fix(nextjs): Make withSentryConfig return type match given config type

packages/nextjs/src/config/index.ts

@@ -11,23 +11,22 @@ import { constructWebpackConfigFunction } from './webpack';
 export function withSentryConfig(
   userNextConfig: ExportedNextConfig = {},
   userSentryWebpackPluginOptions: Partial<SentryWebpackPluginOptions> = {},
-): NextConfigFunction {
-  const newWebpackExport = constructWebpackConfigFunction(userNextConfig, userSentryWebpackPluginOptions);
-
-  const finalNextConfig = (
-    phase: string,
-    defaults: { defaultConfig: { [key: string]: unknown } },
-  ): NextConfigObject => {
-    const materializedUserNextConfig =
-      typeof userNextConfig === 'function' ? userNextConfig(phase, defaults) : userNextConfig;
-
-    return {
-      ...materializedUserNextConfig,
-      // TODO When we add a way to disable the webpack plugin, doing so should turn this off, too
-      productionBrowserSourceMaps: true,
-      webpack: newWebpackExport,
-    };
+): NextConfigFunction | NextConfigObject {
+  const partialConfig = {
+    // TODO When we add a way to disable the webpack plugin, doing so should turn this off, too
+    productionBrowserSourceMaps: true,
+    webpack: constructWebpackConfigFunction(userNextConfig, userSentryWebpackPluginOptions),
   };
 
-  return finalNextConfig;
+  // If the user has passed us a function, we need to return a function, so that we have access to `phase` and
+  // `defaults` in order to pass them along to the user's function
+  if (typeof userNextConfig === 'function') {
+    return (phase: string, defaults: { defaultConfig: { [key: string]: unknown } }): NextConfigObject => ({
+      ...userNextConfig(phase, defaults),
+      ...partialConfig,
+    });
+  }
+
+  // Otherwise, we can just merge their config with ours and return an object.
+  return { ...userNextConfig, ...partialConfig };
 }

packages/nextjs/test/config.test.ts

@@ -50,24 +50,31 @@ const clientWebpackConfig = {
 const buildContext = { isServer: true, dev: false, buildId: 'doGsaREgReaT' };
 
 /**
- * Derive the final values of all next config options, by first applying `withSentryConfig` and then running the
- * resulting function.
+ * Derive the final values of all next config options, by first applying `withSentryConfig` and then, if it returns a
+ *  function, running that function.
  *
  * @param userNextConfig Next config options provided by the user
  * @param userSentryWebpackPluginConfig SentryWebpackPlugin options provided by the user
  *
- * @returns The config values next will receive when it calls the function returned by `withSentryConfig`
+ * @returns The config values next will receive directly from `withSentryConfig` or when it calls the function returned
+ * by `withSentryConfig`
  */
 function materializeFinalNextConfig(
   userNextConfig: ExportedNextConfig,
   userSentryWebpackPluginConfig: SentryWebpackPluginOptions,
 ): NextConfigObject {
-  const configFunction = withSentryConfig(userNextConfig, userSentryWebpackPluginConfig);
-  const finalConfigValues = configFunction('phase-production-build', {
-    defaultConfig: {},
-  });
+  const sentrifiedConfig = withSentryConfig(userNextConfig, userSentryWebpackPluginConfig);
+  let finalConfigValues = sentrifiedConfig;
+
+  if (typeof sentrifiedConfig === 'function') {
+    // for some reason TS won't recognize that `finalConfigValues` is now a NextConfigObject, which is why the cast
+    // below is necessary
+    finalConfigValues = sentrifiedConfig('phase-production-build', {
+      defaultConfig: {},
+    });
+  }
 
-  return finalConfigValues;
+  return finalConfigValues as NextConfigObject;
 }
 
 /**