feat: support data as Function (#648)

Author: evilebottnawi

Diff for: README.md

@@ -274,6 +274,27 @@ If you want to prepend Sass code before the actual entry file, you can set the `
 }
 ```
 
+The `data` option supports `Function` notation:
+
+```javascript
+{
+    loader: "sass-loader",
+    options: {
+        data: (loaderContext) => {
+          // More information about avalaible options https://webpack.js.org/api/loaders/
+          const { resourcePath, rootContext } = loaderContext;
+          const relativePath = path.relative(rootContext,resourcePath);
+          
+          if (relativePath === "styles/foo.scss") {
+             return "$value: 100px;"
+          }
+          
+          return "$value: 200px;"
+        }
+    }
+}
+```
+
 **Please note:** Since you're injecting code, this will break the source mappings in your entry file. Often there's a simpler solution than this, like multiple Sass entry files.
 
 <h2 align="center">Maintainers</h2>

Diff for: lib/normalizeOptions.js

@@ -23,7 +23,13 @@ function normalizeOptions(loaderContext, content, webpackImporter) {
   const options = cloneDeep(utils.getOptions(loaderContext)) || {};
   const { resourcePath } = loaderContext;
 
-  options.data = options.data ? options.data + os.EOL + content : content;
+  let { data } = options;
+
+  if (typeof options.data === 'function') {
+    data = options.data(loaderContext);
+  }
+
+  options.data = data ? data + os.EOL + content : content;
 
   // opt.outputStyle
   if (!options.outputStyle && loaderContext.minimize) {

Diff for: test/index.test.js

@@ -47,6 +47,8 @@ Object.defineProperty(loaderContextMock, 'options', {
   },
 });
 
+/* global should */
+
 implementations.forEach((implementation) => {
   const [implementationName] = implementation.info.split('\t');
 
@@ -185,10 +187,18 @@ implementations.forEach((implementation) => {
             }));
         });
         describe('prepending data', () => {
-          it('should extend the data-option if present', () =>
+          it('should extend the data option if present and it is string', () =>
             execTest('prepending-data', {
               data: `$prepended-data: hotpink${ext === 'sass' ? '\n' : ';'}`,
             }));
+          it('should extend the data option if present and it is function', () =>
+            execTest('prepending-data', {
+              data: (loaderContext) => {
+                should.exist(loaderContext);
+
+                return `$prepended-data: hotpink${ext === 'sass' ? '\n' : ';'}`;
+              },
+            }));
         });
         // See https://github.com/webpack-contrib/sass-loader/issues/21
         describe('empty files', () => {