fix(jest): default isolatedModules to true for faster compilation (fixes #1899)

packages/jest/src/default-config.resolver.spec.ts

@@ -9,27 +9,21 @@ const defaultConfigResolver = new DefaultConfigResolver({});
 describe('Resolve project default configuration', () => {
   it('Should resolve tsconfig relatively to project root', () => {
     const config = defaultConfigResolver.resolveForProject(normalize('/some/cool/directory'));
-    expect(config.transform[defaultConfigResolver.tsJestTransformRegExp]).toEqual([
-      'jest-preset-angular',
-      {
-        stringifyContentPathRegex: '\\.(html|svg)$',
-        tsconfig: getSystemPath(normalize(`/some/cool/directory/${tsConfigName}`)),
-      },
-    ]);
+    const [, transformOptions] = config.transform[defaultConfigResolver.tsJestTransformRegExp];
+    expect(transformOptions.tsconfig).toEqual(
+      getSystemPath(normalize(`/some/cool/directory/${tsConfigName}`))
+    );
   });
 
   it('Should resolve path to the tsconfig if "tsConfig" is provided', () => {
     const defaultConfigResolver = new DefaultConfigResolver({
       tsConfig: './ts-configs/tsconfig.spec.json',
     });
     const config = defaultConfigResolver.resolveForProject(normalize('/some/cool/project'));
-    expect(config.transform[defaultConfigResolver.tsJestTransformRegExp]).toEqual([
-      'jest-preset-angular',
-      {
-        stringifyContentPathRegex: '\\.(html|svg)$',
-        tsconfig: getSystemPath(normalize(`/some/cool/project/ts-configs/tsconfig.spec.json`)),
-      },
-    ]);
+    const [, transformOptions] = config.transform[defaultConfigResolver.tsJestTransformRegExp];
+    expect(transformOptions.tsconfig).toEqual(
+      getSystemPath(normalize(`/some/cool/project/ts-configs/tsconfig.spec.json`))
+    );
   });
 
   it('Should resolve testMatch pattern relatively to project root', () => {

packages/jest/src/default-config.resolver.ts

@@ -47,6 +47,16 @@ export class DefaultConfigResolver {
             stringifyContentPathRegex: '\\.(html|svg)$',
             // Join with the default `tsConfigName` if the `tsConfig` option is not provided
             tsconfig: getTsConfigPath(projectRoot, this.options),
+            // Default to isolatedModules: true for significantly faster compilation.
+            // With isolatedModules: false (the previous implicit default), ts-jest uses the
+            // TypeScript language service to build a full cross-file Program for every test
+            // file, which becomes extremely slow with Angular 19+ code (signals, new control
+            // flow, standalone-by-default). Angular 19+ users report 2min → 15min regressions.
+            // Cross-file type checking is better handled by `tsc --noEmit` or `ng build`.
+            // Users who need the old behaviour can opt out via their jest.config.ts:
+            //   transform: { '...': ['jest-preset-angular', { isolatedModules: false }] }
+            // BREAKING CHANGE: targeted for the next major version.
+            isolatedModules: true,
           },
         ],
       },

packages/jest/tests/integration.js

@@ -15,6 +15,17 @@ module.exports = [
     command: 'yarn test:esm --no-cache',
   },
 
+  // isolatedModules default - behavioral proof
+  {
+    id: 'isolated-modules-default',
+    name: 'jest: isolatedModules:true default works',
+    purpose:
+      'Tests pass correctly with isolatedModules:true (default since v21), regression for #1899',
+    app: 'examples/jest/simple-app',
+    command:
+      'node ../../../packages/jest/tests/validate.js --no-cache --expect-suites=2 --expect-tests=4',
+  },
+
   // CLI passthrough - validated tests
   {
     id: 'cli-no-cache',