Add test for configuration file with TypeScript

src/index.ts

@@ -33,3 +33,7 @@ export interface Config<Engine extends typeof Marpit = typeof Marp>
       options?: ConstructorParameters<Engine>[0]
     }
   > {}
+
+export const defineConfig = <Engine extends typeof Marpit = typeof Marp>(
+  config: Config<Engine>
+) => config

test/__mocks__/cosmiconfig.ts

@@ -17,6 +17,7 @@ cosmiconfig.cosmiconfig = jest.fn((moduleName, options) => {
       '.js': defaultLoadersSync['.js'],
       '.mjs': defaultLoadersSync['.js'],
       '.cjs': defaultLoadersSync['.js'],
+      '.ts': defaultLoadersSync['.ts'],
     },
     ...(options ?? {}),
   })

test/_configs/typescript/marp.config.ts

@@ -0,0 +1,7 @@
+import { defineConfig } from '../../../src/index'
+
+export default defineConfig({
+  title: 'TypeScript configuration',
+})
+
+console.debug('A config file with .ts extension was loaded.')

test/marp-cli.ts

@@ -1154,6 +1154,30 @@ describe('Marp CLI', () => {
           }
         })
       })
+
+      describe('with TypeScript', () => {
+        it('allows loading config with TypeScript', async () => {
+          const debug = jest.spyOn(console, 'debug').mockImplementation()
+          const log = jest.spyOn(console, 'log').mockImplementation()
+
+          try {
+            expect(
+              await marpCli([
+                '-v',
+                '-c',
+                assetFn('_configs/typescript/marp.config.ts'),
+              ])
+            ).toBe(0)
+
+            expect(debug).toHaveBeenCalledWith(
+              expect.stringContaining('loaded')
+            )
+          } finally {
+            debug.mockRestore()
+            log.mockRestore()
+          }
+        })
+      })
     })
 
     describe('with --preview / -p option', () => {