fix(nextjs): Fix conflict with other libraries modifying webpack entry property

packages/nextjs/src/config/types.ts

@@ -54,10 +54,13 @@ export type WebpackEntryProperty = EntryPropertyObject | EntryPropertyFunction;
 // Each value in that object is either a string representing a single entry point, an array of such strings, or an
 // object containing either of those, along with other configuration options. In that third case, the entry point(s) are
 // listed under the key `import`.
-export type EntryPropertyObject =
-  | { [key: string]: string }
-  | { [key: string]: Array<string> }
-  | { [key: string]: EntryPointObject }; // only in webpack 5
+export type EntryPropertyObject = {
+  [key: string]:
+    | string
+    | Array<string>
+    // only in webpack 5
+    | EntryPointObject;
+};
 
 export type EntryPropertyFunction = () => Promise<EntryPropertyObject>;
 

packages/nextjs/src/config/webpack.ts

@@ -4,6 +4,7 @@ import * as SentryWebpackPlugin from '@sentry/webpack-plugin';
 
 import {
   BuildContext,
+  EntryPointObject,
   EntryPropertyObject,
   ExportedNextConfig,
   SentryWebpackPluginOptions,
@@ -151,6 +152,23 @@ async function addSentryToEntryProperty(
   // On the client, it's sufficient to inject it into the `main` JS code, which is included in every browser page.
   else {
     addFileToExistingEntryPoint(newEntryProperty, 'main', SENTRY_CLIENT_CONFIG_FILE);
+
+    // To work around a bug in nextjs, we need to ensure that the `main.js` entry is empty (otherwise it'll choose that
+    // over `main` and we'll lose the change we just made). In case some other library has put something into it, copy
+    // its contents over before emptying it out. See
+    // https://github.com/getsentry/sentry-javascript/pull/3696#issuecomment-863363803.)
+    const mainjsValue = newEntryProperty['main.js'];
+    if (Array.isArray(mainjsValue) && mainjsValue.length > 0) {
+      const mainValue = newEntryProperty.main;
+
+      // copy the `main.js` entries over
+      newEntryProperty.main = Array.isArray(mainValue)
+        ? [...mainjsValue, ...mainValue]
+        : { ...(mainValue as EntryPointObject), import: [...mainjsValue, ...(mainValue as EntryPointObject).import] };
+
+      // nuke the entries
+      newEntryProperty['main.js'] = [];
+    }
   }
 
   return newEntryProperty;

packages/nextjs/test/config.test.ts

@@ -212,7 +212,27 @@ describe('webpack config', () => {
       });
 
       expect(finalWebpackConfig.entry).toEqual(
-        expect.objectContaining({ main: expect.arrayContaining(['./sentry.client.config.js']) }),
+        expect.objectContaining({ main: ['./src/index.ts', './sentry.client.config.js'] }),
+      );
+    });
+
+    // see https://github.com/getsentry/sentry-javascript/pull/3696#issuecomment-863363803
+    it('handles non-empty `main.js` entry point', async () => {
+      const finalWebpackConfig = await materializeFinalWebpackConfig({
+        userNextConfig,
+        userSentryWebpackPluginConfig,
+        incomingWebpackConfig: {
+          ...clientWebpackConfig,
+          entry: () => Promise.resolve({ main: './src/index.ts', 'main.js': ['sitLieDownRollOver.config.js'] }),
+        },
+        incomingWebpackBuildContext: { ...buildContext, isServer: false },
+      });
+
+      expect(finalWebpackConfig.entry).toEqual(
+        expect.objectContaining({
+          main: ['sitLieDownRollOver.config.js', './src/index.ts', './sentry.client.config.js'],
+          'main.js': [],
+        }),
       );
     });
   });