Feat: support Node's --experimental-default-type flag

.vscode/launch.json

@@ -0,0 +1,26 @@
+{
+    // Use IntelliSense to learn about possible attributes.
+    // Hover to view descriptions of existing attributes.
+    // For more information, visit: https://go.microsoft.com/fwlink/?linkid=830387
+    "version": "0.2.0",
+    "configurations": [
+        {
+            "name": "Launch Program",
+            "type": "node",
+            "request": "launch",
+            "skipFiles": [
+                "<node_internals>/**",
+                "**/node_modules/**"
+            ],
+            "runtimeArgs": [
+                "--import=./lib/index.js",
+                "--experimental-default-type module"
+            ],
+            "program": "zz_test.js",
+            "outFiles": [
+                "${workspaceFolder}/lib/*.js"
+            ],
+            "sourceMaps": true
+        }
+    ]
+}

rollup.config.js

@@ -19,7 +19,7 @@ export default defineConfig([
             dir: 'lib',
             format: 'esm',
             generatedCode: 'es2015',
-            sourcemap: "hidden",
+            sourcemap: true,
             sourcemapExcludeSources: true
         },
         plugins: [

source/ambient.d.ts

@@ -1,23 +1,27 @@
 declare global {
-	namespace NodeJS {
-		interface Module {
-			_compile(code: string, filename: string): string
-		}
+    type ModuleType = 'commonjs' | 'module'
 
-		interface NodeError {
-			code: string
-		}
+    // In package.json.
+    interface PkgType {
+        type?: ModuleType
+    }
 
-		type ModuleType = 'commonjs' | 'module'
-		interface PkgType {
-			type?: ModuleType
-		}
-	}
+    // The data passed to the initialize() hook.
+    interface InitializeHookData {
+        self: string
+        defaultModuleType: ModuleType
+    }
+
+    namespace NodeJS {
+        interface Module {
+            _compile(code: string, filename: string): string
+        }
+    }
 }
 
 declare module 'module' {
-	export const _extensions: NodeJS.RequireExtensions;
+    export const _extensions: NodeJS.RequireExtensions
 }
 
 // This file needs to be a module
-export {}
+export { }

source/cjs-hooks.ts

@@ -4,7 +4,7 @@ import { readFileSync } from 'node:fs'
 
 const require = createRequire(import.meta.url)
 
-function transpile(m: Module, format: NodeJS.ModuleType, filePath: string) {
+function transpile(m: Module, format: ModuleType, filePath: string) {
     // Notes:
     // - This function is called by the CJS loader so it must be sync.
     // - We lazy-load Sucrase as the CJS loader may well never be used
@@ -18,8 +18,8 @@ function transpile(m: Module, format: NodeJS.ModuleType, filePath: string) {
 }
 
 const unknownType = Symbol()
-const pkgTypeCache = new Map<string, NodeJS.ModuleType | Symbol>()
-function nearestPackageType(file: string): NodeJS.ModuleType {
+const pkgTypeCache = new Map<string, ModuleType | Symbol>()
+function nearestPackageType(file: string, defaultType: ModuleType): ModuleType {
     for (
         let current = path.dirname(file), previous: string | undefined = undefined;
         previous !== current;
@@ -30,13 +30,13 @@ function nearestPackageType(file: string): NodeJS.ModuleType {
         if (!format) {
             try {
                 const data = readFileSync(pkgFile, 'utf-8')
-                const { type } = JSON.parse(data) as NodeJS.PkgType
+                const { type } = JSON.parse(data) as PkgType
                 format = type === 'module' || type ==='commonjs'
                     ? type
                     : unknownType
             }
             catch(err) {
-                const { code } = err as NodeJS.NodeError
+                const { code } = err as NodeJS.ErrnoException
                 if (code !== 'ENOENT')
                     console.error(err)
                 format = unknownType
@@ -49,12 +49,11 @@ function nearestPackageType(file: string): NodeJS.ModuleType {
             return format
     }
 
-    // TODO: decide default format based on --experimental-default-type
-    return 'commonjs'
+    return defaultType
 }
 
-export function install_cjs_hooks() {
-    Module._extensions['.ts']  = (m, filename) => transpile(m, nearestPackageType(filename), filename)
+export function install_cjs_hooks(defaultType: ModuleType) {
+    Module._extensions['.ts']  = (m, filename) => transpile(m, nearestPackageType(filename, defaultType), filename)
     Module._extensions['.cts'] = (m, filename) => transpile(m, 'commonjs', filename)
     Module._extensions['.mts'] = (m, filename) => transpile(m, 'module', filename)
 }

source/cjs-transform.cts

@@ -1,11 +1,11 @@
-import { transform as sucrase, type Transform } from 'sucrase'
+import { transform as sucrase, type Transform, type TransformResult } from 'sucrase'
 
-const transforms: Record<NodeJS.ModuleType, Transform[]> = {
+const transforms: Record<ModuleType, Transform[]> = {
     commonjs: [ 'typescript', 'imports' ],
     module: [ 'typescript' ]
 }
 
