[api-extractor] More predictable merging of arrays in configuration

apps/api-extractor/src/api/ExtractorConfig.ts

@@ -568,6 +568,17 @@ export class ExtractorConfig {
     let currentConfigFilePath: string = path.resolve(jsonFilePath);
     let configObject: Partial<IConfigFile> = {};
 
+    // Lodash merges array values by default, which is unintuitive for config files (and makes it impossible for derived configurations to overwrite arrays).
+    // For example, given a base config containing an array property with value ["foo", "bar"] and a derived config that specifies ["baz"] for that property, lodash will produce ["baz", "bar"], which is unintuitive.
+    // This customizer function ensures that arrays are always overwritten.
+    const mergeCustomizer: lodash.MergeWithCustomizer = (objValue, srcValue) => {
+      if (Array.isArray(srcValue)) {
+        return srcValue;
+      }
+      // Fall back to default merge behavior.
+      return undefined;
+    };
+
     try {
       do {
         // Check if this file was already processed.
@@ -612,7 +623,7 @@ export class ExtractorConfig {
         ExtractorConfig._resolveConfigFileRelativePaths(baseConfig, currentConfigFolderPath);
 
         // Merge extractorConfig into baseConfig, mutating baseConfig
-        lodash.merge(baseConfig, configObject);
+        lodash.mergeWith(baseConfig, configObject, mergeCustomizer);
         configObject = baseConfig;
 
         currentConfigFilePath = extendsField;
@@ -622,7 +633,11 @@ export class ExtractorConfig {
     }
 
     // Lastly, apply the defaults
-    configObject = lodash.merge(lodash.cloneDeep(ExtractorConfig._defaultConfig), configObject);
+    configObject = lodash.mergeWith(
+      lodash.cloneDeep(ExtractorConfig._defaultConfig),
+      configObject,
+      mergeCustomizer
+    );
 
     ExtractorConfig.jsonSchema.validateObject(configObject, jsonFilePath);
 

apps/api-extractor/src/api/test/ExtractorConfig-merge.test.ts

@@ -0,0 +1,27 @@
+// Copyright (c) Microsoft Corporation. All rights reserved. Licensed under the MIT license.
+// See LICENSE in the project root for license information.
+
+import * as path from 'path';
+
+import { ExtractorConfig } from '../ExtractorConfig';
+
+const testDataFolder: string = path.join(__dirname, 'test-data');
+
+// Tests verifying the merge behavior of ExtractorConfig.loadFile
+describe(`${ExtractorConfig.name}.${ExtractorConfig.loadFile.name}`, () => {
+  it('array properties completely override array properties in the base config', () => {
+    const extractorConfig: ExtractorConfig = ExtractorConfig.loadFileAndPrepare(
+      path.join(testDataFolder, 'override-array-properties', 'api-extractor.json')
+    );
+    // Base config specifies: ["alpha", "beta", "public"]
+    // Derived config specifies: ["complete"]
+    // By default, lodash's merge() function would generate ["complete", "beta", "public"],
+    // but we instead want the derived config's array property to completely override that of the base.
+    expect(extractorConfig.reportConfigs).toEqual([
+      {
+        variant: 'complete',
+        fileName: 'override-array-properties.api.md'
+      }
+    ]);
+  });
+});

apps/api-extractor/src/api/test/test-data/override-array-properties/README.md

@@ -0,0 +1,2 @@
+Reproduction of #4786.
+Merging of configs should *not* merge arrays - the overriding config file's array properties should completely overwrite the base config's array properties.

apps/api-extractor/src/api/test/test-data/override-array-properties/api-extractor-base.json

@@ -0,0 +1,18 @@
+{
+  "$schema": "https://developer.microsoft.com/json-schemas/api-extractor/v7/api-extractor.schema.json",
+
+  "mainEntryPointFilePath": "index.d.ts",
+
+  "apiReport": {
+    "enabled": true,
+    "reportVariants": ["alpha", "beta", "public"]
+  },
+
+  "docModel": {
+    "enabled": true
+  },
+
+  "dtsRollup": {
+    "enabled": true
+  }
+}

apps/api-extractor/src/api/test/test-data/override-array-properties/api-extractor.json

@@ -0,0 +1,7 @@
+{
+  "extends": "./api-extractor-base.json",
+
+  "apiReport": {
+    "reportVariants": ["complete"]
+  }
+}

apps/api-extractor/src/api/test/test-data/override-array-properties/index.d.ts

@@ -0,0 +1 @@
+// empty file

apps/api-extractor/src/api/test/test-data/override-array-properties/package.json

@@ -0,0 +1,4 @@
+{
+  "name": "override-array-properties",
+  "version": "1.0.0"
+}

common/changes/@microsoft/api-extractor/josmithr-api-extractor-fix-config-inheritence_2025-01-09-00-41.json

@@ -0,0 +1,10 @@
+{
+  "changes": [
+    {
+      "packageName": "@microsoft/api-extractor",
+      "comment": "Update merge behavior for derived configurations to allow overriding array properties",
+      "type": "minor"
+    }
+  ],
+  "packageName": "@microsoft/api-extractor"
+}