Testing Config Plugins

Testing config plugins ensures they work correctly and helps prevent regressions. This guide covers testing strategies with examples from the Expo source.

Testing strategies

Config plugins should be tested at multiple levels:

Unit tests: Test plugin logic in isolation
Integration tests: Test with actual native project files
End-to-end tests: Test full prebuild workflow

Unit testing plugins

Unit tests verify plugin logic without touching the filesystem.

Basic plugin test

import { withCustomPlugin } from '../withCustomPlugin';

describe('withCustomPlugin', () => {
  it('should add custom configuration', () => {
    const config = {
      name: 'TestApp',
      slug: 'test-app',
    };
    
    const result = withCustomPlugin(config, { apiKey: 'test123' });
    
    expect(result.name).toBe('TestApp');
    expect(result.customProperty).toBe('test123');
  });
  
  it('should throw error when props are missing', () => {
    const config = { name: 'TestApp', slug: 'test-app' };
    
    expect(() => {
      withCustomPlugin(config, null);
    }).toThrow('Props are required');
  });
});

Testing Info.plist modifications

import { withInfoPlist } from '@expo/config-plugins';

const withCameraPermission = (config) => {
  return withInfoPlist(config, (config) => {
    config.modResults.NSCameraUsageDescription = 'Camera permission';
    return config;
  });
};

describe('withCameraPermission', () => {
  it('should add camera permission to Info.plist', () => {
    let config: any = {
      name: 'TestApp',
      slug: 'test-app',
    };
    
    // Apply plugin
    config = withCameraPermission(config);
    
    // Verify mod was added
    expect(config.mods?.ios?.infoPlist).toBeDefined();
    
    // Execute mod with mock data
    const modResult = config.mods.ios.infoPlist({
      ...config,
      modResults: {},
      modRequest: {
        projectRoot: '/test',
        platformProjectRoot: '/test/ios',
        platform: 'ios',
        modName: 'infoPlist',
        introspect: false,
      },
    });
    
    expect(modResult.modResults.NSCameraUsageDescription).toBe('Camera permission');
  });
});

Testing Android Manifest modifications

import { withAndroidManifest } from '@expo/config-plugins';
import { AndroidConfig } from '@expo/config-plugins';

const withCameraFeature = (config) => {
  return withAndroidManifest(config, (config) => {
    const manifest = config.modResults.manifest;
    
    if (!manifest['uses-feature']) {
      manifest['uses-feature'] = [];
    }
    
    manifest['uses-feature'].push({
      $: {
        'android:name': 'android.hardware.camera',
        'android:required': 'false',
      },
    });
    
    return config;
  });
};

describe('withCameraFeature', () => {
  it('should add camera feature to AndroidManifest', () => {
    let config: any = {
      name: 'TestApp',
      slug: 'test-app',
    };
    
    config = withCameraFeature(config);
    
    expect(config.mods?.android?.manifest).toBeDefined();
    
    // Mock manifest
    const mockManifest = {
      manifest: {
        $: {
          'xmlns:android': 'http://schemas.android.com/apk/res/android',
        },
        application: [],
      },
    };
    
    // Execute mod
    const modResult = config.mods.android.manifest({
      ...config,
      modResults: mockManifest,
      modRequest: {
        projectRoot: '/test',
        platformProjectRoot: '/test/android',
        platform: 'android',
        modName: 'manifest',
        introspect: false,
      },
    });
    
    expect(modResult.modResults.manifest['uses-feature']).toHaveLength(1);
    expect(modResult.modResults.manifest['uses-feature'][0].$['android:name'])
      .toBe('android.hardware.camera');
  });
});

Testing with real fixtures

Use fixture files from actual projects for integration testing.

