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, the productive move is the same as with timers: 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.