fix(@schematics/angular): remove karma config devkit package usages during application migration

When performing an `ng update` to Angular v20, the application migration
will detect the usage of the `karma` builder's `karmaConfig` option and
attempt to remove usages of the no longer needed `@angular-devkit/build-angular`
karma framework and plugin usage. While the karma framework usage will be
specifically ignored when using the `@angular/build:karma` builder, the
plugin usage leverages a direct `require` within the configuration file.
Regardless of the ability of the builder to ignore, neither usage is
needed with `@angular/build:karma` and removing the code aligns
with what would be generated by `ng generate config karma`.

(cherry picked from commit a2aba4c)

Author: clydin

packages/schematics/angular/migrations/use-application-builder/migration.ts

@@ -232,6 +232,8 @@ function updateProjects(tree: Tree, context: SchematicContext) {
 
     // Use @angular/build directly if there is no devkit package usage
     if (!hasAngularDevkitUsage) {
+      const karmaConfigFiles = new Set<string>();
+
       for (const [, target] of allWorkspaceTargets(workspace)) {
         switch (target.builder) {
           case Builders.Application:
@@ -245,9 +247,15 @@ function updateProjects(tree: Tree, context: SchematicContext) {
             break;
           case Builders.Karma:
             target.builder = '@angular/build:karma';
-            // Remove "builderMode" option since the builder will always use "application"
             for (const [, karmaOptions] of allTargetOptions(target)) {
+              // Remove "builderMode" option since the builder will always use "application"
               delete karmaOptions['builderMode'];
+
+              // Collect custom karma configurations for @angular-devkit/build-angular plugin removal
+              const karmaConfig = karmaOptions['karmaConfig'];
+              if (karmaConfig && typeof karmaConfig === 'string') {
+                karmaConfigFiles.add(karmaConfig);
+              }
             }
             break;
           case Builders.NgPackagr:
@@ -292,6 +300,34 @@ function updateProjects(tree: Tree, context: SchematicContext) {
           }),
         );
       }
+
+      for (const karmaConfigFile of karmaConfigFiles) {
+        if (!tree.exists(karmaConfigFile)) {
+          continue;
+        }
+
+        try {
+          const originalKarmaConfigText = tree.readText(karmaConfigFile);
+          const updatedKarmaConfigText = originalKarmaConfigText
+            .replaceAll(`require('@angular-devkit/build-angular/plugins/karma'),`, '')
+            .replaceAll(`require('@angular-devkit/build-angular/plugins/karma')`, '');
+
+          if (updatedKarmaConfigText.includes('@angular-devkit/build-angular/plugins')) {
+            throw new Error(
+              'Migration does not support found usage of "@angular-devkit/build-angular".',
+            );
+          } else {
+            tree.overwrite(karmaConfigFile, updatedKarmaConfigText);
+          }
+        } catch (error) {
+          const reason = error instanceof Error ? `Reason: ${error.message}` : '';
+          context.logger.warn(
+            `Unable to update custom karma configuration file ("${karmaConfigFile}"). ` +
+              reason +
+              '\nReferences to the "@angular-devkit/build-angular" package within the file may need to be removed manually.',
+          );
+        }
+      }
     }
 
     return chain(rules);

packages/schematics/angular/migrations/use-application-builder/migration_spec.ts

@@ -130,6 +130,103 @@ describe(`Migration to use the application builder`, () => {
     expect(builderMode).toBeUndefined();
   });
 
+  it(`should update file for 'karmaConfig' karma option (no require trailing comma)`, async () => {
+    addWorkspaceTarget(tree, 'test', {
+      'builder': Builders.Karma,
+      'options': {
+        'karmaConfig': './karma.conf.js',
+        'polyfills': ['zone.js', 'zone.js/testing'],
+        'tsConfig': 'projects/app-a/tsconfig.spec.json',
+      },
+    });
+    tree.create(
+      './karma.conf.js',
+      `
+        module.exports = function (config) {
+          config.set({
+            basePath: '',
+            frameworks: ['jasmine', '@angular-devkit/build-angular'],
+            plugins: [
+              require('karma-jasmine'),
+              require('karma-chrome-launcher'),
+              require('karma-jasmine-html-reporter'),
+              require('karma-coverage'),
+              require('@angular-devkit/build-angular/plugins/karma')
+            ]
+          });
+        };`,
+    );
+
+    const newTree = await schematicRunner.runSchematic(schematicName, {}, tree);
+    const {
+      projects: { app },
+    } = JSON.parse(newTree.readContent('/angular.json'));
+
+    const { karmaConfig } = app.architect['test'].options;
+    expect(karmaConfig).toBe('./karma.conf.js');
+
+    const karmaConfigText = newTree.readText('./karma.conf.js');
+    expect(karmaConfigText).not.toContain(`require('@angular-devkit/build-angular/plugins/karma')`);
+  });
+
+  it(`should update file for 'karmaConfig' karma option (require trailing comma)`, async () => {
+    addWorkspaceTarget(tree, 'test', {
+      'builder': Builders.Karma,
+      'options': {
+        'karmaConfig': './karma.conf.js',
+        'polyfills': ['zone.js', 'zone.js/testing'],
+        'tsConfig': 'projects/app-a/tsconfig.spec.json',
+      },
+    });
+    tree.create(
+      './karma.conf.js',
+      `
+        module.exports = function (config) {
+          config.set({
+            basePath: '',
+            frameworks: ['jasmine', '@angular-devkit/build-angular'],
+            plugins: [
+              require('karma-jasmine'),
+              require('karma-chrome-launcher'),
+              require('karma-jasmine-html-reporter'),
+              require('karma-coverage'),
+              require('@angular-devkit/build-angular/plugins/karma'),
+            ]
+          });
+        };`,
+    );
+
+    const newTree = await schematicRunner.runSchematic(schematicName, {}, tree);
+    const {
+      projects: { app },
+    } = JSON.parse(newTree.readContent('/angular.json'));
+
+    const { karmaConfig } = app.architect['test'].options;
+    expect(karmaConfig).toBe('./karma.conf.js');
+
+    const karmaConfigText = newTree.readText('./karma.conf.js');
+    expect(karmaConfigText).not.toContain(`require('@angular-devkit/build-angular/plugins/karma')`);
+  });
+
+  it(`should ignore missing file for 'karmaConfig' karma option`, async () => {
+    addWorkspaceTarget(tree, 'test', {
+      'builder': Builders.Karma,
+      'options': {
+        'karmaConfig': './karma.conf.js',
+        'polyfills': ['zone.js', 'zone.js/testing'],
+        'tsConfig': 'projects/app-a/tsconfig.spec.json',
+      },
+    });
+
+    const newTree = await schematicRunner.runSchematic(schematicName, {}, tree);
+    const {
+      projects: { app },
+    } = JSON.parse(newTree.readContent('/angular.json'));
+
+    const { karmaConfig } = app.architect['test'].options;
+    expect(karmaConfig).toBe('./karma.conf.js');
+  });
+
   it('should remove tilde prefix from CSS @import specifiers', async () => {
     // Replace outputPath
     tree.create(