Render a helpful message for null GS(S)P return (#14252)

This makes Next.js render a better error message when `undefined` (or null) is returned from `getStaticProps` or `getServerSideProps`.

---

Fixes #11139

packages/next/lib/constants.ts

@@ -34,6 +34,11 @@ export const PAGES_404_GET_INITIAL_PROPS_ERROR = `\`pages/404\` can not have get
 
 export const SERVER_PROPS_EXPORT_ERROR = `pages with \`getServerSideProps\` can not be exported. See more info here: https://err.sh/next.js/gssp-export`
 
+export const GSP_NO_RETURNED_VALUE =
+  'Your `getStaticProps` function did not return an object. Did you forget to add a `return`?'
+export const GSSP_NO_RETURNED_VALUE =
+  'Your `getServerSideProps` function did not return an object. Did you forget to add a `return`?'
+
 export const UNSTABLE_REVALIDATE_RENAME_ERROR =
   'The `revalidate` property is not yet available for general use.\n' +
   'To try the experimental implementation, please use `unstable_revalidate` instead.\n' +

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

@@ -2,26 +2,30 @@ import { IncomingMessage, ServerResponse } from 'http'
 import { ParsedUrlQuery } from 'querystring'
 import React from 'react'
 import { renderToStaticMarkup, renderToString } from 'react-dom/server'
+import { UnwrapPromise } from '../../lib/coalesced-function'
 import {
+  GSP_NO_RETURNED_VALUE,
+  GSSP_COMPONENT_MEMBER_ERROR,
+  GSSP_NO_RETURNED_VALUE,
   PAGES_404_GET_INITIAL_PROPS_ERROR,
   SERVER_PROPS_GET_INIT_PROPS_CONFLICT,
   SERVER_PROPS_SSG_CONFLICT,
   SSG_GET_INITIAL_PROPS_CONFLICT,
   UNSTABLE_REVALIDATE_RENAME_ERROR,
-  GSSP_COMPONENT_MEMBER_ERROR,
 } from '../../lib/constants'
 import { isSerializableProps } from '../../lib/is-serializable-props'
+import { GetServerSideProps, GetStaticProps } from '../../types'
 import { isInAmpMode } from '../lib/amp'
 import { AmpStateContext } from '../lib/amp-context'
 import {
   AMP_RENDER_TARGET,
-  STATIC_PROPS_ID,
   SERVER_PROPS_ID,
+  STATIC_PROPS_ID,
 } from '../lib/constants'
 import { defaultHead } from '../lib/head'
+import { HeadManagerContext } from '../lib/head-manager-context'
 import Loadable from '../lib/loadable'
 import { LoadableContext } from '../lib/loadable-context'
-import { HeadManagerContext } from '../lib/head-manager-context'
 import mitt, { MittEmitter } from '../lib/mitt'
 import { RouterContext } from '../lib/router-context'
 import { NextRouter } from '../lib/router/router'
@@ -41,8 +45,6 @@ import { tryGetPreviewData, __ApiPreviewProps } from './api-utils'
 import { getPageFiles } from './get-page-files'
 import { LoadComponentsReturnType, ManifestItem } from './load-components'
 import optimizeAmp from './optimize-amp'
-import { UnwrapPromise } from '../../lib/coalesced-function'
-import { GetStaticProps, GetServerSideProps } from '../../types'
 
 function noRouter() {
   const message =
@@ -515,6 +517,10 @@ export async function renderToHTML(
         throw staticPropsError
       }
 
+      if (data == null) {
+        throw new Error(GSP_NO_RETURNED_VALUE)
+      }
+
       const invalidKeys = Object.keys(data).filter(
         (key) => key !== 'unstable_revalidate' && key !== 'props'
       )
@@ -601,6 +607,10 @@ export async function renderToHTML(
         throw serverSidePropsError
       }
 
+      if (data == null) {
+        throw new Error(GSSP_NO_RETURNED_VALUE)
+      }
+
       const invalidKeys = Object.keys(data).filter((key) => key !== 'props')
 
       if (invalidKeys.length) {

test/integration/ssg-component-members-error/test/index.test.js → test/integration/data-fetching-errors/test/index.test.js

 /* eslint-env jest */
 
 import fs from 'fs-extra'
-import { join } from 'path'
 import {
-  nextBuild,
-  launchApp,
   findPort,
-  renderViaHTTP,
   killApp,
+  launchApp,
+  nextBuild,
+  renderViaHTTP,
 } from 'next-test-utils'
-
+import { join } from 'path'
+import {
+  GSP_NO_RETURNED_VALUE,
+  GSSP_NO_RETURNED_VALUE,
+} from '../../../../packages/next/dist/lib/constants'
 jest.setTimeout(1000 * 60 * 2)
 
 const appDir = join(__dirname, '..')
@@ -88,9 +91,37 @@ const runTests = (isDev = false) => {
       `getStaticPaths can not be attached to a page's component and must be exported from the page`
     )
   })
+
+  it('should show error for undefined getStaticProps', async () => {
+    await fs.writeFile(
+      indexPage,
+      `
+        export function getStaticProps() {}
+        export default function Page() {
+          return <div />;
+        }
+      `
+    )
+    expect(await getStderr()).toContain(GSP_NO_RETURNED_VALUE)
+  })
+
+  if (isDev) {
+    it('should show error for undefined getServerSideProps', async () => {
+      await fs.writeFile(
+        indexPage,
+        `
+          export function getServerSideProps() {}
+          export default function Page() {
+            return <div />;
+          }
+        `
+      )
+      expect(await getStderr()).toContain(GSSP_NO_RETURNED_VALUE)
+    })
+  }
 }
 
-describe('GSSP Page Component Member Error', () => {
+describe('GS(S)P Page Errors', () => {
   beforeAll(async () => {
     origIndexPage = await fs.readFile(indexPage, 'utf8')
   })