marp.config.ts support and defineConfig helper

CHANGELOG.md

@@ -2,6 +2,11 @@
 
 ## [Unreleased]
 
+### Added
+
+- Support the project configuration file written in TypeScript `marp.config.ts` ([#548](https://github.com/marp-team/marp-cli/pull/548), [#549](https://github.com/marp-team/marp-cli/pull/549))
+- `defineConfig` helper for writing typed configuration ([#549](https://github.com/marp-team/marp-cli/pull/549))
+
 ### Changed
 
 - Upgrade Marpit to [v2.5.3](https://github.com/marp-team/marpit/releases/tag/v2.5.3) ([#548](https://github.com/marp-team/marp-cli/pull/548))

README.md

@@ -621,9 +621,9 @@ This configuration will set the constructor option for Marp Core as specified:
 
 </details>
 
-### Type annotation
+### Auto completion
 
-For getting better IDE support (such as [IntelliSense](https://code.visualstudio.com/docs/editor/intellisense)) to write a config, you can annotate the config object through JSDoc, with Marp CLI's `Config` type.
+For getting the power of auto completion for the config, such as [IntelliSense](https://code.visualstudio.com/docs/editor/intellisense), you can annotate the config object through JSDoc, with Marp CLI's `Config` type.
 
 ```javascript
 /** @type {import('@marp-team/marp-cli').Config} */
@@ -634,9 +634,19 @@ const config = {
 export default config
 ```
 
+Or you can use Vite-like `defineConfig` helper from Marp CLI instead.
+
+```javascript
+import { defineConfig } from '@marp-team/marp-cli'
+
+export default defineConfig({
+  // ...
+})
+```
+
 #### `Config` type with custom engine
 
-If you've swapped the engine into another Marpit based engine, you also can provide better suggestion for `options` field by passing the engine type to generics.
+If you've swapped the engine into another Marpit based engine, you can provide better suggestion for `options` field by passing the engine type to generics.
 
 ```javascript
 /** @type {import('@marp-team/marp-cli').Config<typeof import('@marp-team/marpit').Marpit>} */
@@ -650,6 +660,25 @@ const config = {
 export default config
 ```
 
+#### TypeScript (`marp.config.ts`)
+
+If you installed `typescript` into your local project together with Marp CLI, you can write a config by TypeScript `marp.config.ts`. Marp CLI will try to transpile `.ts` with the project configuration `tsconfig.json`.
+
+In TypeScript configuration, you can specify the custom engine as the generics for `defineConfig` helper, like this:
+
+```typescript
+// marp.config.ts
+import { Marpit } from '@marp-team/marpit'
+import { defineConfig } from '@marp-team/marp-cli'
+
+export default defineConfig<typeof Marpit>({
+  engine: Marpit,
+  options: {
+    // Suggest only Marpit constructor options
+  },
+})
+```
+
 ## API _(EXPERIMENTAL)_
 
 You can use Marp CLI through Node.js [if installed Marp CLI into your local project](#local-installation).

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', () => {