refactor: Output ESM files w/ .mjs when appropriate (#950)

* fix: Output ESM w/ .mjs ext when pkg type is cjs

* docs: Adding changeset

* Update README.md

* Update README.md

Co-authored-by: Jason Miller <developit@users.noreply.github.com>

* chore: Add warning for new ESM output extension behavior

* Update src/index.js

Co-authored-by: Jason Miller <developit@users.noreply.github.com>

Co-authored-by: Jason Miller <developit@users.noreply.github.com>

.changeset/clean-ducks-push.md

@@ -0,0 +1,5 @@
+---
+'microbundle': minor
+---
+
+Microbundle will now output ESM using `.mjs` as the file extension when the package type is CJS

README.md

@@ -120,8 +120,8 @@ export default function (e, t, r) {
 ```jsonc
 {
 	"main": "./dist/foo.umd.js", // legacy UMD output (for Node & CDN use)
-	"module": "./dist/foo.module.js", // legacy ES Modules output (for bundlers)
-	"exports": "./dist/foo.modern.js", // modern ES2017 output
+	"module": "./dist/foo.module.mjs", // legacy ES Modules output (for bundlers)
+	"exports": "./dist/foo.modern.mjs", // modern ES2017 output
 	"scripts": {
 		"build": "microbundle src/foo.js"
 	}
@@ -134,9 +134,9 @@ The `"exports"` field can also be an object for packages with multiple entry mod
 {
 	"name": "foo",
 	"exports": {
-		".": "./dist/foo.modern.js", // import "foo" (the default)
-		"./lite": "./dist/lite.modern.js", // import "foo/lite"
-		"./full": "./dist/full.modern.js" // import "foo/full"
+		".": "./dist/foo.modern.mjs", // import "foo" (the default)
+		"./lite": "./dist/lite.modern.mjs", // import "foo/lite"
+		"./full": "./dist/full.modern.mjs" // import "foo/full"
 	},
 	"scripts": {
 		"build": "microbundle src/*.js" // build foo.js, lite.js and full.js
@@ -163,15 +163,15 @@ The filenames and paths for generated bundles in each format are defined by the
 
 ```jsonc
 {
-  "source": "src/index.js",            // input
-  "main": "dist/foo.js",               // CommonJS output bundle
-  "umd:main": "dist/foo.umd.js",       // UMD output bundle
-  "module": "dist/foo.m.js",           // ES Modules output bundle 
+  "source": "src/index.js",             // input
+  "main": "dist/foo.js",                // CommonJS output bundle
+  "umd:main": "dist/foo.umd.js",        // UMD output bundle
+  "module": "dist/foo.mjs",           // ES Modules output bundle
   "exports": {
-    "require": "./dist/foo.js",        // CommonJS output bundle
-    "default": "./dist/foo.modern.js", // Modern ES Modules output bundle
+    "require": "./dist/foo.js",         // CommonJS output bundle
+    "default": "./dist/foo.modern.mjs", // Modern ES Modules output bundle
   },
-  "types": "dist/foo.d.ts"             // TypeScript typings directory
+  "types": "dist/foo.d.ts"              // TypeScript typings directory
 }
 ```
 

src/index.js

@@ -2,7 +2,7 @@ import fs from 'fs';
 import { resolve, relative, dirname, basename, extname } from 'path';
 import camelCase from 'camelcase';
 import escapeStringRegexp from 'escape-string-regexp';
-import { blue, red } from 'kleur';
+import { blue, yellow, red } from 'kleur';
 import { map, series } from 'asyncro';
 import glob from 'tiny-glob/sync';
 import autoprefixer from 'autoprefixer';
@@ -282,6 +282,7 @@ function walk(exports, includeDefault) {
 function getMain({ options, entry, format }) {
 	const { pkg } = options;
 	const pkgMain = options['pkg-main'];
+	const pkgTypeModule = pkg.type === 'module';
 
 	if (!pkgMain) {
 		return options.output;
@@ -301,18 +302,23 @@ function getMain({ options, entry, format }) {
 	mainsByFormat.es = replaceName(
 		pkg.module && !pkg.module.match(/src\//)
 			? pkg.module
-			: pkg['jsnext:main'] || 'x.esm.js',
+			: pkg['jsnext:main'] || pkgTypeModule
+			? 'x.esm.js'
+			: 'x.esm.mjs',
 		mainNoExtension,
 	);
+
 	mainsByFormat.modern = replaceName(
-		(pkg.exports && walk(pkg.exports, pkg.type === 'module')) ||
+		(pkg.exports && walk(pkg.exports, pkgTypeModule)) ||
 			(pkg.syntax && pkg.syntax.esmodules) ||
 			pkg.esmodule ||
-			'x.modern.js',
+			pkgTypeModule
+			? 'x.modern.js'
+			: 'x.modern.mjs',
 		mainNoExtension,
 	);
 	mainsByFormat.cjs = replaceName(
-		pkg['cjs:main'] || (pkg.type && pkg.type === 'module' ? 'x.cjs' : 'x.js'),
+		pkg['cjs:main'] || (pkgTypeModule ? 'x.cjs' : 'x.js'),
 		mainNoExtension,
 	);
 	mainsByFormat.umd = replaceName(
@@ -440,6 +446,15 @@ function createConfig(options, entry, format, writeMeta) {
 	const outputDir = dirname(absMain);
 	const outputEntryFileName = basename(absMain);
 
+	// Warn about the (somewhat) breaking change in #950
+	if (format === 'es' && !pkg.module && outputEntryFileName.endsWith('.mjs')) {
+		stdout(
+			yellow(
+				'Warning: your package.json does not specify {"type":"module"}. Microbundle assumes this is a CommonJS package and is generating ES Modules with the ".mjs" file extension.',
+			),
+		);
+	}
+
 	let config = {
 		/** @type {import('rollup').InputOptions} */
 		inputOptions: {