feat: support for ESM (#465)

Author: pmbanugo

packages/micro/package.json

@@ -14,7 +14,7 @@
     "micro": "./dist/src/bin/micro.js"
   },
   "engines": {
-    "node": ">= 14.5.0"
+    "node": ">= 16.0.0"
   },
   "scripts": {
     "build": "tsc",
@@ -34,8 +34,7 @@
   "dependencies": {
     "arg": "4.1.0",
     "content-type": "1.0.4",
-    "raw-body": "2.4.1",
-    "tsimportlib": "0.0.3"
+    "raw-body": "2.4.1"
   },
   "devDependencies": {
     "@types/content-type": "1.1.5",

packages/micro/src/lib/handler.ts

@@ -7,9 +7,7 @@ export const handle = async (file: string) => {
   try {
     mod = await import(file);
 
-    if (mod && typeof mod === 'object') {
-      mod = await (mod as { default: unknown }).default; // Await to support es6 module's default export
-    }
+    mod = await (mod as { default: unknown }).default; // use ES6 module's default export
   } catch (err: unknown) {
     if (isErrorObject(err) && err.stack) {
       logError(`Error when importing ${file}: ${err.stack}`, 'invalid-entry');

packages/micro/tsconfig.json

@@ -1,9 +1,9 @@
 {
   "extends": "@vercel/style-guide/typescript",
   "compilerOptions": {
-    "target": "ES2020",
+    "target": "ES2021",
     "module": "CommonJS",
-    "moduleResolution": "node",
+    "moduleResolution": "Node16",
     "esModuleInterop": true,
     "resolveJsonModule": true,
     "outDir": "dist",

packages/micro/types/src/lib/handler.d.ts.map

@@ -1,6 +1,9 @@
 {
   "name": "test",
   "version": "1.0.0",
+  "engines": {
+    "node": ">= 16.0.0"
+  },
   "scripts": {
     "eslint-check": "eslint --max-warnings=0 .",
     "prettier-check": "prettier --check .",

test/package.json

@@ -20,45 +20,21 @@ void test('handle async function', async (t) => {
   t.type(result, 'function');
 });
 
-void test(`handle Babel's non-async function`, async (t) => {
+void test(`handle ESM's non-async function`, async (t) => {
   const dir = t.testdir({
-    'babel-function-export.js': `"use strict";
-
-    Object.defineProperty(exports, "__esModule", {
-      value: true
-    });
-    exports.default = void 0;
-    
-    var _default = (req, res) => {
-      res.end("Test");
-    };
-    
-    exports.default = _default;
-    `,
+    'esm-function-export.mjs': `export default () => 'Hello ESM';`,
   });
 
-  const result = await handle(`${dir}/babel-function-export.js`);
+  const result = await handle(`${dir}/esm-function-export.mjs`);
   t.type(result, 'function');
 });
 
-void test(`handle Babel's async function`, async (t) => {
+void test(`handle ESM's async function`, async (t) => {
   const dir = t.testdir({
-    'babel-async-export.js': `"use strict";
-
-    Object.defineProperty(exports, "__esModule", {
-      value: true
-    });
-    exports.default = void 0;
-    
-    var _default = async (req, res) => {
-      res.end("Test");
-    };
-    
-    exports.default = _default;
-    `,
+    'esm-async-export.mjs': `export default async () => 'Hello ESM';`,
   });
 
-  const result = await handle(`${dir}/babel-async-export.js`);
+  const result = await handle(`${dir}/esm-async-export.mjs`);
   t.type(result, 'function');
 });
 

test/suite/handler.ts

@@ -1,9 +1,9 @@
 {
   "extends": "@vercel/style-guide/typescript",
   "compilerOptions": {
-    "target": "ES2020",
+    "target": "ES2021",
     "module": "CommonJS",
-    "moduleResolution": "node",
+    "moduleResolution": "Node16",
     "esModuleInterop": true,
     "noEmit": true
   }

test/tsconfig.json

@@ -7379,11 +7379,6 @@ tsconfig-paths@^3.12.0, tsconfig-paths@^3.14.1:
     minimist "^1.2.6"
     strip-bom "^3.0.0"
 
-tsimportlib@0.0.3:
-  version "0.0.3"
-  resolved "https://registry.yarnpkg.com/tsimportlib/-/tsimportlib-0.0.3.tgz#008453c0f0eea6f736f2a4431171de240b21fc0d"
-  integrity sha512-U9sW2/3D0P4IVRnhH2RCqjCP0sG66qvb4ahB0aQln5xGMphDjntz5rdk0rFZ6Fg+lW3L+i+gRnIl4VvNBvxiQw==
-
 tslib@^1.8.1:
   version "1.14.1"
   resolved "https://registry.yarnpkg.com/tslib/-/tslib-1.14.1.tgz#cf2d38bdc34a134bcaf1091c41f6619e2f672d00"