feat(@angular/build): support Istanbul coverage in Vitest runner

This change enables code coverage reporting when running tests in non-Chromium browsers (like Firefox or Safari) with the Vitest runner.

The system now automatically detects the best coverage provider based on the configured browsers and installed packages:
- If non-Chromium browsers are configured, it selects 'istanbul'.
- If only Chromium browsers are used, it selects 'istanbul' if it is the only provider package installed.
- Otherwise, it defaults to 'v8'.
It also respects the provider specified in the user's custom Vitest configuration.

Author: clydin

packages/angular/build/src/builders/unit-test/runners/vitest/index.ts

@@ -9,6 +9,7 @@
 import assert from 'node:assert';
 import type { TestRunner } from '../api';
 import { DependencyChecker } from '../dependency-checker';
+import { normalizeBrowserName } from './browser-provider';
 import { getVitestBuildOptions } from './build-options';
 import { VitestExecutor } from './executor';
 
@@ -50,7 +51,16 @@ const VitestTestRunner: TestRunner = {
     }
 
     if (options.coverage.enabled) {
-      checker.check('@vitest/coverage-v8');
+      const browsersToCheck: string[] = options.browsers ?? [];
+      const hasNonChromium = browsersToCheck
+        .map((b) => normalizeBrowserName(b).browser)
+        .filter((b) => !['chrome', 'chromium', 'edge'].includes(b)).length > 0;
+
+      if (hasNonChromium) {
+        checker.check('@vitest/coverage-istanbul');
+      } else {
+        checker.check('@vitest/coverage-v8');
+      }
     }
 
     checker.report();

packages/angular/build/src/builders/unit-test/runners/vitest/plugins.ts

@@ -69,6 +69,67 @@ async function findTestEnvironment(
   }
 }
 
+function determineCoverageProvider(
+  browser: BrowserConfigOptions | undefined,
+  testConfig: InlineConfig | undefined,
+  optionsCoverageEnabled: boolean | undefined,
+  projectSourceRoot: string,
+): 'istanbul' | 'v8' | 'custom' | undefined {
+  let determinedProvider = testConfig?.coverage?.provider;
+  if (!determinedProvider && (optionsCoverageEnabled || testConfig?.coverage?.enabled)) {
+    const browsersToCheck = getBrowsersToCheck(browser, testConfig?.browser);
+    
+    const hasNonChromium = browsersToCheck
+      .map((b) => normalizeBrowserName(b).browser)
+      .filter((b) => !['chrome', 'chromium', 'edge'].includes(b)).length > 0;
+      
+    if (hasNonChromium) {
+      determinedProvider = 'istanbul';
+    } else {
+      const projectRequire = createRequire(projectSourceRoot + '/');
+      const checkInstalled = (pkg: string) => {
+        try { projectRequire.resolve(pkg); return true; } catch { return false; }
+      };
+      const hasIstanbul = checkInstalled('@vitest/coverage-istanbul');
+      const hasV8 = checkInstalled('@vitest/coverage-v8');
+      
+      if (hasIstanbul && !hasV8) {
+        determinedProvider = 'istanbul';
+      } else {
+        determinedProvider = 'v8';
+      }
+    }
+  }
+
+  return determinedProvider;
+}
+
+function getBrowsersToCheck(
+  browser: BrowserConfigOptions | undefined,
+  testConfigBrowser: BrowserConfigOptions | undefined,
+): string[] {
+  const browsersToCheck: string[] = [];
+
+  // 1. Check browsers passed by the Angular CLI options
+  const cliBrowser = browser as CustomBrowserConfigOptions | undefined;
+  if (cliBrowser?.instances) {
+    browsersToCheck.push(...cliBrowser.instances.map((i) => i.browser));
+  }
+
+  // 2. Check browsers defined in the user's vitest.config.ts
+  const userBrowser = testConfigBrowser as CustomBrowserConfigOptions | undefined;
+  if (userBrowser) {
+    if (userBrowser.instances) {
+      browsersToCheck.push(...userBrowser.instances.map((i) => i.browser));
+    }
+    if (userBrowser.name) {
+      browsersToCheck.push(userBrowser.name);
+    }
+  }
+  
+  return browsersToCheck;
+}
+
 export async function createVitestConfigPlugin(
   options: VitestConfigPluginOptions,
 ): Promise<VitestPlugins[0]> {
@@ -89,6 +150,13 @@ export async function createVitestConfigPlugin(
     async config(config) {
       const testConfig = config.test;
 
