fix: 9919 Add warning when no config is exported from next.con… (verc…

…el#10228)

* fix: 9919 no exported config found

* fix: 9919 remove isolated test, add integration

* fix: 9919 add check for successfull compilation and fix warnin check

* Add test for development output

* fix: 9919 add error page and link to it in warning

* Update empty-configuration.md

Co-authored-by: JJ Kasper <jj@jjsweb.site>
Co-authored-by: Tim Neutkens <tim@timneutkens.nl>

errors/empty-configuration.md

@@ -0,0 +1,19 @@
+# Detected next.config.js, no exported configuration found
+
+#### Why This Warning Occurred
+
+There is no object exported from next.config.js or the object is empty.
+
+#### Possible Ways to Fix It
+
+Check if you correctly export configuration in `next.config.js` file:
+
+```
+module.exports = {
+  /* config options here */
+}
+```
+
+### Useful Links
+
+- [Introduction to next.config.js](https://nextjs.org/docs/api-reference/next.config.js/introduction)

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

@@ -229,6 +229,14 @@ export default function loadConfig(
       phase,
       userConfigModule.default || userConfigModule
     )
+
+    if (Object.keys(userConfig).length === 0) {
+      console.warn(
+        chalk.yellow.bold('Warning: ') +
+          'Detected next.config.js, no exported configuration found. https://err.sh/zeit/next.js/empty-configuration'
+      )
+    }
+
     if (userConfig.target && !targets.includes(userConfig.target)) {
       throw new Error(
         `Specified target is invalid. Provided: "${

test/integration/config-empty/next.config.js

@@ -0,0 +1,6 @@
+/* eslint-disable */
+{
+  experimental: {
+    basePath: '/docs'
+  }
+}

test/integration/config-empty/pages/index.js

@@ -0,0 +1 @@
+export default () => <div>Hello World</div>

test/integration/config-empty/test/index.test.js

@@ -0,0 +1,44 @@
+/* eslint-env jest */
+/* global jasmine */
+import { join } from 'path'
+import {
+  nextBuild,
+  launchApp,
+  findPort,
+  killApp,
+  waitFor,
+} from 'next-test-utils'
+
+jasmine.DEFAULT_TIMEOUT_INTERVAL = 1000 * 60 * 2
+
+const appDir = join(__dirname, '..')
+
+describe('Empty configuration', () => {
+  it('should show relevant warning and compile successfully for next build', async () => {
+    const { stderr, stdout } = await nextBuild(appDir, [], {
+      stderr: true,
+      stdout: true,
+    })
+    expect(stdout).toMatch(/Compiled successfully./)
+    expect(stderr).toMatch(
+      /Warning: Detected next.config.js, no exported configuration found. https:\/\/err.sh\/zeit\/next.js\/empty-configuration/
+    )
+  })
+
+  it('should show relevant warning and compile successfully for next dev', async () => {
+    let stderr = ''
+
+    const appPort = await findPort()
+    const app = await launchApp(appDir, appPort, {
+      onStderr(msg) {
+        stderr += msg || ''
+      },
+    })
+    await waitFor(1000)
+    await killApp(app)
+
+    expect(stderr).toMatch(
+      /Warning: Detected next.config.js, no exported configuration found. https:\/\/err.sh\/zeit\/next.js\/empty-configuration/
+    )
+  })
+})