Summary

The private_output_path configuration (added in PR #592 / #604) works correctly on the Ruby side but is missing its computed JS-side property. This causes React on Rails' generated serverWebpackConfig.js to always fall back to a hardcoded path, breaking the sync between shakapacker.yml and webpack output.

Expected behavior

config.privateOutputPath should be available as a computed absolute path on the JS config object, following the same pattern as config.outputPath:

const { config } = require('shakapacker')
console.log(config.outputPath)        // "/app/public/packs" (works)
console.log(config.privateOutputPath)  // "/app/ssr-generated" (expected)

Actual behavior

const { config } = require('shakapacker')
console.log(config.outputPath)         // "/app/public/packs" (works)
console.log(config.privateOutputPath)  // undefined
console.log(config.private_output_path) // "ssr-generated" (raw YAML string, not absolute)

Reproduction

rails new test_app --minimal --skip-git --skip-bundle --skip-test
cd test_app

# Add shakapacker, install, configure private_output_path
echo 'gem "shakapacker", "9.4.0"' >> Gemfile
bundle install
rails shakapacker:install

# Set private_output_path in config/shakapacker.yml under default:
#   private_output_path: ssr-generated

# Check JS config
node -e "
  const config = require('./node_modules/shakapacker/package/config.js');
  console.log('outputPath:', config.outputPath);
  console.log('privateOutputPath:', config.privateOutputPath);
  console.log('private_output_path:', config.private_output_path);
"
# outputPath: /path/to/test_app/public/packs
# privateOutputPath: undefined        <-- BUG
# private_output_path: ssr-generated   <-- raw YAML, not absolute

Root cause

PR #592 added private_output_path to the Ruby-side Shakapacker::Configuration class (returning an absolute path via root_path.join(private_path)), but did not add the corresponding computed property to the JS-side config.ts.

The existing pattern in config.ts:

// Public output: YAML snake_case -> computed camelCase absolute path
config.outputPath = resolve(config.public_root_path, config.public_output_path)

// Private output: missing equivalent
// Should be:
// if (config.private_output_path) {
//   config.privateOutputPath = resolve(config.private_output_path)
// }

Impact

React on Rails' generated serverWebpackConfig.js (from PR shakacode/react_on_rails#2028) uses config.privateOutputPath for the server bundle output path. Since the property is always undefined:

1. False warning on every build: "Shakapacker 9.0+ detected but private_output_path not configured" fires even when it IS configured
2. Broken config sync: Changing private_output_path in shakapacker.yml has no effect on webpack output. The template always falls back to a hardcoded path. This defeats the purpose of the feature.

User report: shakacode/react_on_rails#2289 (comment)

Suggested fix

In package/config.ts, after the outputPath computation:

config.outputPath = resolve(config.public_root_path, config.public_output_path)

// Add computed privateOutputPath (matching outputPath pattern)
if (config.private_output_path) {
  config.privateOutputPath = resolve(config.private_output_path)
}

And update the Config interface in package/types.ts:

export interface Config {
  // ... existing properties ...
  privateOutputPath?: string  // Computed absolute path for private output
}