+      const determinedProvider = determineCoverageProvider(
+        browser,
+        testConfig,
+        options.coverage.enabled,
+        projectSourceRoot,
+      );
+
       if (reporters !== undefined) {
         delete testConfig?.reporters;
       }
@@ -155,8 +223,8 @@ export async function createVitestConfigPlugin(
         (browser || testConfig?.browser?.enabled) &&
         (options.coverage.enabled || testConfig?.coverage?.enabled)
       ) {
-        // Validate that enabled browsers support V8 coverage
-        validateBrowserCoverage(browser, testConfig?.browser);
+        // Validate that enabled browsers support the selected coverage provider
+        validateBrowserCoverage(browser, testConfig?.browser, determinedProvider);
 
         projectPlugins.unshift(createSourcemapSupportPlugin());
         setupFiles.unshift('virtual:source-map-support');
@@ -208,6 +276,7 @@ export async function createVitestConfigPlugin(
             options.coverage,
             testConfig?.coverage,
             projectName,
+            determinedProvider,
           ),
           // eslint-disable-next-line @typescript-eslint/no-explicit-any
           ...(reporters ? ({ reporters } as any) : {}),
@@ -434,25 +503,12 @@ interface CustomBrowserConfigOptions {
 function validateBrowserCoverage(
   browser: BrowserConfigOptions | undefined,
   testConfigBrowser: BrowserConfigOptions | undefined,
+  provider?: string,
 ): void {
-  const browsersToCheck: string[] = [];
-
-  // 1. Check browsers passed by the Angular CLI options
-  const cliBrowser = browser as CustomBrowserConfigOptions | undefined;
-  if (cliBrowser?.instances) {
-    browsersToCheck.push(...cliBrowser.instances.map((i) => i.browser));
-  }
-
-  // 2. Check browsers defined in the user's vitest.config.ts
-  const userBrowser = testConfigBrowser as CustomBrowserConfigOptions | undefined;
-  if (userBrowser) {
-    if (userBrowser.instances) {
-      browsersToCheck.push(...userBrowser.instances.map((i) => i.browser));
-    }
-    if (userBrowser.name) {
-      browsersToCheck.push(userBrowser.name);
-    }
+  if (provider === 'istanbul') {
+    return;
   }
+  const browsersToCheck = getBrowsersToCheck(browser, testConfigBrowser);
 
   // Normalize and filter unsupported browsers
   const unsupportedBrowsers = browsersToCheck
@@ -473,6 +529,7 @@ async function generateCoverageOption(
   optionsCoverage: NormalizedUnitTestBuilderOptions['coverage'],
   configCoverage: VitestCoverageOption | undefined,
   projectName: string,
+  provider?: 'istanbul' | 'v8' | 'custom',
 ): Promise<VitestCoverageOption> {
   let defaultExcludes: string[] = [];
   // When a coverage exclude option is provided, Vitest's default coverage excludes
@@ -486,6 +543,7 @@ async function generateCoverageOption(
   }
 
   return {
+    provider,
     excludeAfterRemap: true,
     reportsDirectory:
       configCoverage?.reportsDirectory ?? toPosixPath(path.join('coverage', projectName)),

tests/e2e/tests/vitest/browser-coverage-istanbul.ts

@@ -0,0 +1,36 @@
+import { ng } from '../../utils/process';
+import { applyVitestBuilder } from '../../utils/vitest';
+import assert from 'node:assert';
+import { installPackage } from '../../utils/packages';
+import { expectFileToExist, readFile } from '../../utils/fs';
+import { updateJsonFile } from '../../utils/project';
+
+export default async function (): Promise<void> {
+  await applyVitestBuilder();
+
+  // Install ONLY Istanbul coverage package.
+  // This will trigger the auto-detection logic to use Istanbul even for Node tests.
+  await installPackage('@vitest/coverage-istanbul@4');
+
+  // Use the 'json' reporter to get a machine-readable output for assertions.
+  await updateJsonFile('angular.json', (json) => {
+    const project = Object.values(json['projects'])[0] as any;
+    const test = project['architect']['test'];
+    test.options = {
+      coverageReporters: ['json', 'text'],
+    };
+  });
+
+  // Run tests with coverage (defaults to Node/jsdom environment)
+  const { stdout } = await ng('test', '--no-watch', '--coverage');
+  
+  // Verify that tests passed
+  assert.match(stdout, /1 passed/, 'Expected tests to run successfully.');
+
+  // Verify that coverage files are generated
+  const coverageJsonPath = 'coverage/test-project/coverage-final.json';
+  await expectFileToExist(coverageJsonPath);
+
+  const coverageSummary = JSON.parse(await readFile(coverageJsonPath));
+  assert.ok(Object.keys(coverageSummary).length > 0, 'Expected coverage report to not be empty.');
+}

tests/e2e/tests/vitest/browser-coverage-validation.ts

@@ -26,8 +26,8 @@ export default async function (): Promise<void> {
   const output1 = stripVTControlCharacters(error1.message);
   assert.match(
     output1,
-    /Code coverage is enabled, but the following configured browsers do not support the V8 coverage provider: firefox/,
-    'Expected validation error for unsupported browser with coverage (CLI option).',
+    /The following packages are required but were not found:.*@vitest\/coverage-istanbul/,
+    'Expected validation error for missing Istanbul coverage package.',
   );
 
   const configPath = 'vitest.config.ts';
@@ -41,6 +41,7 @@ export default async function (): Promise<void> {
       import { defineConfig } from 'vitest/config';
       export default defineConfig({
         test: {
+          coverage: { provider: 'v8' },
           browser: {
             enabled: true,
             name: 'firefox',
@@ -71,6 +72,7 @@ export default async function (): Promise<void> {
       import { defineConfig } from 'vitest/config';
       export default defineConfig({
         test: {
+          coverage: { provider: 'v8' },
           browser: {
             enabled: true,
             provider: 'playwright',