Support ES modules in bundles config files

build-plugins/esm-dynamic-import.ts

@@ -1,12 +1,14 @@
 import type { Plugin } from 'rollup';
 
+const expectedImports = 3;
+
 export default function esmDynamicImport(): Plugin {
 	let importsFound = 0;
 	return {
 		generateBundle() {
-			if (importsFound !== 2) {
+			if (importsFound !== expectedImports) {
 				throw new Error(
-					'Could not find 2 dynamic import in "loadConfigFile.ts" and "commandPlugin.ts", were the files renamed or modified?'
+					`Could not find ${expectedImports} dynamic imports in "loadConfigFile.ts" and "commandPlugin.ts", found ${importsFound}.`
 				);
 			}
 		},

cli/run/loadConfigFile.ts

@@ -1,21 +1,18 @@
-import { extname, isAbsolute } from 'node:path';
+import { promises as fs } from 'node:fs';
+import { dirname, extname, isAbsolute, join } from 'node:path';
 import { pathToFileURL } from 'node:url';
 import getPackageType from 'get-package-type';
 import * as rollup from '../../src/node-entry';
 import type { MergedRollupOptions } from '../../src/rollup/types';
 import { bold } from '../../src/utils/colors';
-import { errMissingConfig, error, errTranspiledEsmConfig } from '../../src/utils/error';
+import { errMissingConfig, error } from '../../src/utils/error';
 import { mergeOptions } from '../../src/utils/options/mergeOptions';
 import type { GenericConfigObject } from '../../src/utils/options/options';
 import relativeId from '../../src/utils/relativeId';
 import { stderr } from '../logging';
 import batchWarnings, { type BatchWarnings } from './batchWarnings';
 import { addCommandPluginsToInputOptions, addPluginsFromCommandOption } from './commandPlugins';
 
-interface NodeModuleWithCompile extends NodeModule {
-	_compile(code: string, filename: string): any;
-}
-
 export async function loadConfigFile(
 	fileName: string,
 	commandOptions: any = {}
@@ -45,18 +42,18 @@ async function loadConfigsFromFile(
 	const configFileExport =
 		commandOptions.configPlugin ||
 		// We always transpile the .js non-module case because many legacy code bases rely on this
-		(extension === '.js' && getPackageType.sync(fileName) !== 'module')
-			? await getDefaultFromTranspiledConfigFile(fileName, commandOptions)
-			: getDefaultFromCjs((await import(pathToFileURL(fileName).href)).default);
+		(extension === '.js' && (await getPackageType(fileName)) !== 'module')
+			? await loadTranspiledConfigFile(fileName, commandOptions)
+			: (await import(pathToFileURL(fileName).href)).default;
 
-	return getConfigList(configFileExport, commandOptions);
+	return getConfigList(getDefaultFromCjs(configFileExport), commandOptions);
 }
 
 function getDefaultFromCjs(namespace: GenericConfigObject): unknown {
-	return namespace.__esModule ? namespace.default : namespace;
+	return namespace.default || namespace;
 }
 
-async function getDefaultFromTranspiledConfigFile(
+async function loadTranspiledConfigFile(
 	fileName: string,
 	commandOptions: Record<string, unknown>
 ): Promise<unknown> {
@@ -79,7 +76,7 @@ async function getDefaultFromTranspiledConfigFile(
 		output: [{ code }]
 	} = await bundle.generate({
 		exports: 'named',
-		format: 'cjs',
+		format: 'es',
 		plugins: [
 			{
 				name: 'transpile-import-meta',
@@ -97,29 +94,14 @@ async function getDefaultFromTranspiledConfigFile(
 	return loadConfigFromBundledFile(fileName, code);
 }
 
