Use SWC transpiler for all JavaScript and TypeScript

- Remove babel.config.js and all Babel transpilation dependencies
- Configure shakapacker.yml to use javascript_transpiler: swc
- Update swc.config.js with spec-compliant settings for Stimulus:
  - keepClassNames: true to preserve class names
  - loose: false for spec-compliant class transforms
  - externalHelpers: false to avoid binding issues
  - target: es2015 for broad compatibility
- Move @swc/core and swc-loader to dependencies
- Update Jest to use @swc/jest
- Keep @babel/runtime for runtime helpers and @babel/eslint-parser for ESLint

This is a pure SWC migration (not the hybrid Babel/SWC approach).

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

Co-Authored-By: Claude <noreply@anthropic.com>

Author: justin808

babel.config.js

@@ -8,7 +8,7 @@ default: &default
   cache_path: tmp/shakapacker
   webpack_compile_output: true
   nested_entries: true
-  javascript_transpiler: babel
+  javascript_transpiler: swc
 
   # Additional paths webpack should lookup modules
   # ['app/assets', 'engine/foo/app/assets']

config/shakapacker.yml

@@ -4,14 +4,27 @@
 
 module.exports = {
   jsc: {
+    parser: {
+      syntax: "ecmascript",
+      jsx: true,
+      dynamicImport: true,
+      decorators: false
+    },
     transform: {
       react: {
         runtime: "automatic"
       }
     },
-    // Preserve ES6 classes for Stimulus compatibility
+    // Critical for Stimulus: preserve class names and use spec-compliant transforms
     keepClassNames: true,
-    // Disable loose mode to ensure proper class method binding for Stimulus
-    loose: false
+    // Use spec-compliant class transforms (not loose mode)
+    loose: false,
+    // Don't use external helpers to avoid any potential binding issues
+    externalHelpers: false,
+    target: "es2015"
+  },
+  // Preserve original semantics for class fields/methods
+  module: {
+    type: "es6"
   }
 };

config/swc.config.js

@@ -31,13 +31,8 @@
     "build:clean": "rm -rf public/packs || true"
   },
   "dependencies": {
-    "@babel/plugin-transform-runtime": "^7.21.0",
-    "@babel/preset-env": "^7.20.2",
-    "@babel/preset-react": "^7.18.6",
     "@babel/runtime": "^7.17.9",
-    "babel-loader": "^9.1.2",
-    "babel-plugin-macros": "^3.1.0",
-    "babel-plugin-transform-react-remove-prop-types": "^0.4.24",
+    "@swc/core": "^1.3.100",
     "@glennsl/rescript-fetch": "^0.2.0",
     "@glennsl/rescript-json-combinators": "^1.2.1",
     "@hotwired/stimulus": "^3.2.1",
@@ -101,18 +96,15 @@
     "webpack": "5",
     "webpack-assets-manifest": "5",
     "webpack-cli": "5",
-    "webpack-merge": "5"
+    "webpack-merge": "5",
+    "swc-loader": "^0.2.6"
   },
   "devDependencies": {
-    "@babel/cli": "^7.21.0",
-    "@babel/core": "^7.21.0",
     "@babel/eslint-parser": "^7.16.5",
     "@pmmmwh/react-refresh-webpack-plugin": "^0.5.10",
-    "@swc/core": "^1.3.100",
     "@swc/jest": "^0.2.29",
     "@tailwindcss/typography": "^0.5.10",
     "@webpack-cli/serve": "^2.0.5",
-    "babel-jest": "^29.5.0",
     "body-parser": "^1.20.2",
     "eslint": "^8.35.0",
     "eslint-config-prettier": "^8.6.0",
@@ -135,7 +127,6 @@
     "react-refresh": "^0.14.0",
     "react-transform-hmr": "^1.0.4",
     "regenerator-runtime": "^0.13.11",
-    "swc-loader": "^0.2.3",
     "typescript": "^5.1.3",
     "webpack-dev-server": "^4.11.1"
   },
@@ -158,7 +149,7 @@
     ],
     "testRegex": "./app/.*.spec\\.jsx?$",
     "transform": {
-      "^.+\\.jsx?$": "babel-jest"
+      "^.+\\.jsx?$": "@swc/jest"
     }
   },
   "packageManager": "yarn@1.22.22+sha512.a6b2f7906b721bba3d67d4aff083df04dad64c399707841b7acf00f6b133b7ac24255f2652fa22ae3534329dc6180534e98d17432037ff6fd140556e2bb3137e"