Add tests about ESM config resolution

jest.config.js

@@ -32,7 +32,11 @@ module.exports = {
     '\\.s?css$': '<rootDir>/test/_transformers/css.js',
     '\\.png$': '<rootDir>/test/_transformers/png.js',
     '\\.pug$': '<rootDir>/test/_transformers/pug.js',
+
+    // TODO: Remove if Jest did not fail on ESM dynamic imports
     'custom-engine\\.mjs$': 'babel-jest',
+    'config\\.mjs$': 'babel-jest',
+    'esm-project/marp\\.config\\.js$': 'babel-jest',
   },
   transformIgnorePatterns: [`/node_modules/(?!${esModules.join('|')})`],
   testEnvironment: 'node',

src/config.ts

@@ -2,7 +2,11 @@
 import fs from 'fs'
 import path from 'path'
 import chalk from 'chalk'
-import { cosmiconfig, cosmiconfigSync } from 'cosmiconfig'
+import {
+  cosmiconfig,
+  cosmiconfigSync,
+  Options as CosmiconfigOptions,
+} from 'cosmiconfig'
 import { osLocale } from 'os-locale'
 import { info, warn, error as cliError } from './cli'
 import { ConverterOption, ConvertType } from './converter'
@@ -110,6 +114,10 @@ export class MarpCLIConfig {
     return conf
   }
 
+  static isESMAvailable() {
+    return ResolvedEngine.isESMAvailable()
+  }
+
   private constructor() {} // eslint-disable-line @typescript-eslint/no-empty-function
 
   async converterOption(): Promise<ConverterOption> {
@@ -331,11 +339,9 @@ export class MarpCLIConfig {
   }
 
   private async loadConf(confPath?: string) {
-    const generateCosmiconfigExplorer = ResolvedEngine.isESMAvailable()
-      ? cosmiconfig
-      : cosmiconfigSync // sync version is using `require()` instead of `import()` so not expect to meet a trouble with ESM
-
-    const explorer = generateCosmiconfigExplorer(MarpCLIConfig.moduleName)
+    const explorer = MarpCLIConfig.isESMAvailable()
+      ? cosmiconfig(MarpCLIConfig.moduleName)
+      : cosmiconfigSync(MarpCLIConfig.moduleName)
 
     try {
       const ret = await (confPath === undefined

test/__mocks__/cosmiconfig.ts

@@ -0,0 +1,25 @@
+const cosmiconfig: typeof import('cosmiconfig') =
+  jest.requireActual('cosmiconfig')
+
+const {
+  cosmiconfig: originalCosmiconfig,
+  defaultLoaders,
+  defaultLoadersSync,
+} = cosmiconfig
+
+// Because of v8's bug, Jest fails with SIGSEGV if used dynamic import.
+// When using ESM in tests, you have to set up Jest to transpile config files into CommonJS with Babel.
+cosmiconfig.cosmiconfig = jest.fn((moduleName, options) => {
+  return originalCosmiconfig(moduleName, {
+    loaders: {
+      // cosmiconfig sync loader is using `require()` to load JS
+      ...defaultLoaders,
+      '.js': defaultLoadersSync['.js'],
+      '.mjs': defaultLoadersSync['.js'],
+      '.cjs': defaultLoadersSync['.js'],
+    },
+    ...(options ?? {}),
+  })
+})
+
+module.exports = cosmiconfig

test/_configs/esm-project/marp.config.js

@@ -0,0 +1,6 @@
+/** @type {import('../../../src/index').Config} */
+const config = {}
+
+export default config
+
+console.debug('A config file with .mjs extension was loaded.')

test/_configs/esm-project/package.json

@@ -0,0 +1,4 @@
+{
+  "private": true,
+  "type": "module"
+}

test/_configs/mjs/config.mjs

@@ -0,0 +1,3 @@
+export default {}
+
+console.debug('A config file with .mjs extension was loaded.')

test/marp-cli.ts

@@ -37,6 +37,7 @@ const runForObservation = async (argv: string[]) => {
   return ret
 }
 
+jest.mock('cosmiconfig')
 jest.mock('fs')
 jest.mock('../src/preview')
 jest.mock('../src/watcher', () => jest.createMockFromModule('../src/watcher'))
@@ -1110,6 +1111,48 @@ describe('Marp CLI', () => {
           }
         })
       })
+
+      describe('with ES Module', () => {
+        it('allows loading config with mjs extension', async () => {
+          const debug = jest.spyOn(console, 'debug').mockImplementation()
+          const log = jest.spyOn(console, 'log').mockImplementation()
+
+          try {
+            expect(
+              await marpCli(['-v', '-c', assetFn('_configs/mjs/config.mjs')])
+            ).toBe(0)
+
+            expect(debug).toHaveBeenCalledWith(
+              expect.stringContaining('loaded')
+            )
+          } finally {
+            debug.mockRestore()
+            log.mockRestore()
+          }
+        })
+
+        it('allows loading config from ESM project', async () => {
+          const debug = jest.spyOn(console, 'debug').mockImplementation()
+          const log = jest.spyOn(console, 'log').mockImplementation()
+
+          try {
+            expect(
+              await marpCli([
+                '-v',
+                '-c',
+                assetFn('_configs/esm-project/marp.config.js'),
+              ])
+            ).toBe(0)
+
+            expect(debug).toHaveBeenCalledWith(
+              expect.stringContaining('loaded')
+            )
+          } finally {
+            debug.mockRestore()
+            log.mockRestore()
+          }
+        })
+      })
     })
 
     describe('with --preview / -p option', () => {