fix: Restore @babel/register strategy.

src/lib/configuration/loader.ts

@@ -5,6 +5,7 @@ import merge from 'deepmerge';
 
 import validators from 'etc/validators';
 import { babelStrategy } from 'lib/configuration/strategies/babel';
+import { babelRegisterStrategy } from 'lib/configuration/strategies/babel-register';
 import { esbuildStrategy } from 'lib/configuration/strategies/esbuild';
 import log from 'lib/log';
 import { getPackageInfo } from 'lib/package';
@@ -76,7 +77,7 @@ async function ecmaScriptLoader(filePath: string /* , content: string */) {
 
 
   /**
-   * Strategy 3: Babel
+   * Strategy 3: babel
    */
   try {
     const result = await babelStrategy(filePath, pkgInfo);
@@ -87,6 +88,21 @@ async function ecmaScriptLoader(filePath: string /* , content: string */) {
   }
 
 
+  /**
+   * Strategy 3: babel/register
+   *
+   * This strategy will (likely) only need to be used when a configuration file
+   * relies on other files that also need to be transpiled.
+   */
+  try {
+    const result = await babelRegisterStrategy(filePath, pkgInfo);
+    log.verbose(prefix, 'Used strategy:', log.chalk.bold('babel/register'));
+    return getDefaultExport(result);
+  } catch (err: any) {
+    errors.push(new Error(`${prefix} Failed to load file with ${log.chalk.bold('babel/register')}: ${err}`));
+  }
+
+
   if (errors.length > 0) throw new AggregateError(errors, 'All parsing strategies failed.');
 }
 

