open `next.config.mjs` and add the `assetPrefix` config based on the [phase](/docs/app/api-reference/next-config-js#async-configuration):
+
+```js filename="next.config.mjs"
+// @ts-check
+import { PHASE_DEVELOPMENT_SERVER } from 'next/constants'
+
+export default (phase) => {
+  const isDev = phase === PHASE_DEVELOPMENT_SERVER
+  /**
+   * @type {import('next').NextConfig}
+   */
+  const nextConfig = {
+    assetPrefix: isDev ? undefined : 'https://cdn.mydomain.com',
+  }
+  return nextConfig
 }
 ```
 

packages/next/src/client/components/react-dev-overlay/internal/helpers/get-socket-url.ts

@@ -8,19 +8,19 @@ function getSocketProtocol(assetPrefix: string): string {
     protocol = new URL(assetPrefix).protocol
   } catch {}
 
-  return protocol === 'http:' ? 'ws' : 'wss'
+  return protocol === 'http:' ? 'ws:' : 'wss:'
 }
 
 export function getSocketUrl(assetPrefix: string | undefined): string {
-  const { hostname, port } = window.location
-  const protocol = getSocketProtocol(assetPrefix || '')
   const prefix = normalizedAssetPrefix(assetPrefix)
+  const protocol = getSocketProtocol(assetPrefix || '')
 
-  // if original assetPrefix is a full URL with protocol
-  // we just update to use the correct `ws` protocol
-  if (assetPrefix?.replace(/^\/+/, '').includes('://')) {
-    return `${protocol}://${prefix}`
+  if (URL.canParse(prefix)) {
+    // since normalized asset prefix is ensured to be a URL format,
+    // we can safely replace the protocol
+    return prefix.replace(/^http/, 'ws')
   }
 
-  return `${protocol}://${hostname}:${port}${prefix}`
+  const { hostname, port } = window.location
+  return `${protocol}//${hostname}${port ? `:${port}` : ''}${prefix}`
 }

packages/next/src/server/lib/router-server.ts

@@ -657,7 +657,15 @@ export async function initialize(opts: {
         // assetPrefix overrides basePath for HMR path
         if (assetPrefix) {
           hmrPrefix = normalizedAssetPrefix(assetPrefix)
+
+          if (URL.canParse(hmrPrefix)) {
+            // remove trailing slash from pathname
+            // return empty string if pathname is '/'
+            // to avoid conflicts with '/_next' below
+            hmrPrefix = new URL(hmrPrefix).pathname.replace(/\/$/, '')
+          }
         }
+
         const isHMRRequest = req.url.startsWith(
           ensureLeadingSlash(`${hmrPrefix}/_next/webpack-hmr`)
         )

packages/next/src/shared/lib/normalized-asset-prefix.test.ts

@@ -0,0 +1,44 @@
+import { normalizedAssetPrefix } from './normalized-asset-prefix'
+
+describe('normalizedAssetPrefix', () => {
+  it('should return an empty string when assetPrefix is nullish', () => {
+    expect(normalizedAssetPrefix(undefined)).toBe('')
+  })
+
+  it('should return an empty string when assetPrefix is an empty string', () => {
+    expect(normalizedAssetPrefix('')).toBe('')
+  })
+
+  it('should return an empty string when assetPrefix is a single slash', () => {
+    expect(normalizedAssetPrefix('/')).toBe('')
+  })
+
+  // we expect an empty string because it could be an unnecessary trailing slash
+  it('should remove leading slash(es) when assetPrefix has more than one', () => {
+    expect(normalizedAssetPrefix('///path/to/asset')).toBe('/path/to/asset')
+  })
+
+  it('should not remove the leading slash when assetPrefix has only one', () => {
+    expect(normalizedAssetPrefix('/path/to/asset')).toBe('/path/to/asset')
+  })
+
+  it('should add a leading slash when assetPrefix is missing one', () => {
+    expect(normalizedAssetPrefix('path/to/asset')).toBe('/path/to/asset')
+  })
+
+  it('should remove all trailing slash(es) when assetPrefix has one', () => {
+    expect(normalizedAssetPrefix('/path/to/asset///')).toBe('/path/to/asset')
+  })
+
+  it('should return the URL when assetPrefix is a URL', () => {
+    expect(normalizedAssetPrefix('https://example.com/path/to/asset')).toBe(
+      'https://example.com/path/to/asset'
+    )
+  })
+
+  it('should not leave a trailing slash when assetPrefix is a URL with no pathname', () => {
+    expect(normalizedAssetPrefix('https://example.com')).toBe(
+      'https://example.com'
+    )
+  })
+})

packages/next/src/shared/lib/normalized-asset-prefix.ts

@@ -1,16 +1,19 @@
 export function normalizedAssetPrefix(assetPrefix: string | undefined): string {
-  const escapedAssetPrefix = assetPrefix?.replace(/^\/+/, '') || false
+  // remove all leading slashes and trailing slashes
+  const escapedAssetPrefix = assetPrefix?.replace(/^\/+|\/+$/g, '') || false
 
-  // assetPrefix as a url
-  if (escapedAssetPrefix && escapedAssetPrefix.startsWith('://')) {
-    return escapedAssetPrefix.split('://', 2)[1]
-  }
-
-  // assetPrefix is set to `undefined` or '/'
+  // if an assetPrefix was '/', we return empty string
+  // because it could be an unnecessary trailing slash
   if (!escapedAssetPrefix) {
     return ''
   }
 
-  // assetPrefix is a common path but escaped so let's add one leading slash
+  if (URL.canParse(escapedAssetPrefix)) {
+    const url = new URL(escapedAssetPrefix).toString()
+    return url.endsWith('/') ? url.slice(0, -1) : url
+  }
+
+  // assuming assetPrefix here is a pathname-style,
+  // restore the leading slash
   return `/${escapedAssetPrefix}`
 }

test/development/app-dir/hmr-asset-prefix-full-url/app/layout.tsx

@@ -0,0 +1,7 @@
+export default function Root({ children }) {
+  return (
+    <html>
+      <body>{children}</body>
+    </html>
+  )
+}

test/development/app-dir/hmr-asset-prefix-full-url/app/page.tsx

@@ -0,0 +1,3 @@
+export default function Page() {
+  return <p>before edit</p>
+}

test/development/app-dir/hmr-asset-prefix-full-url/asset-prefix.test.ts

@@ -0,0 +1,32 @@
+import { createNext } from 'e2e-utils'
+import { findPort, retry } from 'next-test-utils'
+
+describe('app-dir assetPrefix full URL', () => {
+  let next, forcedPort
+  beforeAll(async () => {
+    forcedPort = ((await findPort()) ?? '54321').toString()
+
+    next = await createNext({
+      files: __dirname,
+      forcedPort,
+      nextConfig: {
+        assetPrefix: `http://localhost:${forcedPort}`,
+      },
+    })
+  })
+  afterAll(() => next.destroy())
+
+  it('should not break HMR when asset prefix set to full URL', async () => {
+    const browser = await next.browser('/')
+    const text = await browser.elementByCss('p').text()
+    expect(text).toBe('before edit')
+
+    await next.patchFile('app/page.tsx', (content) => {
+      return content.replace('before', 'after')
+    })
+
+    await retry(async () => {
+      expect(await browser.elementByCss('p').text()).toBe('after edit')
+    })
+  })
+})