Setting up fixtures

__tests__/
  fixtures/
    ios/
      Info.plist
      TestApp.entitlements
    android/
      AndroidManifest.xml
      strings.xml
  withCustomPlugin.test.ts

Loading and testing with fixtures

import fs from 'fs';
import path from 'path';
import { XMLParser, XMLBuilder } from 'fast-xml-parser';

function loadFixture(name: string): string {
  return fs.readFileSync(
    path.join(__dirname, 'fixtures', name),
    'utf-8'
  );
}

function parseXML(xml: string) {
  const parser = new XMLParser({
    ignoreAttributes: false,
    attributeNamePrefix: '',
  });
  return parser.parse(xml);
}

describe('withCameraFeature with fixtures', () => {
  it('should add camera feature to real AndroidManifest', () => {
    // Load fixture
    const manifestXml = loadFixture('android/AndroidManifest.xml');
    const manifest = parseXML(manifestXml);
    
    // Apply modifications
    if (!manifest.manifest['uses-feature']) {
      manifest.manifest['uses-feature'] = [];
    }
    manifest.manifest['uses-feature'].push({
      $: {
        'android:name': 'android.hardware.camera',
        'android:required': 'false',
      },
    });
    
    // Verify
    expect(manifest.manifest['uses-feature']).toContainEqual({
      $: {
        'android:name': 'android.hardware.camera',
        'android:required': 'false',
      },
    });
  });
});

Testing code generation

Test that code injection is idempotent.

import { mergeContents } from '@expo/config-plugins/build/utils/generateCode';

describe('code generation', () => {
  const originalCode = `
import React from 'react';

function App() {
  return <View />;
}
`;

  const codeToInsert = `  console.log('Hello from plugin');`;
  
  it('should insert code once', () => {
    const result = mergeContents({
      src: originalCode,
      newSrc: codeToInsert,
      tag: 'my-plugin',
      anchor: /function App\(\) {/,
      offset: 1,
      comment: '//',
    });
    
    expect(result.didMerge).toBe(true);
    expect(result.contents).toContain(codeToInsert);
    expect(result.contents).toContain('// @generated begin my-plugin');
    expect(result.contents).toContain('// @generated end my-plugin');
  });
  
  it('should not duplicate code on second run', () => {
    // First insertion
    let result = mergeContents({
      src: originalCode,
      newSrc: codeToInsert,
      tag: 'my-plugin',
      anchor: /function App\(\) {/,
      offset: 1,
      comment: '//',
    });
    
    const firstPass = result.contents;
    
    // Second insertion (should be idempotent)
    result = mergeContents({
      src: firstPass,
      newSrc: codeToInsert,
      tag: 'my-plugin',
      anchor: /function App\(\) {/,
      offset: 1,
      comment: '//',
    });
    
    expect(result.didMerge).toBe(false);
    expect(result.contents).toBe(firstPass);
    
    // Should only appear once
    const occurrences = (result.contents.match(/console\.log/g) || []).length;
    expect(occurrences).toBe(1);
  });
});

Testing with mock filesystem

Use memfs to test filesystem operations without touching disk.

import { vol } from 'memfs';
import { withDangerousMod } from '@expo/config-plugins';
import path from 'path';

jest.mock('fs', () => require('memfs').fs);
jest.mock('fs/promises', () => require('memfs').fs.promises);

const withFileWriter = (config) => {
  return withDangerousMod(config, ['ios', async (config) => {
    const filePath = path.join(
      config.modRequest.platformProjectRoot,
      'custom-file.txt'
    );
    await require('fs/promises').writeFile(filePath, 'test content');
    return config;
  }]);
};

describe('withFileWriter', () => {
  beforeEach(() => {
    vol.reset();
    // Set up mock filesystem
    vol.fromJSON({
      '/test/ios/': null,
    });
  });
  
  it('should write file to ios directory', async () => {
    let config: any = {
      name: 'TestApp',
      slug: 'test-app',
    };
    
    config = withFileWriter(config);
    
    // Execute dangerous mod
    await config.mods.ios.dangerous({
      ...config,
      modResults: null,
      modRequest: {
        projectRoot: '/test',
        platformProjectRoot: '/test/ios',
        platform: 'ios',
        modName: 'dangerous',
        introspect: false,
      },
    });
    
    // Verify file was written
    const fs = require('fs');
    expect(fs.existsSync('/test/ios/custom-file.txt')).toBe(true);
    expect(fs.readFileSync('/test/ios/custom-file.txt', 'utf-8')).toBe('test content');
  });
});

Example tests from Expo source

Here are patterns from Expo’s test suite:

