feat: throw when jest globals are used

Author: V3RON

apps/playground/src/__tests__/jest-throw.harness.ts

@@ -0,0 +1,16 @@
+import { describe, it, expect } from 'react-native-harness';
+
+describe('Jest globals warning', () => {
+  it('should throw when accessing jest global', () => {
+    expect(() => {
+      // @ts-expect-error - jest is not available in Harness tests
+      jest.fn();
+    }).toThrow('Jest globals are not available in Harness tests');
+  });
+
+  it('should throw when importing @jest/globals', () => {
+    expect(() => require('@jest/globals')).toThrow(
+      "Importing '@jest/globals' is not supported in Harness tests"
+    );
+  });
+});

packages/metro/src/jest-globals-mock.ts

@@ -0,0 +1,6 @@
+// Mock module for @jest/globals imports
+// This module throws immediately when imported to warn users about using Jest APIs
+
+throw new Error(
+  "Importing '@jest/globals' is not supported in Harness tests. Import from 'react-native-harness' instead."
+);

packages/metro/src/resolver.ts

@@ -30,6 +30,14 @@ export const getHarnessResolver = (
       };
     }
 
+    // Intercept @jest/globals imports and redirect to mock module
+    if (moduleName === '@jest/globals') {
+      return {
+        type: 'sourceFile',
+        filePath: require.resolve('./jest-globals-mock'),
+      };
+    }
+
     return resolvedModule;
   };
 };

packages/runtime/assets/moduleSystem.flow.js

@@ -588,11 +588,14 @@ function loadModuleImplementation(
     // keep args in sync with with defineModuleCode in
     // metro/src/Resolver/index.js
     // and metro/src/ModuleGraph/worker.js
+    const capturedRequire = (...args) => global.__r(...args);
+    Object.assign(capturedRequire, global.__r);
+
     factory(
       global,
-      (...args) => global.__r(...args),
-      (...args) => global.__r.importDefault(...args),
-      (...args) => global.__r.importAll(...args),
+      capturedRequire,
+      capturedRequire.importDefault,
+      capturedRequire.importAll,
       moduleObject,
       moduleObject.exports,
       dependencyMap

packages/runtime/src/initialize.ts

@@ -1,18 +1,23 @@
 import { getDeviceDescriptor } from './client/getDeviceDescriptor.js';
 import { getClient } from './client/index.js';
+import { setupJestMock } from './jest-mock.js';
 
 // Polyfill for EventTarget
 const Shim = require('event-target-shim');
 globalThis.Event = Shim.Event;
 globalThis.EventTarget = Shim.EventTarget;
 
+// Setup jest mock to warn users about using Jest APIs
+setupJestMock();
+
 // Turn off LogBox
 const { LogBox } = require('react-native');
 LogBox.ignoreAllLogs(true);
 
 // Turn off HMR
 const HMRClientModule = require('react-native/Libraries/Utilities/HMRClient');
-const HMRClient = 'default' in HMRClientModule ? HMRClientModule.default : HMRClientModule;
+const HMRClient =
+  'default' in HMRClientModule ? HMRClientModule.default : HMRClientModule;
 
 // Wait for HMRClient to be initialized
 setTimeout(() => {
@@ -23,3 +28,8 @@ setTimeout(() => {
     client.rpc.reportReady(getDeviceDescriptor())
   );
 });
+
+// Re-throw fatal errors
+ErrorUtils.setGlobalHandler((error) => {
+  throw error;
+});

packages/runtime/src/jest-mock.ts

@@ -0,0 +1,32 @@
+// Mock jest global to warn users about using Jest APIs in Harness tests
+export const setupJestMock = (): void => {
+  function throwError(): never {
+    throw new Error(
+      `Jest globals are not available in Harness tests. Import from 'react-native-harness' instead (e.g., import { harness } from 'react-native-harness'; harness.fn())`
+    );
+  }
+
+  const jestMock = new Proxy(
+    {},
+    {
+      get() {
+        throwError();
+      },
+      set() {
+        throwError();
+      },
+      has() {
+        throwError();
+      },
+      ownKeys() {
+        throwError();
+      },
+    }
+  );
+
+  Object.defineProperty(globalThis, 'jest', {
+    value: jestMock,
+    writable: false,
+    configurable: false,
+  });
+};