src/lib/configuration/strategies/babel-register.ts

@@ -0,0 +1,125 @@
+import path from 'path';
+
+import fs from 'fs-extra';
+import resolvePkg from 'resolve-pkg';
+import * as tsConfck from 'tsconfck';
+
+import log from 'lib/log';
+
+import type { PackageInfo } from 'lib/package';
+
+
+/**
+ * Creates a temporary module in the nearest node_modules folder that loads
+ * @babel/register, then loads the provided configuration file, and returns the
+ * results.
+ */
+export async function babelRegisterStrategy(filePath: string, pkgInfo: PackageInfo) {
+  const prefix = log.prefix('strategy:babel-register');
+
+  try {
+    if (!pkgInfo.root) throw new Error('Unable to determine host package root directory.');
+
+    const babelRegisterPath = resolvePkg('@babel/register', { cwd: pkgInfo.root });
+    if (!babelRegisterPath) throw new Error('Unable to resolve path to @babel/register.');
+
+    const babelPresetEnvPath = resolvePkg('@babel/preset-env', { cwd: pkgInfo.root });
+    if (!babelPresetEnvPath) throw new Error('Unable to resolve path to @babel/preset-env.');
+
+    const babelPresetTypeScriptPath = resolvePkg('@babel/preset-typescript', { cwd: pkgInfo.root });
+    if (!babelPresetTypeScriptPath) throw new Error('Unable to resolve path to @babel/preset-typescript.');
+
+    const babelPluginModuleResolverTsConfigPath = resolvePkg('babel-plugin-module-resolver-tsconfig', { cwd: pkgInfo.root });
+    if (!babelPluginModuleResolverTsConfigPath) throw new Error('Unable to resolve path to babel-plugin-module-resolver-tsconfig.');
+
+    const tsConfigFilePath = await tsConfck.find(filePath);
+
+    const wrapperWithTsConfig = `
+      const { setModuleResolverPluginForTsConfig } = require('${babelPluginModuleResolverTsConfigPath}');
+
+      const extensions =  ['.ts', '.tsx', '.js', '.jsx', '.cts', '.cjs', '.mjs', '.mts'];
+
+      require('${babelRegisterPath}')({
+        extensions,
+        // Treat files that contain import statements as modules and require statements as CommonJS.
+        sourceType: 'unambiguous',
+        // Let's Babel transpile files that may be above process.cwd(), which are ignored when using the
+        // default settings.
+        // See: https://github.com/babel/babel/issues/8321
+        ignore: [/node_modules/],
+        // Apply the minimum amount of transforms required to make code compatible with the local Node
+        // installation.
+        targets: { node: 'current' },
+        presets: [
+          ['${babelPresetEnvPath}', {
+            // Tell Babel to not transpile dynamic import statements into require() calls as this is the
+            // mechanism by which CommonJS can import ESM.
+            exclude: ['proposal-dynamic-import']
+          }],
+          '${babelPresetTypeScriptPath}'
+        ],
+        plugins: [
+          // If the project has set up path mappings using tsconfig.json, this plugin will allow those
+          // path specifiers to work as expected.
+          setModuleResolverPluginForTsConfig({
+            tsconfigPath: '${tsConfigFilePath}',
+            extensions
+          })
+        ]
+      });
+
+      const configExport = require('${filePath}');
+      module.exports = configExport.default ?? configExport;
+    `;
+
+    const wrapperWithoutTsConfig = `
+      const extensions =  ['.ts', '.tsx', '.js', '.jsx', '.cts', '.cjs', '.mjs', '.mts'];
+
+      require('${babelRegisterPath}')({
+        extensions,
+        // Treat files that contain import statements as modules and require statements as CommonJS.
+        sourceType: 'unambiguous',
+        // Let's Babel transpile files that may be above process.cwd(), which are ignored when using the
+        // default settings.
+        // See: https://github.com/babel/babel/issues/8321
+        ignore: [/node_modules/],
+        // Apply the minimum amount of transforms required to make code compatible with the local Node
+        // installation.
+        targets: { node: 'current' },
+        presets: [
+          ['${babelPresetEnvPath}', {
+            // Tell Babel to not transpile dynamic import statements into require() calls as this is the
+            // mechanism by which CommonJS can import ESM.
+            exclude: ['proposal-dynamic-import']
+          }]
+        ]
+      });
+
+      const configExport = require('${filePath}');
+      module.exports = configExport.default ?? configExport;
+    `;
+
+    if (tsConfigFilePath) {
+      log.silly(prefix, `Loaded tsconfig.json from: ${log.chalk.green(tsConfigFilePath)}`);
+    }
+
+    const tempDir = path.resolve(pkgInfo.root, 'node_modules', '.saffron-config');
+    await fs.ensureDir(tempDir);
+    const loaderPath = path.resolve(tempDir, 'loader.cjs');
+    await fs.writeFile(
+      loaderPath,
+      tsConfigFilePath
+        ? wrapperWithTsConfig
+        : wrapperWithoutTsConfig
+    );
+    const result = await import(loaderPath);
+    await fs.remove(tempDir);
+
+    return result.default ?? result;
+  } catch (err: any) {
+    throw new Error(
+      `${log.chalk.red(`[${prefix}] Failed to import() configuration file ${filePath}:`)} ${err.message}`,
+      { cause: err }
+    );
+  }
+}

src/lib/configuration/strategies/babel.ts

@@ -27,20 +27,12 @@ const EXT_MAP: Record<string, string> = {
 };
 
 
-const extensions =  [
-  '.ts',
-  '.tsx',
-  '.js',
-  '.jsx',
-  '.cts',
-  '.cjs',
-  '.mjs',
-  '.mts'
-];
-
-
 /**
- * WIP
+ * Uses Babel to transpile the file at `filePath` by creating a temporary
+ * file in the same directory, then attempts to dynamically import it. An
+ * output format and extension are chosen based on the host project's
+ * "type" setting that are the least likely to produce errors. Once imported,
+ * the temporary file is removed.
  */
 export async function babelStrategy(filePath: string, pkgInfo: PackageInfo) {
   const prefix = log.prefix('strategy:babel');
@@ -100,7 +92,16 @@ export async function babelStrategy(filePath: string, pkgInfo: PackageInfo) {
       log.verbose(prefix, `Using TypeScript configuration: ${log.chalk.green(tsConfigFilePath)}`);
       transformOptions.plugins?.push(setModuleResolverPluginForTsConfig({
         tsconfigPath: tsConfigFilePath,
-        extensions
+        extensions: [
+          '.ts',
+          '.tsx',
+          '.js',
+          '.jsx',
+          '.cts',
+          '.cjs',
+          '.mjs',
+          '.mts'
+        ]
       }));
     }