-export function transform(source: string, format: NodeJS.ModuleType, filePath: string) {
+export function transform(source: string, format: ModuleType, filePath: string) {
     const { code, sourceMap } = sucrase(source, {
         filePath,
         transforms: transforms[format],
@@ -16,10 +16,10 @@ export function transform(source: string, format: NodeJS.ModuleType, filePath: s
         sourceMapOptions: {
             compiledFilename: filePath
         }
-    })
+    }) as Required<TransformResult>
 
-    sourceMap!.sourceRoot = ''
-    sourceMap!.sources = [ filePath ]
+    sourceMap.sourceRoot = ''
+    sourceMap.sources = [ filePath ]
     // sourceMap.sourcesContent = [ source ]
 
     return code + '\n//# sourceMappingURL=data:application/json;charset=utf-8;base64,'

source/esm-hooks.ts

@@ -1,13 +1,15 @@
 import path from 'node:path'
 import { fileURLToPath } from 'node:url'
 import { readFile } from 'node:fs/promises'
-import { type InitializeHook, type ResolveHook, type LoadHook, type ModuleSource, createRequire } from 'node:module'
+import { createRequire, type InitializeHook, type ResolveHook, type LoadHook } from 'node:module'
 
 const { transform } = createRequire(import.meta.url)('./cjs-transform.cjs') as typeof import('./cjs-transform.cjs')
 
 let self: string
-export const initialize: InitializeHook<string> = data => {
-    self = data
+let defaultModuleType: ModuleType
+export const initialize: InitializeHook<InitializeHookData> = data => {
+    self = data.self
+    defaultModuleType = data.defaultModuleType
 }
 
 export const resolve: ResolveHook = async (specifier, context, nextResolve) => {
@@ -37,8 +39,8 @@ export const resolve: ResolveHook = async (specifier, context, nextResolve) => {
 }
 
 const unknownType = Symbol()
-const pkgTypeCache = new Map<string, NodeJS.ModuleType | Symbol>()
-async function nearestPackageType(file: string): Promise<NodeJS.ModuleType> {
+const pkgTypeCache = new Map<string, ModuleType | Symbol>()
+async function nearestPackageType(file: string): Promise<ModuleType> {
     for (
         let current = path.dirname(file), previous: string | undefined = undefined;
         previous !== current;
@@ -48,9 +50,9 @@ async function nearestPackageType(file: string): Promise<NodeJS.ModuleType> {
         let format = pkgTypeCache.get(pkgFile)
         if (!format) {
             format = await readFile(pkgFile, 'utf-8')
-                .then(data => (JSON.parse(data) as NodeJS.PkgType).type ?? unknownType)
+                .then(data => (JSON.parse(data) as PkgType).type ?? unknownType)
                 .catch(err => {
-                    const { code } = err as NodeJS.NodeError
+                    const { code } = err as NodeJS.ErrnoException
                     if (code !== 'ENOENT')
                         console.error(err)
                     return unknownType
@@ -62,8 +64,7 @@ async function nearestPackageType(file: string): Promise<NodeJS.ModuleType> {
             return format
     }
 
-    // TODO: decide default format based on --experimental-default-type
-    return 'commonjs'
+    return defaultModuleType
 }
 
 export const load: LoadHook = async (url, context, nextLoad) => {
@@ -77,7 +78,7 @@ export const load: LoadHook = async (url, context, nextLoad) => {
     // Determine the output format based on the file's extension
     // or the nearest package.json's `type` field.
     const filePath = fileURLToPath(url)
-    const format: NodeJS.ModuleType = (
+    const format: ModuleType = (
         ext[1] === '.ts'
             ? await nearestPackageType(filePath)
             : ext[1] === '.mts'

source/index.ts

@@ -11,16 +11,31 @@ if (
     || (major === 20 && minor >= 6)
     || (major === 18 && minor >= 19)
 ) {
-    const self = import.meta.url
+
+    // Determine the default module type.
+    let defaultModuleType: ModuleType = 'commonjs'
+    const argc = process.execArgv.findIndex(arg => arg.startsWith('--experimental-default-type'))
+    if (argc >= 0) {
+        const argv = process.execArgv[argc].split('=')
+        const type = argv.length === 1
+            ? process.execArgv[argc + 1]
+            : argv[1]
+        if (type === 'module' || type === 'commonjs')
+            defaultModuleType = type
+    }
 
     // Install the esm hooks -- those are run in a worker thread.
-    Module.register('./esm-hooks.js', {
+    const self = import.meta.url
+    Module.register<InitializeHookData>('./esm-hooks.js', {
         parentURL: self,
-        data: self
+        data: {
+            self,
+            defaultModuleType
+        }
     })
 
     // Install the cjs hooks -- those are run synchronously in the main thread.
-    install_cjs_hooks()
+    install_cjs_hooks(defaultModuleType)
 
     // Enable source map support.
     process.setSourceMapsEnabled(true)