wenn package.json "type":"module" switch default type to ".cjs" for C…

…ommonJS build (#802)

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

README.md

@@ -153,7 +153,7 @@ Neither require any options, but you can tailor things to suit your needs a bit
 >
 > Read more about [How Microbundle decides which dependencies to bundle](https://github.com/developit/microbundle/wiki/How-Microbundle-decides-which-dependencies-to-bundle), including some example configurations.
 
-### Configuration
+### Specifying filenames in package.json
 
 Unless overridden via the command line, microbundle uses the `source` property in your `package.json` to determine which of your JavaScript files to start bundling from (your "entry module").
 The filenames and paths for generated bundles in each format are defined by the `main`, `umd:main`, `module` and `exports` properties in your `package.json`.
@@ -178,6 +178,20 @@ For more information about the meaning of the different properties, refer to the
 For UMD builds, microbundle will use a camelCase version of the `name` field in your `package.json` as export name.
 Alternatively, this can be explicitly by adding an `"amdName"` key in your `package.json`, or passing the `--name` command line argument.
 
+### Usage with `{"type":"module"}` in `package.json`
+
+Node.js 12.16+ adds a new "ES Module package", which can be enabled by adding `{"type":"module"}` to your package.json.
+This property [changes the default source type](https://nodejs.org/api/packages.html#packages_determining_module_system) of `.js` files to be ES Modules instead of CommonJS.
+When using `{"type":"module"}`, the file extension for CommonJS bundles generated by Microbundle must be changed to `.cjs`:
+
+```
+{
+  "type": "module",
+  "module": "dist/foo.js",  // ES Module bundle
+  "main": "dist/foo.cjs",   // CommonJS bundle
+}
+```
+
 ### Additional Configuration Options
 
 Config also can be overridded by the [`publishConfig`](https://docs.npmjs.com/cli/v7/configuring-npm/package-json#publishconfig) property in your `package.json`.
@@ -263,8 +277,6 @@ The `--define` option can be used to inject or replace build-time constants when
 | `microbundle --define API_KEY='abc123'`      | `console.log(API_KEY)` | `console.log("abc123")` |
 | `microbundle --define @assign=Object.assign` | `assign(a, b)`         | `Object.assign(a, b)`   |
 
-
-
 ### All CLI Options <a name="options"></a>
 
 ```

src/index.js

@@ -266,13 +266,15 @@ function getMain({ options, entry, format }) {
 
 	let mainNoExtension = options.output;
 	if (options.multipleEntries) {
-		let name = entry.match(/([\\/])index(\.(umd|cjs|es|m))?\.(mjs|[tj]sx?)$/)
+		let name = entry.match(
+			/([\\/])index(\.(umd|cjs|es|m))?\.(mjs|cjs|[tj]sx?)$/,
+		)
 			? mainNoExtension
 			: entry;
 		mainNoExtension = resolve(dirname(mainNoExtension), basename(name));
 	}
 	mainNoExtension = mainNoExtension.replace(
-		/(\.(umd|cjs|es|m))?\.(mjs|[tj]sx?)$/,
+		/(\.(umd|cjs|es|m))?\.(mjs|cjs|[tj]sx?)$/,
 		'',
 	);
 
@@ -288,7 +290,10 @@ function getMain({ options, entry, format }) {
 		(pkg.syntax && pkg.syntax.esmodules) || pkg.esmodule || 'x.modern.js',
 		mainNoExtension,
 	);
-	mainsByFormat.cjs = replaceName(pkg['cjs:main'] || 'x.js', mainNoExtension);
+	mainsByFormat.cjs = replaceName(
+		pkg['cjs:main'] || (pkg.type && pkg.type === 'module' ? 'x.cjs' : 'x.js'),
+		mainNoExtension,
+	);
 	mainsByFormat.umd = replaceName(
 		pkg['umd:main'] || pkg.unpkg || 'x.umd.js',
 		mainNoExtension,