fix: enforce esm

src/loader.js

@@ -209,6 +209,8 @@ export function pitch(request) {
       ? `\n${esModule ? 'export default' : 'module.exports ='} ${JSON.stringify(
           locals
         )};`
+      : esModule
+      ? `\nexport {};`
       : '';
 
     let resultSource = `// extracted by ${pluginName}`;

test/enforce-esm.test.js

@@ -0,0 +1,36 @@
+import { getCompiler, source, compile } from './helpers';
+
+it('should enforce esm for empty module with options.esModule', async (done) => {
+  const compiler = getCompiler(
+    './esm.js',
+    { esModule: true },
+    {
+      mode: 'production',
+      optimization: { minimize: false },
+    }
+  );
+  const stats = await compile(compiler);
+  expect(stats.hasErrors()).toBe(false);
+  expect(source('./simple.css', stats)).toMatchInlineSnapshot(`
+    "// extracted by mini-css-extract-plugin
+    export {};"
+  `);
+  done();
+});
+
+it('should keep empty module without options.esModule', async (done) => {
+  const compiler = getCompiler(
+    './esm.js',
+    {},
+    {
+      mode: 'production',
+      optimization: { minimize: false },
+    }
+  );
+  const stats = await compile(compiler);
+  expect(stats.hasErrors()).toBe(false);
+  expect(source('./simple.css', stats)).toMatchInlineSnapshot(
+    `"// extracted by mini-css-extract-plugin"`
+  );
+  done();
+});

test/fixtures/esm.js

@@ -0,0 +1 @@
+import "./simple.css";

test/helpers/getCompiler.js

@@ -6,9 +6,11 @@ import { createFsFromVolume, Volume } from 'memfs';
 import MiniCssExtractPlugin from '../../src';
 
 export default (fixture, loaderOptions = {}, config = {}) => {
+  const { outputFileSystem, ...cnfg } = config;
+
   const fullConfig = {
     mode: 'development',
-    devtool: config.devtool || false,
+    devtool: cnfg.devtool || false,
     context: path.resolve(__dirname, '../fixtures'),
     entry: path.resolve(__dirname, '../fixtures', fixture),
     output: {
@@ -40,16 +42,18 @@ export default (fixture, loaderOptions = {}, config = {}) => {
         chunkFilename: '[id].css',
       }),
     ],
-    ...config,
+    ...cnfg,
   };
 
   const compiler = webpack(fullConfig);
 
-  if (!config.outputFileSystem) {
-    const outputFileSystem = createFsFromVolume(new Volume());
+  if (!outputFileSystem) {
+    const outputFS = createFsFromVolume(new Volume());
     // Todo remove when we drop webpack@4 support
-    outputFileSystem.join = path.join.bind(path);
+    outputFS.join = path.join.bind(path);
 
+    compiler.outputFileSystem = outputFS;
+  } else {
     compiler.outputFileSystem = outputFileSystem;
   }
 

test/helpers/index.js

@@ -1,4 +1,5 @@
 import compile from './compile';
 import getCompiler from './getCompiler';
+import source from './source';
 
-export { compile, getCompiler };
+export { source, compile, getCompiler };

test/helpers/source.js

@@ -0,0 +1,19 @@
+export default function getSource(name, stats) {
+  const { modules } = stats.toJson({ source: true });
+
+  for (let i = 0; i < modules.length; i++) {
+    const module = modules[i];
+
+    if (module.modules && module.modules.length > 0) {
+      for (let j = 0; j < module.modules.length; j++) {
+        if (module.modules[j].name === name) {
+          return module.modules[j].source;
+        }
+      }
+    } else if (module.name === name) {
+      return module.source;
+    }
+  }
+
+  return undefined;
+}