fix(@angular-devkit/build-webpack): correctly handle ESM webpack configurations

Previoiusly, we didn't correctly handle ESM configurations as the `import` was always downlevelled to `require` by TypeScript.

Closes #22547

(cherry picked from commit cb73c0b)

Author: alan-agius4

goldens/public-api/angular_devkit/build_webpack/src/index.md

@@ -24,7 +24,7 @@ export type DevServerBuildOutput = BuildResult & {
     address: string;
 };
 
-// @public
+// @public (undocumented)
 export interface EmittedFiles {
     // (undocumented)
     asset?: boolean;

packages/angular_devkit/build_webpack/src/utils.ts

@@ -6,7 +6,10 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
+import { existsSync } from 'fs';
 import * as path from 'path';
+import { URL, pathToFileURL } from 'url';
+import { Compilation, Configuration } from 'webpack';
 
 export interface EmittedFiles {
   id?: string;
@@ -17,7 +20,7 @@ export interface EmittedFiles {
   extension: string;
 }
 
-export function getEmittedFiles(compilation: import('webpack').Compilation): EmittedFiles[] {
+export function getEmittedFiles(compilation: Compilation): EmittedFiles[] {
   const files: EmittedFiles[] = [];
   const chunkFileNames = new Set<string>();
 
@@ -51,3 +54,51 @@ export function getEmittedFiles(compilation: import('webpack').Compilation): Emi
 
   return files;
 }
+
+/**
+ * This uses a dynamic import to load a module which may be ESM.
+ * CommonJS code can load ESM code via a dynamic import. Unfortunately, TypeScript
+ * will currently, unconditionally downlevel dynamic import into a require call.
+ * require calls cannot load ESM code and will result in a runtime error. To workaround
+ * this, a Function constructor is used to prevent TypeScript from changing the dynamic import.
+ * Once TypeScript provides support for keeping the dynamic import this workaround can
+ * be dropped.
+ *
+ * @param modulePath The path of the module to load.
+ * @returns A Promise that resolves to the dynamically imported module.
+ */
+function loadEsmModule<T>(modulePath: string | URL): Promise<T> {
+  return new Function('modulePath', `return import(modulePath);`)(modulePath) as Promise<T>;
+}
+
+export async function getWebpackConfig(configPath: string): Promise<Configuration> {
+  if (!existsSync(configPath)) {
+    throw new Error(`Webpack configuration file ${configPath} does not exist.`);
+  }
+
+  switch (path.extname(configPath)) {
+    case '.mjs':
+      // Load the ESM configuration file using the TypeScript dynamic import workaround.
+      // Once TypeScript provides support for keeping the dynamic import this workaround can be
+      // changed to a direct dynamic import.
+      return (await loadEsmModule<{ default: Configuration }>(pathToFileURL(configPath))).default;
+    case '.cjs':
+      return require(configPath);
+    default:
+      // The file could be either CommonJS or ESM.
+      // CommonJS is tried first then ESM if loading fails.
+      try {
+        return require(configPath);
+      } catch (e) {
+        if (e.code === 'ERR_REQUIRE_ESM') {
+          // Load the ESM configuration file using the TypeScript dynamic import workaround.
+          // Once TypeScript provides support for keeping the dynamic import this workaround can be
+          // changed to a direct dynamic import.
+          return (await loadEsmModule<{ default: Configuration }>(pathToFileURL(configPath)))
+            .default;
+        }
+
+        throw e;
+      }
+  }
+}

packages/angular_devkit/build_webpack/src/webpack-dev-server/index.ts

@@ -12,7 +12,7 @@ import { Observable, from, isObservable, of } from 'rxjs';
 import { switchMap } from 'rxjs/operators';
 import webpack from 'webpack';
 import WebpackDevServer from 'webpack-dev-server';
-import { getEmittedFiles } from '../utils';
+import { getEmittedFiles, getWebpackConfig } from '../utils';
 import { BuildResult, WebpackFactory, WebpackLoggingCallback } from '../webpack';
 import { Schema as WebpackDevServerBuilderSchema } from './schema';
 
@@ -112,10 +112,8 @@ export default createBuilder<WebpackDevServerBuilderSchema, DevServerBuildOutput
   (options, context) => {
     const configPath = pathResolve(context.workspaceRoot, options.webpackConfig);
 
-    return from(import(configPath)).pipe(
-      switchMap(({ default: config }: { default: webpack.Configuration }) =>
-        runWebpackDevServer(config, context),
-      ),
+    return from(getWebpackConfig(configPath)).pipe(
+      switchMap((config) => runWebpackDevServer(config, context)),
     );
   },
 );

packages/angular_devkit/build_webpack/src/webpack-dev-server/index_spec.ts

@@ -47,8 +47,23 @@ describe('Dev Server Builder', () => {
     await createArchitect(workspaceRoot);
   });
 