Testing permission helpers

Permissions-test.ts

import * as Permissions from '../Permissions';

describe('Android Permissions', () => {
  it('should prefix permission names', () => {
    const permissions = ['CAMERA', 'LOCATION'];
    const result = Permissions.prefixAndroidPermissionsIfNecessary(permissions);
    
    expect(result).toEqual([
      'android.permission.CAMERA',
      'android.permission.LOCATION',
    ]);
  });
  
  it('should not duplicate prefixes', () => {
    const permissions = ['android.permission.CAMERA'];
    const result = Permissions.prefixAndroidPermissionsIfNecessary(permissions);
    
    expect(result).toEqual(['android.permission.CAMERA']);
  });
  
  it('should add permission to manifest', () => {
    const manifest = {
      manifest: {
        $: {},
        'uses-permission': [],
      },
    };
    
    Permissions.addPermission(manifest, 'android.permission.CAMERA');
    
    expect(manifest.manifest['uses-permission']).toContainEqual({
      $: { 'android:name': 'android.permission.CAMERA' },
    });
  });
  
  it('should ensure permissions are unique', () => {
    const manifest = {
      manifest: {
        $: {},
        'uses-permission': [
          { $: { 'android:name': 'android.permission.CAMERA' } },
        ],
      },
    };
    
    const added = Permissions.ensurePermission(manifest, 'android.permission.CAMERA');
    
    expect(added).toBe(false);
    expect(manifest.manifest['uses-permission']).toHaveLength(1);
  });
});

Testing entitlements

Entitlements-test.ts

import { setAssociatedDomains } from '../Entitlements';

describe('iOS Entitlements', () => {
  it('should set associated domains', () => {
    const config = {
      ios: {
        associatedDomains: ['applinks:example.com', 'applinks:www.example.com'],
      },
    };
    
    const entitlements = {};
    const result = setAssociatedDomains(config, entitlements);
    
    expect(result['com.apple.developer.associated-domains']).toEqual([
      'applinks:example.com',
      'applinks:www.example.com',
    ]);
  });
  
  it('should not modify entitlements when config is empty', () => {
    const config = {};
    const entitlements = { existingKey: 'value' };
    
    const result = setAssociatedDomains(config, entitlements);
    
    expect(result).toEqual(entitlements);
    expect(result['com.apple.developer.associated-domains']).toBeUndefined();
  });
});

Integration testing with prebuild

Test the full prebuild process in a temporary directory.

import { execSync } from 'child_process';
import fs from 'fs';
import path from 'path';
import os from 'os';

describe('prebuild integration', () => {
  let tempDir: string;
  
  beforeEach(() => {
    tempDir = fs.mkdtempSync(path.join(os.tmpdir(), 'expo-test-'));
  });
  
  afterEach(() => {
    fs.rmSync(tempDir, { recursive: true, force: true });
  });
  
  it('should apply plugin during prebuild', () => {
    // Create minimal app.json
    const appJson = {
      expo: {
        name: 'TestApp',
        slug: 'test-app',
        version: '1.0.0',
        plugins: ['./custom-plugin.js'],
      },
    };
    
    fs.writeFileSync(
      path.join(tempDir, 'app.json'),
      JSON.stringify(appJson, null, 2)
    );
    
    // Create plugin
    const pluginCode = `
module.exports = function withCustomPlugin(config) {
  config.ios = config.ios || {};
  config.ios.bundleIdentifier = 'com.test.custom';
  return config;
};
`;
    
    fs.writeFileSync(
      path.join(tempDir, 'custom-plugin.js'),
      pluginCode
    );
    
    // Run prebuild
    execSync('npx expo prebuild --no-install', {
      cwd: tempDir,
      stdio: 'pipe',
    });
    
    // Verify modifications
    expect(fs.existsSync(path.join(tempDir, 'ios'))).toBe(true);
    expect(fs.existsSync(path.join(tempDir, 'android'))).toBe(true);
    
    // Could parse Info.plist or other files to verify changes
  }, 60000); // 60s timeout for slow CI
});

Continuous integration

Add plugin tests to your CI pipeline. GitHub Actions example:

