Support .mjs files by default.

Since rollup prefers ESM formatted modules, it should look for node's new .mjs file extension before looking for .js files by default. This offers support for deployed modules intended to be used in node's ESM mode.

Folks can work around this today by manually supplying the `extensions` option, however it would be great if this worked by default.

Note that looking for `.mjs` before `.js` is important for rollup which prefers ESM, however the `resolve` dependency should not use the same order by default since it is used in many other tools which do not yet support ESM or would not expect it by default.

Encountered this as the root cause behind graphql/graphql-js#1293

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' );