feat: support coverage (#10)

This pull request adds support for code coverage reporting in the Jest integration. You can now use the --coverage parameter when running Jest with Harness to collect data on how much of your code is covered by your test suites.

Author: V3RON

apps/playground/jest.config.js

@@ -9,4 +9,5 @@ module.exports = {
       setupFilesAfterEnv: ['./src/setupFileAfterEnv.ts'],
     },
   ],
+  collectCoverageFrom: ['./src/**/*.(ts|tsx)'],
 };

packages/babel-preset/package.json

@@ -15,7 +15,8 @@
     }
   },
   "dependencies": {
-    "@babel/plugin-transform-class-static-block": "^7.27.1"
+    "@babel/plugin-transform-class-static-block": "^7.27.1",
+    "babel-plugin-istanbul": "^7.0.1"
   },
   "peerDependencies": {
     "@babel/core": "^7.22.0"

packages/babel-preset/src/preset.ts

@@ -3,7 +3,8 @@ import resolveWeakPlugin from './resolve-weak-plugin';
 export const rnHarnessPlugins = [
   '@babel/plugin-transform-class-static-block',
   resolveWeakPlugin,
-];
+  process.env.RN_HARNESS_COLLECT_COVERAGE ? 'babel-plugin-istanbul' : null,
+].filter((plugin): plugin is string => plugin !== null);
 
 export const rnHarnessPreset = () => {
   if (!process.env.RN_HARNESS) {

packages/bridge/src/shared/test-runner.ts

@@ -80,4 +80,5 @@ export type TestSuiteResult = {
   status: TestResultStatus;
   error?: SerializedError;
   duration: number;
+  coverage?: unknown;
 };

packages/jest/src/index.ts

@@ -66,6 +66,13 @@ export default class JestHarness implements CallbackTestRunnerInterface {
     const { config: harnessConfig } = await getConfig(projectRoot);
     const cliArgs = getAdditionalCliArgs();
     const selectedRunner = getHarnessRunner(harnessConfig, cliArgs);
+
+    if (this.#globalConfig.collectCoverage) {
+      // This is going to be used by @react-native-harness/babel-preset
+      // to enable instrumentation of test files.
+      process.env.RN_HARNESS_COLLECT_COVERAGE = 'true';
+    }
+
     const harness = await getHarness(selectedRunner);
 
     try {

packages/jest/src/run.ts

@@ -111,5 +111,6 @@ export const runHarnessTestFile: RunHarnessTestFile = async ({
     errorMessage,
     tests,
     jestTestPath: testPath,
+    coverage: results.coverage as JestTestResult['coverage'],
   });
 };

packages/runtime/src/runner/factory.ts

@@ -9,10 +9,17 @@ export const getTestRunner = (): TestRunner => {
   return {
     events,
     run: async (testSuite, testFilePath) => {
-      return runSuite(testSuite, {
+      const result = await runSuite(testSuite, {
         events,
         testFilePath,
       });
+
+      // If coverage is enabled, there will be a global variable called __coverage__
+      if ('__coverage__' in global && !!global.__coverage__) {
+        result.coverage = global.__coverage__;
+      }
+
+      return result;
     },
     dispose: () => {
       events.clearAllListeners();