From 15e8edd828972cad274caab88ac91081414e82c5 Mon Sep 17 00:00:00 2001
From: Frederik Wessberg <frederikwessberg@gmail.com>
Date: Tue, 7 Jun 2022 23:51:14 +0200
Subject: [PATCH] feat: add support for ESM-based Babel configs. Closes #167

---
 src/constant/constant.ts        |  1 +
 src/plugin/typescript-plugin.ts |  4 ++--
 src/transpiler/babel.ts         |  6 ++---
 test/babel.test.ts              | 40 ++++++++++++++++++++++++++++++++-
 4 files changed, 45 insertions(+), 6 deletions(-)

diff --git a/src/constant/constant.ts b/src/constant/constant.ts
index ef7bdede..1dc56742 100644
--- a/src/constant/constant.ts
+++ b/src/constant/constant.ts
@@ -83,6 +83,7 @@ export const BABEL_RUNTIME_PREFIX_2 = "babel-runtime/";
 export const SWC_HELPERS_PREFIX = "@swc/helpers";
 
 export const BABEL_CONFIG_JS_FILENAME = "babel.config.js";
+export const BABEL_CONFIG_MJS_FILENAME = "babel.config.mjs";
 export const BABEL_CONFIG_JSON_FILENAME = "babel.config.json";
 export const BABELRC_FILENAME = ".babelrc";
 
diff --git a/src/plugin/typescript-plugin.ts b/src/plugin/typescript-plugin.ts
index 79e7963c..39266ad7 100644
--- a/src/plugin/typescript-plugin.ts
+++ b/src/plugin/typescript-plugin.ts
@@ -144,7 +144,7 @@ export default function typescriptRollupPlugin(pluginInputOptions: Partial<Types
 		// Conditionally initialize babel at this point.
 		// Only require @babel/preset-typescript if relevant for the initial emit that may include TypeScript specific syntax
 		const babel = await loadBabel(initial);
-		const babelConfigResult = babelConfigFileFactory!(fileName, initial);
+		const babelConfigResult = await babelConfigFileFactory!(fileName, initial);
 
 		const transpilationResult = await babel.transformAsync(input.code, {
 			...babelConfigResult.config,
@@ -339,7 +339,7 @@ export default function typescriptRollupPlugin(pluginInputOptions: Partial<Types
 			let updatedSourceDescription: SourceDescription | undefined;
 
 			if (transpilerOptions.otherSyntax === "babel") {
-				const {config} = babelConfigChunkFactory!(chunk.fileName);
+				const {config} = await babelConfigChunkFactory!(chunk.fileName);
 				const babel = await loadBabel();
 
 				// When targeting CommonJS and using babel as a transpiler, we may need to rewrite forced ESM paths for preserved external helpers to paths that are compatible with CommonJS.
diff --git a/src/transpiler/babel.ts b/src/transpiler/babel.ts
index cafd8233..1ef3982b 100644
--- a/src/transpiler/babel.ts
+++ b/src/transpiler/babel.ts
@@ -160,16 +160,16 @@ export interface GetBabelConfigResult {
 	config: FullConfig | undefined;
 }
 
-export type BabelConfigFactory = (filename: string, inTypescriptStep?: boolean) => GetBabelConfigResult;
+export type BabelConfigFactory = (filename: string, inTypescriptStep?: boolean) => Promise<GetBabelConfigResult>;
 
 /**
  * Gets a Babel Config based on the given options
  */
 export function getBabelConfig({babel, babelConfig, cwd, forcedOptions = {}, defaultOptions = {}, browserslist, phase, hook}: GetBabelConfigOptions): BabelConfigFactory {
-	return (filename: string, inTypescriptStep = false) => {
+	return async (filename: string, inTypescriptStep = false) => {
 
 		// Load a partial Babel config based on the input options
-		const partialConfig = babel.loadPartialConfig(
+		const partialConfig = await babel.loadPartialConfigAsync(
 			// If babel options are provided directly
 			isBabelConfig(babelConfig) && Object.keys(babelConfig).length > 0
 				? // If the given babelConfig is an object of input options, use that as the basis for the full config
diff --git a/test/babel.test.ts b/test/babel.test.ts
index 7c94a575..5698c75c 100644
--- a/test/babel.test.ts
+++ b/test/babel.test.ts
@@ -2,7 +2,7 @@ import test, {ExecutionContext} from "ava";
 import {withTypeScript, withTypeScriptVersions} from "./util/ts-macro.js";
 import {ConfigItem} from "@babel/core";
 import {generateRollupBundle} from "./setup/setup-rollup.js";
-import {BABEL_CONFIG_JS_FILENAME, BABEL_CONFIG_JSON_FILENAME, BABELRC_FILENAME} from "../src/constant/constant.js";
+import {BABEL_CONFIG_JS_FILENAME, BABEL_CONFIG_JSON_FILENAME, BABELRC_FILENAME, BABEL_CONFIG_MJS_FILENAME} from "../src/constant/constant.js";
 import {areTempFilesEqual, createTemporaryFile} from "./util/create-temporary-file.js";
 import {getAppropriateEcmaVersionForBrowserslist} from "browserslist-generator";
 import {formatCode} from "./util/format-code.js";
@@ -270,6 +270,44 @@ test.serial("Can resolve the nearest file-relative babel config. #1", withTypeSc
 	}
 });
 
+test.serial("Can handle ESM-based babel configs. #1", withTypeScript, async (t, {typescript}) => {
+	const unlinker = createTemporaryFile(BABEL_CONFIG_MJS_FILENAME, `export default {}`);
+	let configFileName: string | undefined;
+	let forcePass = false;
+
+	try {
+		await generateRollupBundle(
+			[
+				{
+					entry: true,
+					fileName: "index.ts",
+					text: `\
+					`
+				}
+			],
+			{
+				debug: false,
+				typescript,
+				cwd: unlinker.dir,
+				transpiler: "babel",
+				hook: {
+					babelConfig: (config, fileName) => {
+						configFileName = fileName;
+						return config;
+					}
+				}
+			}
+		);
+	} catch (ex) {
+		if (handlePotentiallyAllowedFailingBabelError(t, ex)) {
+			forcePass = true;
+		}
+	} finally {
+		unlinker.cleanup();
+		t.true(forcePass || (configFileName != null && areTempFilesEqual(configFileName, unlinker.path)));
+	}
+});
+
 test.serial("Won't apply @babel/preset-env if the browserslist option is 'false'. #1", withTypeScript, async (t, {typescript}) => {
 	let hasPresetEnv: boolean | undefined;
 	await generateRollupBundle(