Fix ESM TS config loading in a CJS project (#15694)

Co-authored-by: Christoph Nakazawa <[email protected]>

Author: phryneas

e2e/__tests__/__snapshots__/jest.config.ts.test.ts.snap

@@ -1,4 +1,4 @@
-// Jest Snapshot v1, https://jestjs.io/docs/snapshot-testing
+// Jest Snapshot v1, https://goo.gl/fbAQLP
 
 exports[`on node >=24 invalid JS in jest.config.ts (node with native TS support) 1`] = `
 "Error: Jest: Failed to parse the TypeScript config file <<REPLACED>>
@@ -54,6 +54,19 @@ Time:        <<REPLACED>>
 Ran all test suites."
 `;
 
+exports[`works with jest.config.ts with cjs contents 1`] = `
+"PASS __tests__/a-giraffe.js
+  ✓ giraffe"
+`;
+
+exports[`works with jest.config.ts with cjs contents 2`] = `
+"Test Suites: 1 passed, 1 total
+Tests:       1 passed, 1 total
+Snapshots:   0 total
+Time:        <<REPLACED>>
+Ran all test suites."
+`;
+
 exports[`works with tsconfig.json 1`] = `
 "PASS __tests__/a-giraffe.js
   ✓ giraffe"

e2e/__tests__/jest.config.ts.test.ts

@@ -33,6 +33,23 @@ test('works with jest.config.ts', () => {
   expect(summary).toMatchSnapshot();
 });
 
+test('falls back to a loader if we encounter a ESM TS config file in a CommonJs project', () => {
+  writeFiles(DIR, {
+    '__tests__/a-giraffe.js': "test('giraffe', () => expect(1).toBe(1));",
+    'jest.config.ts':
+      "export default {testEnvironment: 'jest-environment-node', testRegex: '.*-giraffe.js'};",
+    'package.json': '{"type":"commonjs"}',
+  });
+
+  const {stderr, exitCode} = runJest(DIR, ['-w=1', '--ci=false'], {
+    nodeOptions: '--no-warnings',
+  });
+  const {rest, summary} = extractSummary(stderr);
+  expect(exitCode).toBe(0);
+  expect(rest).toMatchSnapshot();
+  expect(summary).toMatchSnapshot();
+});
+
 test('works with tsconfig.json', () => {
   writeFiles(DIR, {
     '__tests__/a-giraffe.js': "test('giraffe', () => expect(1).toBe(1));",

packages/jest-config/src/readConfigFileAndSetRootDir.ts

@@ -39,9 +39,29 @@ export default async function readConfigFileAndSetRootDir(
   let configObject;
 
   try {
-    // @ts-expect-error: type assertion can be removed once @types/node is updated to 23 https://nodejs.org/api/process.html#processfeaturestypescript
-    if (isTS && !process.features.typescript) {
-      configObject = await loadTSConfigFile(configPath);
+    if (isTS) {
+      // @ts-expect-error: Type assertion can be removed once @types/node is updated to 23 https://nodejs.org/api/process.html#processfeaturestypescript
+      if (process.features.typescript) {
+        try {
+          // Try native node TypeScript support first.
+          configObject = await requireOrImportModule<any>(configPath);
+        } catch (error) {
+          if (
+            !(
+              error instanceof SyntaxError &&
+              // Likely ESM in a file interpreted as CJS, which means it needs to be
+              // compiled. We ignore the error and try to load it with a loader.
+              error.message.match(/Unexpected token '(export|import)'/)
+            )
+          ) {
+            throw error;
+          }
+        }
+      }
+      // Fall back to `ts-node` etc. if this cannot be natively parsed/executed.
+      if (!configObject) {
+        configObject = await loadTSConfigFile(configPath);
+      }
     } else if (isJSON) {
       const fileContent = fs.readFileSync(configPath, 'utf8');
       configObject = parseJson(stripJsonComments(fileContent), configPath);