.github/workflows/test.yml

name: Test Config Plugins

on: [push, pull_request]

jobs:
  test:
    runs-on: ubuntu-latest
    
    steps:
      - uses: actions/checkout@v3
      
      - uses: actions/setup-node@v3
        with:
          node-version: 18
          
      - name: Install dependencies
        run: npm ci
        
      - name: Run tests
        run: npm test -- --coverage
        
      - name: Upload coverage
        uses: codecov/codecov-action@v3

Best practices

1. Test edge cases

describe('withCustomPlugin edge cases', () => {
  it('should handle missing config properties', () => {
    const config = {};
    expect(() => withCustomPlugin(config)).not.toThrow();
  });
  
  it('should handle invalid props', () => {
    const config = { name: 'Test' };
    expect(() => withCustomPlugin(config, { invalid: true })).toThrow();
  });
  
  it('should handle empty arrays', () => {
    const config = { name: 'Test', plugins: [] };
    const result = withCustomPlugin(config);
    expect(result.plugins).toEqual([]);
  });
});

2. Test idempotency

it('should be idempotent', () => {
  let config = { name: 'Test' };
  
  // Apply plugin multiple times
  config = withCustomPlugin(config);
  const firstResult = JSON.stringify(config);
  
  config = withCustomPlugin(config);
  const secondResult = JSON.stringify(config);
  
  expect(firstResult).toBe(secondResult);
});

3. Use snapshots for complex output

it('should match snapshot', () => {
  const config = { name: 'Test' };
  const result = withCustomPlugin(config);
  
  expect(result).toMatchSnapshot();
});

4. Mock external dependencies

jest.mock('@expo/config-plugins', () => ({
  ...jest.requireActual('@expo/config-plugins'),
  WarningAggregator: {
    addWarningIOS: jest.fn(),
  },
}));

it('should log warnings', () => {
  const { WarningAggregator } = require('@expo/config-plugins');
  
  withCustomPlugin(config);
  
  expect(WarningAggregator.addWarningIOS).toHaveBeenCalledWith(
    'custom-plugin',
    'Some warning message'
  );
});

Debugging tests

Enable debug output in tests:

process.env.EXPO_DEBUG = '1';

it('should log debug info', () => {
  const config = withCustomPlugin(baseConfig);
  // Check console output
});

Next steps

Creating plugins - Build testable plugins
Modifying native projects - Test complex modifications
Plugin API reference - API documentation

Introduction

Building Plugins

Common Patterns

Testing Config Plugins

Testing strategies

Unit testing plugins

Basic plugin test

Testing Info.plist modifications

Testing Android Manifest modifications

Testing with real fixtures

Setting up fixtures

Loading and testing with fixtures

Testing code generation

Testing with mock filesystem

Example tests from Expo source

Testing permission helpers

Testing entitlements

Integration testing with prebuild

Continuous integration

Best practices

1. Test edge cases

2. Test idempotency

3. Use snapshots for complex output

4. Mock external dependencies

Debugging tests

Next steps

Introduction

Building Plugins

Common Patterns

​Testing strategies

​Unit testing plugins

​Basic plugin test

​Testing Info.plist modifications

​Testing Android Manifest modifications

​Testing with real fixtures

​Setting up fixtures

​Loading and testing with fixtures

​Testing code generation

​Testing with mock filesystem

​Example tests from Expo source

​Testing permission helpers

​Testing entitlements

​Integration testing with prebuild

​Continuous integration

​Best practices

​1. Test edge cases

​2. Test idempotency

​3. Use snapshots for complex output

​4. Mock external dependencies

​Debugging tests

​Next steps

Testing strategies

Unit testing plugins

Basic plugin test

Testing Info.plist modifications

Testing Android Manifest modifications

Testing with real fixtures

Setting up fixtures

Loading and testing with fixtures

Testing code generation

Testing with mock filesystem

Example tests from Expo source

Testing permission helpers

Testing entitlements

Integration testing with prebuild

Continuous integration

Best practices

1. Test edge cases

2. Test idempotency

3. Use snapshots for complex output

4. Mock external dependencies

Debugging tests

Next steps