Generator fails on Shakapacker < 9.3.0: missing @babel/preset-react dependency

[ihabadham]

Environment

1. Ruby version: 3.3.7
2. Rails version: 8.0.4
3. Shakapacker version: 8.4.0 (any version < 9.3.0)
4. React on Rails version: 16.2.0-rc.0

Summary

The react_on_rails:install generator fails on fresh installations when using Shakapacker < 9.3.0 because it creates a babel.config.js that requires @babel/preset-react, but doesn't install that dependency.

Expected behavior

Running rails generate react_on_rails:install should produce a working application that builds successfully with webpack.

Actual behavior

Webpack build fails with:

Error: Cannot find package '@babel/preset-react' imported from babel-virtual-resolve-base.js

Root Cause

PR #2051 removed @babel/preset-react from the generator's installed dependencies with the intent that:

1. SWC is now the default transpiler (has built-in JSX support)
2. Babel users should install presets manually

However, this creates issues because:

1. Shakapacker < 9.3.0 uses Babel by default (not SWC)
2. The generator still creates babel.config.js that requires @babel/preset-react:

   // Generated babel.config.js line 11
   '@babel/preset-react',

3. The dependency is not installed, so webpack fails

Why CI doesn't catch this

The dummy app (spec/dummy) has @babel/preset-react pre-installed in its package.json (legacy from before PR #2051). CI tests against the existing dummy app, not a fresh generator output.

Steps to Reproduce

rails new test_app --minimal
cd test_app

# Add to Gemfile:
# gem "react_on_rails"
# gem "shakapacker", "~> 8.2"

bundle install
git init && git add . && git commit -m "Initial"

rails generate react_on_rails:install

# Start the app
foreman start -f Procfile.dev
# -> Webpack fails with "Cannot find package '@babel/preset-react'"

Proposed Fix

Install @babel/preset-react when NOT using SWC:

# js_dependency_manager.rb
BABEL_REACT_DEPENDENCIES = %w[
  @babel/preset-react
].freeze

def add_js_dependencies
  # ... existing code ...
  add_babel_react_dependencies unless using_swc?
  # ...
end

This aligns with:

• Shakapacker's own documentation (docs/react.md tells users to npm install @babel/preset-react)
• The gemspec supporting Shakapacker >= 6.0
• CI testing against Shakapacker 8.2.0 as minimum

Related

• PR Refactor: Extract JS dependency management into shared module #2051 introduced this change
• Shakapacker docs/react.md explicitly requires @babel/preset-react for Babel users
• Shakapacker docs/using_swc_loader.md confirms SWC has built-in JSX support (no Babel presets needed)