-function loadConfigFromBundledFile(fileName: string, bundledCode: string): unknown {
-	const resolvedFileName = require.resolve(fileName);
-	const extension = extname(resolvedFileName);
-	const defaultLoader = require.extensions[extension];
-	require.extensions[extension] = (module: NodeModule, requiredFileName: string) => {
-		if (requiredFileName === resolvedFileName) {
-			(module as NodeModuleWithCompile)._compile(bundledCode, requiredFileName);
-		} else {
-			if (defaultLoader) {
-				defaultLoader(module, requiredFileName);
-			}
-		}
-	};
-	delete require.cache[resolvedFileName];
+async function loadConfigFromBundledFile(fileName: string, bundledCode: string): Promise<unknown> {
+	const bundledFileName = join(dirname(fileName), `rollup.config-${Date.now()}.mjs`);
+	await fs.writeFile(bundledFileName, bundledCode);
 	try {
-		const config = getDefaultFromCjs(require(fileName));
-		require.extensions[extension] = defaultLoader;
-		return config;
-	} catch (err: any) {
-		if (err.code === 'ERR_REQUIRE_ESM') {
-			return error(errTranspiledEsmConfig(fileName));
-		}
-		throw err;
+		return (await import(pathToFileURL(bundledFileName).href)).default;
+	} finally {
+		// Not awaiting here saves some ms while potentially hiding a non-critical error
+		fs.unlink(bundledFileName);
 	}
 }
 

src/utils/error.ts

@@ -100,7 +100,6 @@ const ADDON_ERROR = 'ADDON_ERROR',
 	SOURCEMAP_ERROR = 'SOURCEMAP_ERROR',
 	SYNTHETIC_NAMED_EXPORTS_NEED_NAMESPACE_EXPORT = 'SYNTHETIC_NAMED_EXPORTS_NEED_NAMESPACE_EXPORT',
 	THIS_IS_UNDEFINED = 'THIS_IS_UNDEFINED',
-	TRANSPILED_ESM_CONFIG = 'TRANSPILED_ESM_CONFIG',
 	UNEXPECTED_NAMED_IMPORT = 'UNEXPECTED_NAMED_IMPORT',
 	UNKNOWN_OPTION = 'UNKNOWN_OPTION',
 	UNRESOLVED_ENTRY = 'UNRESOLVED_ENTRY',
@@ -756,15 +755,6 @@ export function errThisIsUndefined(): RollupLog {
 	};
 }
 
-export function errTranspiledEsmConfig(fileName: string): RollupLog {
-	return {
-		code: TRANSPILED_ESM_CONFIG,
-		message: `While loading the Rollup configuration from "${relativeId(
-			fileName
-		)}", Node tried to require an ES module from a CommonJS file, which is not supported. A common cause is if there is a package.json file with "type": "module" in the same folder. You can try to fix this by changing the extension of your configuration file to ".cjs" or ".mjs" depending on the content, which will prevent Rollup from trying to preprocess the file but rather hand it to Node directly.`
-	};
-}
-
 export function errUnexpectedNamedImport(
 	id: string,
 	imported: string,

test/cli/node_modules_rename_me/esm-dep/index.js

@@ -0,0 +1 @@
+export default 'esm-default';

test/cli/node_modules_rename_me/esm-dep/package.json

@@ -0,0 +1,4 @@
+{
+	"main": "index.js",
+	"type": "module"
+}

test/cli/samples/import-esm-package/_config.js

@@ -0,0 +1,4 @@
+module.exports = {
+	description: 'allows to import ESM dependencies from transpiled config files',
+	command: "rollup --config --configPlugin '{transform: c => c}'"
+};

test/cli/samples/import-esm-package/_expected/main.js

@@ -0,0 +1,2 @@
+/* esm-default */
+assert.ok(true);

test/cli/samples/import-esm-package/main.js

@@ -0,0 +1 @@
+assert.ok(true);

test/cli/samples/import-esm-package/rollup.config.js

@@ -0,0 +1,10 @@
+import esmDep from 'esm-dep';
+
+export default {
+	input: 'main.js',
+	output: {
+		banner: `/* ${esmDep} */`,
+		format: 'es',
+		dir: '_actual'
+	}
+};

typings/declarations.d.ts

@@ -102,7 +102,8 @@ declare module 'is-reference' {
 
 declare module 'get-package-type' {
 	interface GetPackageType {
-		sync(fileName: string): 'module' | 'commonjs';
+		(fileName: string): Promise<string>;
+		sync(fileName: string): string;
 	}
 	const getPackageType: GetPackageType;
 	export default getPackageType;