feat: stringify Errors properly with --json flag

CHANGELOG.md

@@ -20,7 +20,9 @@
 - `[@jest/core]` Add `perfStats` to surface test setup overhead ([#14622](https://github.com/jestjs/jest/pull/14622))
 - `[@jest/core]` [**BREAKING**] Changed `--filter` to accept an object with shape `{ filtered: Array<string> }` to match [documentation](https://jestjs.io/docs/cli#--filterfile) ([#13319](https://github.com/jestjs/jest/pull/13319))
 - `[@jest/core]` Support `--outputFile` option for [`--listTests`](https://jestjs.io/docs/cli#--listtests) ([#14980](https://github.com/jestjs/jest/pull/14980))
+- `[@jest/core]` Stringify Errors properly with `--json` flag ([#15329](https://github.com/jestjs/jest/pull/15329))
 - `[@jest/core, @jest/test-sequencer]` [**BREAKING**] Exposes `globalConfig` & `contexts` to `TestSequencer` ([#14535](https://github.com/jestjs/jest/pull/14535), & [#14543](https://github.com/jestjs/jest/pull/14543))
+- `[jest-each]` Introduce `%$` option to add number of the test to its title ([#14710](https://github.com/jestjs/jest/pull/14710))
 - `[@jest/environment]` [**BREAKING**] Remove deprecated `jest.genMockFromModule()` ([#15042](https://github.com/jestjs/jest/pull/15042))
 - `[@jest/environment]` [**BREAKING**] Remove unnecessary defensive code ([#15045](https://github.com/jestjs/jest/pull/15045))
 - `[jest-environment-jsdom]` [**BREAKING**] Upgrade JSDOM to v22 ([#13825](https://github.com/jestjs/jest/pull/13825))
@@ -39,15 +41,14 @@
 - `[jest-runtime]` Support `import.meta.resolve` ([#14930](https://github.com/jestjs/jest/pull/14930))
 - `[jest-runtime]` [**BREAKING**] Make it mandatory to pass `globalConfig` to the `Runtime` constructor ([#15044](https://github.com/jestjs/jest/pull/15044))
 - `[jest-runtime]` Add `unstable_unmockModule` ([#15080](https://github.com/jestjs/jest/pull/15080))
+- `[jest-runtime]` Add `onGenerateMock` transformer callback for auto generated callbacks ([#15433](https://github.com/jestjs/jest/pull/15433))
 - `[@jest/schemas]` Upgrade `@sinclair/typebox` to v0.34 ([#15450](https://github.com/jestjs/jest/pull/15450))
 - `[@jest/types]` `test.each()`: Accept a readonly (`as const`) table properly ([#14565](https://github.com/jestjs/jest/pull/14565))
 - `[@jest/types]` Improve argument type inference passed to `test` and `describe` callback functions from `each` tables ([#14920](https://github.com/jestjs/jest/pull/14920))
 - `[jest-snapshot]` [**BREAKING**] Add support for [Error causes](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Error/cause) in snapshots ([#13965](https://github.com/facebook/jest/pull/13965))
 - `[jest-snapshot]` Support Prettier 3 ([#14566](https://github.com/facebook/jest/pull/14566))
 - `[@jest/util-snapshot]` Extract utils used by tooling from `jest-snapshot` into its own package ([#15095](https://github.com/facebook/jest/pull/15095))
 - `[pretty-format]` [**BREAKING**] Do not render empty string children (`''`) in React plugin ([#14470](https://github.com/facebook/jest/pull/14470))
-- `[jest-each]` Introduce `%$` option to add number of the test to its title ([#14710](https://github.com/jestjs/jest/pull/14710))
-- `[jest-runtime]` Add `onGenerateMock` transformer callback for auto generated callbacks ([#15433](https://github.com/jestjs/jest/pull/15433))
 
 ### Fixes
 

packages/jest-core/src/lib/__tests__/serializeToJSON.test.ts

@@ -0,0 +1,49 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+import serializeToJSON from '../serializeToJSON';
+
+// populate an object with all basic JavaScript datatypes
+const object = {
+  chillness: 100,
+  flaws: null,
+  hopOut: {
+    atThe: 'after party',
+    when: new Date('2000-07-14'),
+  },
+  i: ['pull up'],
+  location: undefined,
+  ok: true,
+  species: 'capybara',
+  weight: 9.5,
+};
+
+it('serializes regular objects like JSON.stringify', () => {
+  expect(serializeToJSON(object)).toEqual(JSON.stringify(object));
+});
+
+it('serializes errors', () => {
+  const objectWithError = {
+    ...object,
+    error: new Error('too cool'),
+  };
+  const withError = serializeToJSON(objectWithError);
+  const withoutError = JSON.stringify(objectWithError);
+
+  expect(withoutError).not.toEqual(withError);
+
+  expect(withError).toContain('"message":"too cool"');
+  expect(withError).toContain('"name":"Error"');
+  expect(withError).toContain('"stack":"Error:');
+
+  expect(JSON.parse(withError)).toMatchObject({
+    error: {
+      message: 'too cool',
+      name: 'Error',
+    },
+  });
+});

packages/jest-core/src/lib/serializeToJSON.ts

@@ -0,0 +1,37 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+import {isNativeError} from 'node:util/types';
+
+/**
+ * When we're asked to give a JSON output with the --json flag or otherwise,
+ * some data we need to return don't serialize well with a basic
+ * `JSON.stringify`, particularly Errors returned in `.openHandles`.
+ *
+ * This function handles the extended serialization wanted above.
+ */
+export default function serializeToJSON(
+  value: unknown,
+  space?: string | number,
+): string {
+  return JSON.stringify(
+    value,
+    (_, value) => {
+      // There might be more in Error, but pulling out just the message, name,
+      // and stack should be good enough
+      if (isNativeError(value)) {
+        return {
+          message: value.message,
+          name: value.name,
+          stack: value.stack,
+        };
+      }
+      return value;
+    },
+    space,
+  );
+}

packages/jest-core/src/runJest.ts

@@ -33,6 +33,7 @@ import collectNodeHandles, {
   type HandleCollectionResult,
 } from './collectHandles';
 import getNoTestsFoundMessage from './getNoTestsFoundMessage';
+import serializeToJSON from './lib/serializeToJSON';
 import runGlobalHook from './runGlobalHook';
 import type {Filter, TestRunData} from './types';
 
@@ -111,21 +112,17 @@ const processResults = async (
     runResults = await processor(runResults);
   }
   if (isJSON) {
+    const jsonString = serializeToJSON(formatTestResults(runResults));
     if (outputFile) {
       const cwd = tryRealpath(process.cwd());
       const filePath = path.resolve(cwd, outputFile);
 
-      fs.writeFileSync(
-        filePath,
-        `${JSON.stringify(formatTestResults(runResults))}\n`,
-      );
+      fs.writeFileSync(filePath, `${jsonString}\n`);
       outputStream.write(
         `Test results written to: ${path.relative(cwd, filePath)}\n`,
       );
     } else {
-      process.stdout.write(
-        `${JSON.stringify(formatTestResults(runResults))}\n`,
-      );
+      process.stdout.write(`${jsonString}\n`);
     }
   }