Implement experimental pages/404.js for custom 404 page

errors/404-get-initial-props.md

@@ -0,0 +1,15 @@
+# 404.js Cannot Have getInitialProps
+
+#### Why This Error Occurred
+
+In your `404.js` page you added `getInitialProps` or `getServerProps` which is not allowed as this prevents the page from being static and `404.js` is meant to provide more flexibility for a static 404 page. Having a static 404 page is the most ideal as this page can be served very often and if not static puts extra strain on your server and more invocations for serverless functions which increase the cost of hosting your site unnecessarily.
+
+#### Possible Ways to Fix It
+
+Remove `getInitialProps` from `404.js` and make sure no HOC's used in `404.js` attach `getInitialProps`.
+
+If you want to fetch data for your `404.js` page move it to a client side fetch inside of `componentDidMount` or `useEffect(() => {}, [])`.
+
+### Useful Links
+
+- [Automatic Static Optimization](https://nextjs.org/docs/advanced-features/automatic-static-optimization)

packages/next/build/index.ts

@@ -16,7 +16,10 @@ import checkCustomRoutes, {
   Rewrite,
   Header,
 } from '../lib/check-custom-routes'
-import { PUBLIC_DIR_MIDDLEWARE_CONFLICT } from '../lib/constants'
+import {
+  PUBLIC_DIR_MIDDLEWARE_CONFLICT,
+  PAGES_404_GET_INITIAL_PROPS_ERROR,
+} from '../lib/constants'
 import { findPagesDir } from '../lib/find-pages-dir'
 import { recursiveDelete } from '../lib/recursive-delete'
 import { recursiveReadDir } from '../lib/recursive-readdir'
@@ -202,6 +205,10 @@ export default async function build(dir: string, conf = null): Promise<void> {
   const hasCustomErrorPage = mappedPages['/_error'].startsWith(
     'private-next-pages'
   )
+  const hasPages404 =
+    config.experimental.pages404 &&
+    mappedPages['/404'] &&
+    mappedPages['/404'].startsWith('private-next-pages')
 
   if (hasPublicDir) {
     try {
@@ -262,6 +269,7 @@ export default async function build(dir: string, conf = null): Promise<void> {
   const routesManifestPath = path.join(distDir, ROUTES_MANIFEST)
   const routesManifest: any = {
     version: 1,
+    pages404: !!hasPages404,
     basePath: config.experimental.basePath,
     redirects: redirects.map(r => buildCustomRoute(r, 'redirect')),
     rewrites: rewrites.map(r => buildCustomRoute(r, 'rewrite')),
@@ -511,6 +519,17 @@ export default async function build(dir: string, conf = null): Promise<void> {
             staticPages.add(page)
             isStatic = true
           }
+
+          if (hasPages404 && page === '/404') {
+            if (!result.isStatic) {
+              throw new Error(PAGES_404_GET_INITIAL_PROPS_ERROR)
+            }
+            // we need to ensure the 404 lambda is present since we use
+            // it when _app has getInitialProps
+            if (customAppGetInitialProps) {
+              staticPages.delete(page)
+            }
+          }
         } catch (err) {
           if (err.message !== 'INVALID_DEFAULT_EXPORT') throw err
           invalidPages.add(page)
@@ -569,8 +588,7 @@ export default async function build(dir: string, conf = null): Promise<void> {
   // Only export the static 404 when there is no /_error present
   const useStatic404 =
     !customAppGetInitialProps &&
-    !hasCustomErrorPage &&
-    config.experimental.static404
+    ((!hasCustomErrorPage && config.experimental.static404) || hasPages404)
 
   if (invalidPages.size > 0) {
     throw new Error(
@@ -640,7 +658,9 @@ export default async function build(dir: string, conf = null): Promise<void> {
         })
 
         if (useStatic404) {
-          defaultMap['/_errors/404'] = { page: '/_error' }
+          defaultMap['/_errors/404'] = {
+            page: hasPages404 ? '/404' : '/_error',
+          }
         }
 
         return defaultMap

packages/next/lib/constants.ts

@@ -29,3 +29,5 @@ export const SSG_GET_INITIAL_PROPS_CONFLICT = `You can not use getInitialProps w
 export const SERVER_PROPS_GET_INIT_PROPS_CONFLICT = `You can not use getInitialProps with unstable_getServerProps. Please remove one or the other`
 
 export const SERVER_PROPS_SSG_CONFLICT = `You can not use unstable_getStaticProps with unstable_getServerProps. To use SSG, please remove your unstable_getServerProps`
+
+export const PAGES_404_GET_INITIAL_PROPS_ERROR = `\`pages/404\` can not have getInitialProps/getServerProps, https://err.sh/zeit/next.js/404-get-initial-props`

packages/next/next-server/server/config.ts

@@ -53,6 +53,7 @@ const defaultConfig: { [key: string]: any } = {
     workerThreads: false,
     basePath: '',
     static404: true,
+    pages404: false,
   },
   future: {
     excludeDefaultMomentLocales: false,

packages/next/next-server/server/next-server.ts

@@ -101,6 +101,7 @@ export default class Server {
     documentMiddlewareEnabled: boolean
     hasCssMode: boolean
     dev?: boolean
+    pages404?: boolean
   }
   private compression?: Middleware
   private onErrorMiddleware?: ({ err }: { err: Error }) => Promise<void>
@@ -148,6 +149,7 @@ export default class Server {
       staticMarkup,
       buildId: this.buildId,
       generateEtags,
+      pages404: this.nextConfig.experimental.pages404,
     }
 
