ref(nextjs): Don't assert existance of pageProps in _app (#5945)

Co-authored-by: Lukas Stracke <lukas.stracke@sentry.io>

packages/nextjs/src/config/wrappers/withSentryServerSideAppGetInitialProps.ts

@@ -45,6 +45,15 @@ export function withSentryServerSideAppGetInitialProps(origAppGetInitialProps: A
       });
 
       const requestTransaction = getTransactionFromRequest(req);
+
+      // Per definition, `pageProps` is not optional, however an increased amount of users doesn't seem to call
+      // `App.getInitialProps(appContext)` in their custom `_app` pages which is required as per
+      // https://nextjs.org/docs/advanced-features/custom-app - resulting in missing `pageProps`.
+      // For this reason, we just handle the case where `pageProps` doesn't exist explicitly.
+      if (!appGetInitialProps.pageProps) {
+        appGetInitialProps.pageProps = {};
+      }
+
       if (requestTransaction) {
         appGetInitialProps.pageProps._sentryTraceData = requestTransaction.toTraceparent();
 

packages/nextjs/test/integration/pages/_app.tsx

@@ -0,0 +1,18 @@
+import App, { AppContext, AppProps } from 'next/app';
+
+const MyApp = ({ Component, pageProps }: AppProps) => {
+  return <Component {...pageProps} />;
+};
+
+MyApp.getInitialProps = async (appContext: AppContext) => {
+  // This simulates user misconfiguration. Users should always call `App.getInitialProps(appContext)`, but they don't,
+  // so we have a test for this so we don't break their apps.
+  if (appContext.ctx.pathname === '/faultyAppGetInitialProps') {
+    return {};
+  }
+
+  const appProps = await App.getInitialProps(appContext);
+  return { ...appProps };
+};
+
+export default MyApp;

packages/nextjs/test/integration/pages/faultyAppGetInitialProps.tsx

@@ -0,0 +1,4 @@
+// See _app.tsx for more information why this file exists.
+const Page = (): JSX.Element => <h1>Hello World!</h1>;
+
+export default Page;

packages/nextjs/test/integration/test/client/faultyAppGetInitialPropsConfiguration.js

@@ -0,0 +1,11 @@
+const expect = require('expect');
+
+// This test verifies that a faulty configuration of `getInitialProps` in `_app` will not cause our
+// auto - wrapping / instrumentation to throw an error.
+// See `_app.tsx` for more information.
+
+module.exports = async ({ page, url }) => {
+  await page.goto(`${url}/faultyAppGetInitialProps`);
+  const serverErrorText = await page.$x('//*[contains(text(), "Internal Server Error")]');
+  expect(serverErrorText).toHaveLength(0);
+};