Support .mjs files by default.

README.md

@@ -44,7 +44,7 @@ export default {
       browser: true,  // Default: false
 
       // not all files you want to resolve are .js files
-      extensions: [ '.js', '.json' ],  // Default: ['.js']
+      extensions: [ '.mjs', '.js', '.jsx', '.json' ],  // Default: [ '.mjs', '.js', '.json', '.node' ]
 
       // whether to prefer built-in modules (e.g. `fs`, `path`) or
       // local ones with the same names
@@ -53,7 +53,7 @@ export default {
       // Lock the module search in this path (like a chroot). Module defined
       // outside this path will be marked as external
       jail: '/my/jail/path', // Default: '/'
-      
+
       // Set to an array of strings and/or regexps to lock the module search
       // to modules that match at least one entry. Modules not matching any
       // entry will be marked as external

src/index.js

@@ -6,7 +6,9 @@ import fs from 'fs';
 
 const ES6_BROWSER_EMPTY = resolve( __dirname, '../src/empty.js' );
 const CONSOLE_WARN = ( ...args ) => console.warn( ...args ); // eslint-disable-line no-console
-const exts = [ '.js', '.json', '.node' ];
+// It is important that .mjs occur before .js so that Rollup will interpret npm modules
+// which deploy both ESM .mjs and CommonJS .js files as ESM.
+const DEFAULT_EXTS = [ '.mjs', '.js', '.json', '.node' ];
 
 let readFileCache = {};
 const readFileAsync = file => new Promise((fulfil, reject) => fs.readFile(file, (err, contents) => err ? reject(err) : fulfil(contents)));
@@ -110,6 +112,7 @@ export default function nodeResolve ( options = {} ) {
 			return new Promise( ( fulfil, reject ) => {
 				let disregardResult = false;
 				let packageBrowserField = false;
+				const extensions = options.extensions || DEFAULT_EXTS;
 
 				const resolveOptions = {
 					basedir: dirname( importer ),
@@ -123,7 +126,7 @@ export default function nodeResolve ( options = {} ) {
 									const absoluteKey = resolve( pkgRoot, key );
 									browser[ absoluteKey ] = resolved;
 									if ( !extname(key) ) {
-										exts.reduce( ( browser, ext ) => {
+										extensions.reduce( ( browser, ext ) => {
 											browser[ absoluteKey + ext ] = browser[ key ];
 											return browser;
 										}, browser );
@@ -146,7 +149,7 @@ export default function nodeResolve ( options = {} ) {
 					},
 					readFile: cachedReadFile,
 					isFile: cachedIsFile,
-					extensions: options.extensions
+					extensions: extensions
 				};
 
 				if (preserveSymlinks !== undefined) {

test/samples/dual-cjs-mjs/main.js

@@ -0,0 +1,3 @@
+import module from 'dual-cjs-mjs';
+
+export default module; // MODULE

test/samples/module-mjs/main.js

@@ -0,0 +1,3 @@
+import module from 'module-mjs';
+
+export default module; // MODULE

test/test.js

@@ -436,6 +436,28 @@ describe( 'rollup-plugin-node-resolve', function () {
 		});
 	});
 
+	it('finds and uses an .mjs module', function () {
+		return rollup.rollup({
+			input: 'samples/module-mjs/main.js',
+			plugins: [
+				nodeResolve({ preferBuiltins: false })
+			]
+		}).then( executeBundle ).then( module => {
+			assert.equal( module.exports, 'MODULE-MJS' );
+		});
+	});
+
+	it('finds and uses a dual-distributed .js & .mjs module', function () {
+		return rollup.rollup({
+			input: 'samples/dual-cjs-mjs/main.js',
+			plugins: [
+				nodeResolve({ preferBuiltins: false })
+			]
+		}).then( executeBundle ).then( module => {
+			assert.equal( module.exports, 'DUAL-MJS' );
+		});
+	});
+
 	describe( 'symlinks', () => {
 		function createMissingDirectories () {
 			createDirectory( './samples/symlinked/first/node_modules' );