[0.76 RC] Babel preset is incomplete without `hermes-parser`

[jbroma]

Description

Problem

JavaScript files using newer Flow syntax (e.g., as type casts) fail to transpile correctly in React Native's Metro preset when hermesParser is disabled. This is because babel-parser no longer fully supports Flow, and Hermes parser is now required for handling the latest Flow features.

Background

1. babel-parser no longer fully supports newer Flow features due to the Flow plugin being unmaintained:
   [Discussion] Remove flow support from @babel/parser babel/babel#16264.
2. React Native has enabled hermesParser by default in 0.76.0:
   RN: Enable Hermes Parser in RNTester / CI #46318

Proposed Solution

To make the Metro preset self-sufficient, we should consider including babel-plugin-syntax-hermes-parser in the preset. This would ensure that even when used without metro, babel can still transpile newer flow syntax properly.

Steps to reproduce

1. yarn install
2. yarn react-native start
3. Trigger compilation by going to http://localhost:8081/index.bundle?platform=ios in the browser or by running the app

React Native Version

0.76.0-rc.1

Affected Platforms

Build - MacOS, Build - Windows, Build - Linux

Output of npx react-native info

System:
  OS: macOS 14.6.1
  CPU: (10) arm64 Apple M1 Pro
  Memory: 148.48 MB / 32.00 GB
  Shell:
    version: "5.9"
    path: /bin/zsh
Binaries:
  Node:
    version: 20.13.1
    path: ~/Library/Caches/fnm_multishells/27637_1727087645332/bin/node
  Yarn:
    version: 4.5.0
    path: ~/Library/Caches/fnm_multishells/27637_1727087645332/bin/yarn
  npm:
    version: 10.5.2
    path: ~/Library/Caches/fnm_multishells/27637_1727087645332/bin/npm
  Watchman:
    version: 2024.09.09.00
    path: /opt/homebrew/bin/watchman
Managers:
  CocoaPods:
    version: 1.15.2
    path: /Users/jbroma/.rbenv/shims/pod
SDKs:
  iOS SDK:
    Platforms:
      - DriverKit 24.0
      - iOS 18.0
      - macOS 15.0
      - tvOS 18.0
      - visionOS 2.0
      - watchOS 11.0
  Android SDK: Not Found
IDEs:
  Android Studio: 2024.1 AI-241.18034.62.2412.12266719
  Xcode:
    version: 16.0/16A242d
    path: /usr/bin/xcodebuild
Languages:
  Java:
    version: 17.0.12
    path: /Users/jbroma/.jenv/shims/javac
  Ruby:
    version: 2.7.6
    path: /Users/jbroma/.rbenv/shims/ruby
npmPackages:
  "@react-native-community/cli":
    installed: 15.0.0-alpha.2
    wanted: 15.0.0-alpha.2
  react:
    installed: 18.3.1
    wanted: 18.3.1
  react-native:
    installed: 0.76.0-rc.1
    wanted: 0.76.0-rc.1
  react-native-macos: Not Found
npmGlobalPackages:
  "*react-native*": Not Found
Android:
  hermesEnabled: true
  newArchEnabled: true
iOS:
  hermesEnabled: Not found
  newArchEnabled: false

Stacktrace or Logs

error: SyntaxError: /Users/jbroma/Developer/RN76BabelPresetReproducer/node_modules/react-native/Libraries/vendor/emitter/EventEmitter.js: Unexpected token, expected "]" (39:5)

  37 |
  38 | type Registry<TEventToArgsMap: {...}> = {
> 39 |   [K in keyof TEventToArgsMap]: Set<Registration<TEventToArgsMap[K]>>,
     |      ^
  40 | };
  41 |
  42 | /**

Reproducer

https://github.com/jbroma/RN76BabelPresetReproducer

Screenshots and Videos

No response

[cipolleschi]

We just landed a couple of picks to fix that. We are going to release RC.2 between today and tomorrow morning.
Is it okay to wait for that?

[jbroma]

We just landed a couple of picks to fix that. We are going to release RC.2 between today and tomorrow morning. Is it okay to wait for that?

absolutely, this just needs to be addressed before stable release of 0.76 👍

[cipolleschi]

can you double check if this is fixed, now that RC2 is out? If yes, can we close this issue? 🙏

[jbroma]

tried the RC.2 today and it's still not fixed, btw I've upgraded the repro to RC.2

[jpudysz]

I can confirm that! I was trying to test out the new debugger, but I'm also seeing flow errors

[huntie]

Looks like Riccardo was referencing these picked commits which made it into RC2 — these affect Jest only.

I think I came across this also being hit by our CI, recently in #45960 — but I forget my own context 😅. Anyway, will try and repro + land the fix.