fix: do not transpile ESM for v2 functions

.eslintrc.cjs

@@ -6,6 +6,7 @@ module.exports = {
     sourceType: 'module',
   },
   rules: {
+    complexity: 'off',
     'import/extensions': ['error', 'ignorePackages'],
     'max-lines': 'off',
     'n/no-missing-import': 'off',

src/runtimes/node/bundlers/nft/es_modules.ts

@@ -5,10 +5,13 @@ import { NodeFileTraceReasons } from '@vercel/nft'
 import type { FunctionConfig } from '../../../../config.js'
 import { FeatureFlags } from '../../../../feature_flags.js'
 import type { RuntimeCache } from '../../../../utils/cache.js'
+import { FunctionBundlingUserError } from '../../../../utils/error.js'
 import { cachedReadFile } from '../../../../utils/fs.js'
+import { RUNTIME } from '../../../runtime.js'
 import { ModuleFormat, MODULE_FILE_EXTENSION, MODULE_FORMAT } from '../../utils/module_format.js'
 import { getNodeSupportMatrix } from '../../utils/node_version.js'
 import { getPackageJsonIfAvailable, PackageJson } from '../../utils/package_json.js'
+import { NODE_BUNDLER } from '../types.js'
 
 import { transpile } from './transpile.js'
 
@@ -72,7 +75,10 @@ export const processESM = async ({
 
   // If this is a .mjs file and we want to output pure ESM files, we don't need
   // to transpile anything.
-  if (extension === MODULE_FILE_EXTENSION.MJS && featureFlags.zisi_pure_esm_mjs) {
+  if (
+    extension === MODULE_FILE_EXTENSION.MJS &&
+    (featureFlags.zisi_pure_esm_mjs || featureFlags.zisi_functions_api_v2)
+  ) {
     return {
       moduleFormat: MODULE_FORMAT.ESM,
     }
@@ -81,6 +87,17 @@ export const processESM = async ({
   const entrypointIsESM = isEntrypointESM({ basePath, esmPaths, mainFile })
 
   if (!entrypointIsESM) {
+    if (featureFlags.zisi_functions_api_v2) {
+      throw new FunctionBundlingUserError(
+        `The function '${name}' must use the ES module syntax. To learn more, visit https://ntl.fyi/esm.`,
+        {
+          functionName: name,
+          runtime: RUNTIME.JAVASCRIPT,
+          bundler: NODE_BUNDLER.NFT,
+        },
+      )
+    }
+
     return {
       moduleFormat: MODULE_FORMAT.COMMONJS,
     }
@@ -89,12 +106,27 @@ export const processESM = async ({
   const packageJson = await getPackageJsonIfAvailable(dirname(mainFile))
   const nodeSupport = getNodeSupportMatrix(config.nodeVersion)
 
-  if (featureFlags.zisi_pure_esm && packageJson.type === 'module' && nodeSupport.esm) {
+  if (
+    (featureFlags.zisi_pure_esm || featureFlags.zisi_functions_api_v2) &&
+    packageJson.type === 'module' &&
+    nodeSupport.esm
+  ) {
     return {
       moduleFormat: MODULE_FORMAT.ESM,
     }
   }
 
+  if (featureFlags.zisi_functions_api_v2) {
+    throw new FunctionBundlingUserError(
+      `The function '${name}' must use the ES module syntax. To learn more, visit https://ntl.fyi/esm.`,
+      {
+        functionName: name,
+        runtime: RUNTIME.JAVASCRIPT,
+        bundler: NODE_BUNDLER.NFT,
+      },
+    )
+  }
+
   const rewrites = await transpileESM({ basePath, cache, config, esmPaths, reasons, name })
 
   return {

src/runtimes/node/utils/entry_file.ts

@@ -26,7 +26,7 @@ const getEntryFileContents = (mainPath: string, moduleFormat: string, featureFla
 
   if (featureFlags.zisi_functions_api_v2) {
     return [
-      `import func from '${importPath}'`,
+      `import * as func from '${importPath}'`,
       `import { getLambdaHandler } from './${BOOTSTRAP_FILE_NAME}'`,
       `export const handler = getLambdaHandler(func)`,
     ].join(';')

tests/fixtures/v2-api/package.json

@@ -0,0 +1,3 @@
+{
+  "type": "module"
+}

tests/list_functions_files.test.ts

@@ -95,7 +95,6 @@ describe('listFunctionsFiles', () => {
   testMany(
     'Can list all function files from multiple source directories with listFunctionsFiles()',
     [...allBundleConfigs, 'bundler_none'],
-    // eslint-disable-next-line complexity
     async (options) => {
       const fixtureDir = `${FIXTURES_DIR}/multiple-src-directories`
       const opts = merge(options, {