POC for opt-in SASS loader

packages/react-dev-utils/optinSassWebpackLoader.js

@@ -0,0 +1,18 @@
+module.exports.default = function(source) {
+  try {
+    // XXX performance hit of calling require / require.resolve this for every file passed to this loader?
+    require.resolve('node-sass');
+
+    // Options are passed to loader using the function `this`. We use `bind`
+    // to transparently pass options coming from the webpack config to
+    // sass-loader:
+    // XXX performance hit of bind?
+    const sassLoader = require('sass-loader').bind(this);
+
+    return sassLoader(source);
+  } catch (er) {
+    throw new Error(
+      'It looks like you have required a .scss file. In order for this to work, you need to install node-sass and sass-loader first.'
+    );
+  }
+};

packages/react-dev-utils/package.json

@@ -34,7 +34,8 @@
     "printHostingInstructions.js",
     "WatchMissingNodeModulesPlugin.js",
     "WebpackDevServerUtils.js",
-    "webpackHotDevClient.js"
+    "webpackHotDevClient.js",
+    "optinSassWebpackLoader.js"
   ],
   "dependencies": {
     "@babel/code-frame": "7.0.0-beta.38",
@@ -59,7 +60,9 @@
     "text-table": "0.2.0"
   },
   "devDependencies": {
-    "jest": "22.1.2"
+    "jest": "22.1.2",
+    "node-sass": "^4.7.2",
+    "sass-loader": "^6.0.6"
   },
   "scripts": {
     "test": "jest"

packages/react-scripts/config/env.js

@@ -48,6 +48,30 @@ dotenvFiles.forEach(dotenvFile => {
   }
 });
 
+const appDirectory = fs.realpathSync(process.cwd());
+
+/**
+ * Utility to parse PATH-like strings to array
+ *
+ * Separators:
+ * : in Unix (colon)
+ * ; in Windows (semicolon)
+ *
+ * @param {String} paths
+ * @returns {string}
+ */
+function parsePath(paths) {
+  if (paths) {
+    return paths
+      .split(path.delimiter)
+      .filter(folder => folder && !path.isAbsolute(folder))
+      .map(folder => path.resolve(appDirectory, folder))
+      .join(path.delimiter);
+  }
+
+  return '';
+}
+
 // We support resolving modules according to `NODE_PATH`.
 // This lets you use absolute paths in imports inside large monorepos:
 // https://github.com/facebookincubator/create-react-app/issues/253.
@@ -57,12 +81,14 @@ dotenvFiles.forEach(dotenvFile => {
 // Otherwise, we risk importing Node.js core modules into an app instead of Webpack shims.
 // https://github.com/facebookincubator/create-react-app/issues/1023#issuecomment-265344421
 // We also resolve them to make sure all tools using them work consistently.
-const appDirectory = fs.realpathSync(process.cwd());
-process.env.NODE_PATH = (process.env.NODE_PATH || '')
-  .split(path.delimiter)
-  .filter(folder => folder && !path.isAbsolute(folder))
-  .map(folder => path.resolve(appDirectory, folder))
-  .join(path.delimiter);
+process.env.NODE_PATH = parsePath(process.env.NODE_PATH);
+
+// node-sass supports additional paths to resolve imports from.
+// https://github.com/sass/node-sass/tree/v4.7.2#includepaths
+// It works similar to NODE_PATH, but for importing scss files.
+// We would like to assign an array to process.env variable, but
+// Node will implicitly convert value to string.
+process.env.SASS_INCLUDE_PATHS = parsePath(process.env.SASS_INCLUDE_PATHS);
 
 // Grab NODE_ENV and REACT_APP_* environment variables and prepare them to be
 // injected into the application via DefinePlugin in Webpack configuration.
@@ -87,6 +113,7 @@ function getClientEnvironment(publicUrl) {
         PUBLIC_URL: publicUrl,
       }
     );
+
   // Stringify all values so we can feed into Webpack DefinePlugin
   const stringified = {
     'process.env': Object.keys(raw).reduce((env, key) => {

packages/react-scripts/config/webpack.config.dev.js

@@ -264,6 +264,39 @@ module.exports = {
               },
             ],
           },
+          // "react-dev-utils/optinSassWebpackLoader" wraps sass-loader, trying to require
+          // node-sass and sass-loader, and prompting the user to install them if it fails
+          // "postcss" loader applies autoprefixer to our CSS.
+          // "css" loader resolves paths in CSS and adds assets as dependencies.
+          // "style" loader turns CSS into JS modules that inject <style> tags.
+          // In production, we use a plugin to extract that CSS to a file, but
+          // in development "style" loader enables hot editing of CSS.
+          {
+            test: /\.scss$/,
+            use: [
+              require.resolve('style-loader'),
+              {
+                loader: require.resolve('css-loader'),
+                options: {
+                  importLoaders: 2,
+                },
+              },
+              {
+                loader: require.resolve('postcss-loader'),
+                options: postCSSLoaderOptions,
+              },
+              {
+                loader: require.resolve(
+                  'react-dev-utils/optinSassWebpackLoader'
+                ),
+                options: {
+                  includePaths: process.env.SASS_INCLUDE_PATHS.split(
+                    path.delimiter
+                  ),
+                },
+              },
+            ],
+          },
           // Allows you to use two kinds of imports for SVG:
           // import logoUrl from './logo.svg'; gives you the URL.
           // import { ReactComponent as Logo } from './logo.svg'; gives you a component.