Native modules

Standalone Hermes has no React Native runtime. There is no bridge, no TurboModule infrastructure, and nothing behind NativeModules.

Argus aliases the react-native specifier to an in-realm shim at bundle time, so code that imports from it resolves to something real — and you decide what each native module returns.

Registering a module

import { NativeModules, TurboModuleRegistry } from 'react-native';

describe('BiometricsGate', () => {
  beforeEach(() => {
    argus.resetNativeModules();
    argus.mockNativeModule('Biometrics', () => ({
      isAvailable: argus.fn().mockReturnValue(true),
      authenticate: argus.fn().mockResolvedValue({ ok: true }),
    }));
  });

  test('authenticates when hardware is available', async () => {
    const gate = new BiometricsGate();

    await expect(gate.unlock()).resolves.toBe(true);

    const native = TurboModuleRegistry.getEnforcing('Biometrics');
    expect(native.authenticate).toHaveBeenCalled();
  });
});

mockNativeModule(name, factory) calls the factory immediately and stores the result. The same object is then returned by all three access paths:

Access	Returns
NativeModules.Biometrics	The registered object
TurboModuleRegistry.get('Biometrics')	The registered object, or null if unregistered
TurboModuleRegistry.getEnforcing('Biometrics')	The registered object, or throws if unregistered

getEnforcing on an unregistered name throws TurboModuleRegistry.getEnforcing('X'): module not registered — which is the same failure shape production code sees when a native module is genuinely missing, so error paths are testable.

Reset between tests

The registry is global to the file’s realm. argus.resetNativeModules() empties it.

beforeEach(() => {
  argus.resetNativeModules();
});

Do it in beforeEach rather than afterEach: a test that fails midway then still starts the next one from a clean registry.

The component shim

The same react-native alias exports a minimal set of host components, which is what component testing renders against:

import { Pressable, Text, TextInput, View } from 'react-native';

Component	Props the queries and events understand
View	testID, accessibilityRole, children, anything else
Text	as View; its text content is what getByText matches
Pressable	onPress, disabled
TextInput	value, defaultValue, placeholder, editable, onChangeText

UIManager is exported as an empty object so imports of it resolve without a crash.

These are host-component names, not implementations. Nothing lays out, measures, or paints — see Limitations.

What this is not

This shim covers native-module infrastructure, not the React Native JavaScript library. AppState, Dimensions, Animated, Linking, AsyncStorage and friends are not implemented.

When your code depends on one of those, take it as a dependency instead of importing it deep in the call stack, and pass a fake in tests. Code written that way is testable on every runner, not just this one.