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Hermes transforms not enabled by default when creating a new 0.70.0 project

See original GitHub issue

Description

I can tell that the Hermes Babel transforms are not enabled in brand new 0.70.0 projects.

Version

0.70.0

Output of `npx react-native info`

System:
    OS: macOS 12.5.1
    CPU: (8) arm64 Apple M1
    Memory: 112.94 MB / 16.00 GB
    Shell: 5.8.1 - /bin/zsh
  Binaries:
    Node: 18.7.0 - /opt/homebrew/bin/node
    Yarn: 1.22.15 - /usr/local/bin/yarn
    npm: 8.15.0 - /opt/homebrew/bin/npm
    Watchman: 2022.07.04.00 - /opt/homebrew/bin/watchman
  Managers:
    CocoaPods: 1.11.3 - /opt/homebrew/bin/pod
  SDKs:
    iOS SDK:
      Platforms: DriverKit 21.4, iOS 15.5, macOS 12.3, tvOS 15.4, watchOS 8.5
    Android SDK: Not Found
  IDEs:
    Android Studio: 2021.2 AI-212.5712.43.2112.8512546
    Xcode: 13.4.1/13F100 - /usr/bin/xcodebuild
  Languages:
    Java: 11.0.15 - /usr/bin/javac
  npmPackages:
    @react-native-community/cli: Not Found
    react: 18.1.0 => 18.1.0 
    react-native: 0.70.0 => 0.70.0 
    react-native-macos: Not Found
  npmGlobalPackages:
    *react-native*: Not Found

Steps to reproduce

Create a new project

npx react-native@0.70.0 init ExampleApp
cd ExampleApp

Add the following code to App.js
```
console.log(2n ** 42n);
```
Start the project
```
yarn android
```

Observe that the app crashes with:

 ERROR  TypeError: Cannot convert BigInt to number

This is because the project is using the non Hermes transforms that convert the code you added to this invalid code:

Math.pow(2n, 42n);  // You can't use `Math.pow` with `BigInt`!

The workaround is to enable the transforms in babel.config.js, but it would be preferable if 0.70.0 did this by virtue of Hermes being enabled:

module.exports = {
   presets: [
     [
       'module:metro-react-native-babel-preset',
       {unstable_transformProfile: 'hermes-stable'},
     ],
   ],
};

Snack, code example, screenshot, or link to a repository

https://snack.expo.dev/haG_X4VvG

Issue Analytics

State:
Created a year ago
Comments:8 (4 by maintainers)

Top GitHub Comments

4reactions

motiz88commented, Sep 20, 2022

To add to @huntie’s comment: Longer-term solutions aside, we’re looking to unblock the use of BigInt by removing the exponentiation operator transform (a ** b → Math.pow(a, b)) from metro-react-native-babel-preset altogether, given that both JSC and Hermes support this syntax natively. This will likely depend on https://github.com/facebook/metro/pull/871, which will be part of the next Metro version.

4reactions

lexercommented, Sep 19, 2022

@kelset is there any update or another issue we should track?