     // Only the `publicRuntimeConfig` key is exposed to the client side
@@ -857,6 +859,11 @@ export default class Server {
     result: LoadComponentsReturnType,
     opts: any
   ): Promise<string | null> {
+    // we need to ensure the status code if /404 is visited directly
+    if (this.nextConfig.experimental.pages404 && pathname === '/404') {
+      res.statusCode = 404
+    }
+
     // handle static page
     if (typeof result.Component === 'string') {
       return result.Component
@@ -1115,13 +1122,32 @@ export default class Server {
   ) {
     let result: null | LoadComponentsReturnType = null
 
+    const { static404, pages404 } = this.nextConfig.experimental
+    const is404 = res.statusCode === 404
+    let using404Page = false
+
     // use static 404 page if available and is 404 response
-    if (this.nextConfig.experimental.static404 && res.statusCode === 404) {
-      try {
-        result = await this.findPageComponents('/_errors/404')
-      } catch (err) {
-        if (err.code !== 'ENOENT') {
-          throw err
+    if (is404) {
+      if (static404) {
+        try {
+          result = await this.findPageComponents('/_errors/404')
+        } catch (err) {
+          if (err.code !== 'ENOENT') {
+            throw err
+          }
+        }
+      }
+
+      // use 404 if /_errors/404 isn't available which occurs
+      // during development and when _app has getInitialProps
+      if (!result && pages404) {
+        try {
+          result = await this.findPageComponents('/404')
+          using404Page = true
+        } catch (err) {
+          if (err.code !== 'ENOENT') {
+            throw err
+          }
         }
       }
     }
@@ -1135,7 +1161,7 @@ export default class Server {
       html = await this.renderToHTMLWithComponents(
         req,
         res,
-        '/_error',
+        using404Page ? '/404' : '/_error',
         query,
         result,
         {

packages/next/next-server/server/render.tsx

@@ -28,6 +28,7 @@ import {
   SSG_GET_INITIAL_PROPS_CONFLICT,
   SERVER_PROPS_GET_INIT_PROPS_CONFLICT,
   SERVER_PROPS_SSG_CONFLICT,
+  PAGES_404_GET_INITIAL_PROPS_ERROR,
 } from '../../lib/constants'
 import { AMP_RENDER_TARGET } from '../lib/constants'
 import { LoadComponentsReturnType, ManifestItem } from './load-components'
@@ -135,6 +136,7 @@ type RenderOpts = LoadComponentsReturnType & {
   documentMiddlewareEnabled?: boolean
   isDataReq?: boolean
   params?: ParsedUrlQuery
+  pages404?: boolean
 }
 
 function renderDocument(
@@ -263,6 +265,7 @@ export async function renderToHTML(
     unstable_getServerProps,
     isDataReq,
     params,
+    pages404,
   } = renderOpts
 
   const callMiddleware = async (method: string, args: any[], props = false) => {
@@ -363,6 +366,10 @@ export async function renderToHTML(
       req.url = pathname
       renderOpts.nextExport = true
     }
+
+    if (pages404 && pathname === '/404' && !isAutoExport) {
+      throw new Error(PAGES_404_GET_INITIAL_PROPS_ERROR)
+    }
   }
   if (isAutoExport) renderOpts.autoExport = true
   if (isSpr) renderOpts.nextExport = false

packages/next/server/next-dev-server.ts

@@ -432,6 +432,16 @@ export default class DevServer extends Server {
       })
     } catch (err) {
       if (err.code === 'ENOENT') {
+        if (this.nextConfig.experimental.pages404) {
+          try {
+            await this.hotReloader!.ensurePage('/404')
+          } catch (err) {
+            if (err.code !== 'ENOENT') {
+              throw err
+            }
+          }
+        }
+
         res.statusCode = 404
         return this.renderErrorToHTML(null, req, res, pathname, query)
       }

test/integration/404-page/next.config.js

@@ -0,0 +1,5 @@
+module.exports = {
+  experimental: {
+    pages404: true,
+  },
+}

test/integration/404-page/pages/404.js

@@ -0,0 +1,2 @@
+const page = () => 'custom 404 page'
+export default page

test/integration/404-page/pages/err.js

@@ -0,0 +1,5 @@
+const page = () => 'custom 404 page'
+page.getInitialProps = () => {
+  throw new Error('oops')
+}
+export default page

test/integration/404-page/pages/index.js

@@ -0,0 +1 @@
+export default () => 'hello from index'