Fix RSC test infrastructure: Convert jest.setup.js to ESM and re-enable tests (#2124)

## Summary

Fixes #2122

- Convert `jest.setup.js` from CommonJS `require()` to ESM imports for
compatibility with ESM mode
- Re-enable RSC tests with automatic React version detection
- Remove unnecessary `script/convert` override since `package.json` now
handles version checking automatically

## Changes

### `packages/react-on-rails-pro/tests/jest.setup.js`
Moved `require()` statements to top-level ESM imports:
- `import { TextEncoder, TextDecoder } from 'util'`
- `import { Readable } from 'stream'`
- `import { ReadableStream, ReadableStreamDefaultReader } from
'stream/web'`
- `import { jest } from '@jest/globals'`

### `packages/react-on-rails-pro/package.json`
Re-enabled `test:rsc` script with React version detection:
- Tests run normally on React 19+ where RSC features are available
- Tests skip gracefully on React 18 with informative message: `RSC tests
skipped (requires React 19+, found 18.0.0)`

### `script/convert`
Removed the override that disabled RSC tests since the version check is
now built into `package.json`.

## Test plan

- [x] Run `yarn test` in `packages/react-on-rails-pro` - passes with
React 18 (RSC tests skipped)
- [x] Verify RSC tests will run on React 19 in CI

🤖 Generated with [Claude Code](https://claude.com/claude-code)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

## Release Notes

* **Chores**
* Updated RSC test infrastructure to conditionally run based on React
version; tests now execute for React 19+ and skip for earlier versions.
* Modernized Jest setup configuration to use ES module imports instead
of dynamic requires, improving compatibility with modern JavaScript
standards.

<sub>✏️ Tip: You can customize this high-level summary in your review
settings.</sub>

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Co-authored-by: Claude <noreply@anthropic.com>

Author: justin808

.gitignore

@@ -1,3 +1,6 @@
+# Root Gemfile.lock (delegates to react_on_rails/Gemfile, lock is there)
+/Gemfile.lock
+
 .bundle/
 /.yardoc
 /_yardoc/

packages/react-on-rails-pro/package.json

@@ -9,7 +9,7 @@
     "clean": "rm -rf ./lib",
     "test": "pnpm run test:non-rsc && pnpm run test:rsc",
     "test:non-rsc": "jest tests --testPathIgnorePatterns=\".*(RSC|stream|registerServerComponent|serverRenderReactComponent|SuspenseHydration).*\"",
-    "test:rsc": "echo 'TODO: RSC tests disabled - jest.setup.js uses require() incompatible with ESM mode. Re-enable after converting setup to ESM.' && exit 0",
+    "test:rsc": "node scripts/check-react-version.cjs || NODE_CONDITIONS=react-server jest tests/*.rsc.test.*",
     "type-check": "tsc --noEmit --noErrorTruncation",
     "prepare": "[ -f lib/ReactOnRails.full.js ] || (rm -rf ./lib && tsc)",
     "prepublishOnly": "pnpm run build",

packages/react-on-rails-pro/scripts/check-react-version.cjs

@@ -0,0 +1,19 @@
+/**
+ * Check if React version supports RSC features (React 19+).
+ * Exits with code 0 if React < 19 (skip RSC tests).
+ * Exits with code 1 if React >= 19 (run RSC tests).
+ *
+ * This is a CommonJS file (.cjs) because it needs to use require()
+ * and the parent package has "type": "module".
+ */
+const v = require('react/package.json').version;
+
+const majorVersion = parseInt(v, 10);
+
+if (majorVersion < 19) {
+  console.log(`RSC tests skipped (requires React 19+, found ${v})`);
+  process.exit(0);
+}
+
+// Exit with code 1 so the || chain continues to run Jest
+process.exit(1);

packages/react-on-rails-pro/tests/jest.setup.js

@@ -1,3 +1,8 @@
+import { TextEncoder, TextDecoder } from 'util';
+import { Readable } from 'stream';
+import { ReadableStream, ReadableStreamDefaultReader } from 'stream/web';
+import { jest } from '@jest/globals';
+
 // If jsdom environment is set and TextEncoder is not defined, then define TextEncoder and TextDecoder
 // The current version of jsdom does not support TextEncoder and TextDecoder
 // The following code will tell us when jsdom supports TextEncoder and TextDecoder
@@ -11,13 +16,6 @@ if (typeof window !== 'undefined' && typeof window.MessageChannel !== 'undefined
 }
 
 if (typeof window !== 'undefined') {
-  // eslint-disable-next-line global-require
-  const { TextEncoder, TextDecoder } = require('util');
-  // eslint-disable-next-line global-require
-  const { Readable } = require('stream');
-  // eslint-disable-next-line global-require
-  const { ReadableStream, ReadableStreamDefaultReader } = require('stream/web');
-
   // Mock the fetch function to return a ReadableStream instead of Node's Readable stream
   // This matches browser behavior where fetch responses have ReadableStream bodies
   // Node's fetch and polyfills like jest-fetch-mock return Node's Readable stream,

script/convert

@@ -66,12 +66,8 @@ gsub_file_content(
   '"test:non-rsc": "jest tests --testPathIgnorePatterns=\".*(RSC|stream|' \
   'registerServerComponent|serverRenderReactComponent|SuspenseHydration).*\"",'
 )
-# Make test:rsc script do nothing
-gsub_file_content(
-  "../packages/react-on-rails-pro/package.json",
-  /"test:rsc": "(?:\\"|[^"])*",/,
-  '"test:rsc": "exit 0",'
-)
+# test:rsc script now automatically detects React version and skips on React 18
+# No override needed - the script checks React version and exits cleanly if < 19
 # Keep modern JSX transform for React 18+
 # gsub_file_content("../tsconfig.json", "react-jsx", "react")
 # gsub_file_content("../spec/dummy/babel.config.js", "runtime: 'automatic'", "runtime: 'classic'")