-  it('works', async () => {
-    const run = await architect.scheduleTarget(webpackTargetSpec);
+  it('works with CJS config', async () => {
+    const run = await architect.scheduleTarget(webpackTargetSpec, {
+      webpackConfig: 'webpack.config.cjs',
+    });
+    const output = (await run.result) as DevServerBuildOutput;
+    expect(output.success).toBe(true);
+
+    const response = await fetch(`http://${output.address}:${output.port}/bundle.js`);
+    expect(await response.text()).toContain(`console.log('hello world')`);
+
+    await run.stop();
+  }, 30000);
+
+  it('works with ESM config', async () => {
+    const run = await architect.scheduleTarget(webpackTargetSpec, {
+      webpackConfig: 'webpack.config.mjs',
+    });
     const output = (await run.result) as DevServerBuildOutput;
     expect(output.success).toBe(true);
 

packages/angular_devkit/build_webpack/src/webpack/index.ts

@@ -11,7 +11,7 @@ import { resolve as pathResolve } from 'path';
 import { Observable, from, isObservable, of } from 'rxjs';
 import { switchMap } from 'rxjs/operators';
 import webpack from 'webpack';
-import { EmittedFiles, getEmittedFiles } from '../utils';
+import { EmittedFiles, getEmittedFiles, getWebpackConfig } from '../utils';
 import { Schema as RealWebpackBuilderSchema } from './schema';
 
 export type WebpackBuilderSchema = RealWebpackBuilderSchema;
@@ -118,9 +118,7 @@ export function runWebpack(
 export default createBuilder<WebpackBuilderSchema>((options, context) => {
   const configPath = pathResolve(context.workspaceRoot, options.webpackConfig);
 
-  return from(import(configPath)).pipe(
-    switchMap(({ default: config }: { default: webpack.Configuration }) =>
-      runWebpack(config, context),
-    ),
+  return from(getWebpackConfig(configPath)).pipe(
+    switchMap((config) => runWebpack(config, context)),
   );
 });

packages/angular_devkit/build_webpack/src/webpack/index_spec.ts

@@ -47,8 +47,23 @@ describe('Webpack Builder basic test', () => {
       await createArchitect(workspaceRoot);
     });
 
-    it('works', async () => {
-      const run = await architect.scheduleTarget({ project: 'app', target: 'build' });
+    it('works with CJS config', async () => {
+      const run = await architect.scheduleTarget(
+        { project: 'app', target: 'build' },
+        { webpackConfig: 'webpack.config.cjs' },
+      );
+      const output = await run.result;
+
+      expect(output.success).toBe(true);
+      expect(await vfHost.exists(join(outputPath, 'bundle.js')).toPromise()).toBe(true);
+      await run.stop();
+    });
+
+    it('works with ESM config', async () => {
+      const run = await architect.scheduleTarget(
+        { project: 'app', target: 'build' },
+        { webpackConfig: 'webpack.config.mjs' },
+      );
       const output = await run.result;
 
       expect(output.success).toBe(true);

packages/angular_devkit/build_webpack/test/basic-app/angular.json

@@ -15,13 +15,13 @@
         "build": {
           "builder": "../../:webpack",
           "options": {
-            "webpackConfig": "webpack.config.js"
+            "webpackConfig": "webpack.config.cjs"
           }
         },
         "serve": {
           "builder": "../../:webpack-dev-server",
           "options": {
-            "webpackConfig": "webpack.config.js"
+            "webpackConfig": "webpack.config.cjs"
           }
         }
       }

packages/angular_devkit/build_webpack/test/basic-app/webpack.config.js renamed to packages/angular_devkit/build_webpack/test/basic-app/webpack.config.cjs

@@ -0,0 +1,19 @@
+import { resolve } from 'path';
+import { fileURLToPath } from 'url';
+
+export default {
+  mode: 'development',
+  entry: resolve(fileURLToPath(import.meta.url), '../src/main.js'),
+  module: {
+    rules: [
+      // rxjs 6 requires directory imports which are not support in ES modules.
+      // Disabling `fullySpecified` allows Webpack to ignore this but this is
+      // not ideal because it currently disables ESM behavior import for all JS files.
+      { test: /\.[m]?js$/, resolve: { fullySpecified: false } },
+    ],
+  },
+  output: {
+    path: resolve(fileURLToPath(import.meta.url), '../dist'),
+    filename: 'bundle.js',
+  },
+};