Show a better error when someone throws undefined (#6646)

* Show a better error when someone throws undefined

* Update error wording

Co-Authored-By: ijjk <22380829+ijjk@users.noreply.github.com>

* Update error wording in test

Co-Authored-By: ijjk <22380829+ijjk@users.noreply.github.com>

* Update test and add check for statusCode
before updating error

package.json

@@ -51,7 +51,8 @@
       "packages/next/build/webpack/plugins/terser-webpack-plugin/**",
       "examples/with-ioc/**",
       "examples/with-kea/**",
-      "examples/with-mobx/**"
+      "examples/with-mobx/**",
+      "test/integration/basic/pages/throw-undefined.js"
     ]
   },
   "devDependencies": {

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

@@ -307,7 +307,7 @@ export default class Server {
       )
       return html
     } catch (err) {
-      if (err.code === 'ENOENT') {
+      if (err && err.code === 'ENOENT') {
         res.statusCode = 404
         return this.renderErrorToHTML(null, req, res, pathname, query)
       } else {

packages/next/server/next-dev-server.js

@@ -120,6 +120,10 @@ export default class DevServer extends Server {
       return super.renderErrorToHTML(compilationErr, req, res, pathname, query)
     }
 
+    if (!err && res.statusCode === 500) {
+      err = new Error('An undefined error was thrown sometime during render...')
+    }
+
     try {
       const out = await super.renderErrorToHTML(err, req, res, pathname, query)
       return out

test/integration/basic/pages/throw-undefined.js

@@ -0,0 +1,11 @@
+function ThrowUndefined (props) {
+  return (
+    <div>throw-undefined</div>
+  )
+}
+
+ThrowUndefined.getInitialProps = () => {
+  throw undefined
+}
+
+export default ThrowUndefined

test/integration/basic/test/rendering.js

@@ -289,5 +289,10 @@ export default function ({ app }, suiteName, render, fetch) {
         expect($('#asPath').text()).toBe('Current asPath: /nav/with-hoc')
       })
     })
+
+    it('should show a valid error when undefined is thrown', async () => {
+      const $ = await get$('/throw-undefined')
+      expect($('body').text()).toMatch('An undefined error was thrown sometime during render...')
+    })
   })
 }