Fix #4115: Upgrade to webpack 4.x.

https://github.com/rails/webpacker/blob/master/docs/v4-upgrade.md

.babelrc

@@ -1,18 +0,0 @@
-{
-  "presets": [
-    ["env", {
-      "modules": false,
-      "targets": {
-        "browsers": "> 1%",
-        "uglify": true
-      },
-      "useBuiltIns": true
-    }]
-  ],
-
-  "plugins": [
-    "syntax-dynamic-import",
-    "transform-object-rest-spread",
-    ["transform-class-properties", { "spec": true }]
-  ]
-}

.postcssrc.yml

@@ -1,3 +0,0 @@
-plugins:
-  postcss-import: {}
-  postcss-cssnext: {}

babel.config.js

@@ -0,0 +1,72 @@
+module.exports = function(api) {
+  var validEnv = ['development', 'test', 'production']
+  var currentEnv = api.env()
+  var isDevelopmentEnv = api.env('development')
+  var isProductionEnv = api.env('production')
+  var isTestEnv = api.env('test')
+
+  if (!validEnv.includes(currentEnv)) {
+    throw new Error(
+      'Please specify a valid `NODE_ENV` or ' +
+        '`BABEL_ENV` environment variables. Valid values are "development", ' +
+        '"test", and "production". Instead, received: ' +
+        JSON.stringify(currentEnv) +
+        '.'
+    )
+  }
+
+  return {
+    presets: [
+      isTestEnv && [
+        '@babel/preset-env',
+        {
+          targets: {
+            node: 'current'
+          }
+        }
+      ],
+      (isProductionEnv || isDevelopmentEnv) && [
+        '@babel/preset-env',
+        {
+          forceAllTransforms: true,
+          useBuiltIns: 'entry',
+          corejs: 3,
+          modules: false,
+          exclude: ['transform-typeof-symbol']
+        }
+      ]
+    ].filter(Boolean),
+    plugins: [
+      'babel-plugin-macros',
+      '@babel/plugin-syntax-dynamic-import',
+      isTestEnv && 'babel-plugin-dynamic-import-node',
+      '@babel/plugin-transform-destructuring',
+      [
+        '@babel/plugin-proposal-class-properties',
+        {
+          loose: true
+        }
+      ],
+      [
+        '@babel/plugin-proposal-object-rest-spread',
+        {
+          useBuiltIns: true
+        }
+      ],
+      [
+        '@babel/plugin-transform-runtime',
+        {
+          helpers: false,
+          regenerator: true,
+          corejs: false
+        }
+      ],
+      [
+        '@babel/plugin-transform-regenerator',
+        {
+          async: false
+        }
+      ]
+    ].filter(Boolean)
+  }
+}

config/webpacker.yml

@@ -3,8 +3,11 @@
 default: &default
   source_path: app/javascript
   source_entry_path: packs
+  public_root_path: public
   public_output_path: packs
   cache_path: tmp/cache/webpacker
+  check_yarn_integrity: false
+  webpack_compile_output: false
 
   # Additional paths webpack should lookup modules
   # ['app/assets', 'engine/foo/app/assets']
@@ -13,8 +16,26 @@ default: &default
   # Reload manifest.json on all requests so we reload latest compiled packs
   cache_manifest: false
 
+  # Extract and emit a css file
+  extract_css: false
+
+  static_assets_extensions:
+    - .jpg
+    - .jpeg
+    - .png
+    - .gif
+    - .tiff
+    - .ico
+    - .svg
+    - .eot
+    - .otf
+    - .ttf
+    - .woff
+    - .woff2
+
   extensions:
     - .erb
+    - .mjs
     - .js
     - .sass
     - .scss
@@ -32,6 +53,9 @@ development:
   <<: *default
   compile: true
 
+  # Verifies that correct packages and versions are installed by inspecting package.json, yarn.lock, and node_modules
+  check_yarn_integrity: true
+
   # Reference: https://webpack.js.org/configuration/dev-server/
   dev_server:
     https: false
@@ -49,7 +73,7 @@ development:
     headers:
       'Access-Control-Allow-Origin': '*'
     watch_options:
-      ignored: /node_modules/
+      ignored: '**/node_modules/**'
 
 
 test:
@@ -65,5 +89,8 @@ production:
   # Production depends on precompilation of packs prior to booting for performance.
   compile: false
 
+  # Extract and emit a css file
+  extract_css: true
+
   # Cache manifest.json for performance
   cache_manifest: true

package.json

@@ -1,7 +1,7 @@
 {
   "license": "MIT",
   "dependencies": {
-    "@rails/webpacker": "^3.5.3",
+    "@rails/webpacker": "^4.0.0",
     "debug-loader": "^0.0.1",
     "expose-loader": "^0.7.5",
     "hammerjs": "^2.0.8",

postcss.config.js

@@ -0,0 +1,12 @@
+module.exports = {
+  plugins: [
+    require('postcss-import'),
+    require('postcss-flexbugs-fixes'),
+    require('postcss-preset-env')({
+      autoprefixer: {
+        flexbox: 'no-2009'
+      },
+      stage: 3
+    })
+  ]
+}