Add handling for invalid page config (vercel#7921)

errors/invalid-page-config.md

@@ -0,0 +1,26 @@
+# Invalid Page Config
+
+#### Why This Error Occurred
+
+In one of your pages you did `export const config` with an invalid value.
+
+#### Possible Ways to Fix It
+
+The page's config must be an object initialized directly when being exported.
+
+This is not allowed
+
+```js
+export const config = 'hello world'
+```
+
+This is allowed
+
+```js
+export const config = { amp: true }
+```
+
+### Useful Links
+
+- [Enabling AMP Support](https://github.com/zeit/next.js/#enabling-amp-support)
+- [API Middlewares](https://github.com/zeit/next.js/#api-middlewares)

packages/next/build/babel/plugins/next-page-config.ts

@@ -60,6 +60,18 @@ export default function nextPageConfig({
                 for (const declaration of declarations) {
                   if (declaration.id.name !== 'config') continue
 
+                  if (declaration.init.type !== 'ObjectExpression') {
+                    const pageName =
+                      (state.filename || '').split(state.cwd || '').pop() ||
+                      'unknown'
+
+                    throw new Error(
+                      `Invalid page config export found. Expected object but got ${
+                        declaration.init.type
+                      } in file ${pageName}. See: https://err.sh/zeit/next.js/invalid-page-config`
+                    )
+                  }
+
                   for (const prop of declaration.init.properties) {
                     const { name } = prop.key
                     if (configKeys.has(name)) {

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

@@ -1,6 +1,8 @@
 import { config as hello } from '../something'
 import { config as world } from '../config'
 
+// export const config = 'hello world'
+
 export default () => (
   <p>
     {hello} {world}

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

@@ -1,14 +1,32 @@
 /* eslint-env jest */
 /* global jasmine */
+import fs from 'fs-extra'
 import { join } from 'path'
 import { nextBuild } from 'next-test-utils'
 
 const appDir = join(__dirname, '..')
+const indexPage = join(appDir, 'pages/index.js')
+
 jasmine.DEFAULT_TIMEOUT_INTERVAL = 1000 * 60 * 2
 
 describe('Page Config', () => {
   it('builds without error when export const config is used outside page', async () => {
     const { stderr } = await nextBuild(appDir, undefined, { stderr: true })
     expect(stderr).not.toMatch(/Failed to compile\./)
   })
+
+  it('shows valid error on invalid page config', async () => {
+    const origContent = await fs.readFile(indexPage, 'utf8')
+    const newContent = origContent.replace('// export', 'export')
+    await fs.writeFile(indexPage, newContent, 'utf8')
+
+    try {
+      const { stderr } = await nextBuild(appDir, undefined, { stderr: true })
+      expect(stderr).toMatch(
+        /https:\/\/err\.sh\/zeit\/next\.js\/invalid-page-config/
+      )
+    } finally {
+      await fs.writeFile(indexPage, origContent, 'utf8')
+    }
+  })
 })