keep non .js extensions, e.g. .json

src/utils/defaults.js

@@ -1,21 +1,26 @@
 import { readFileSync } from './fs';
-import { dirname, isAbsolute, resolve } from './path';
+import { dirname, extname, isAbsolute, resolve } from './path';
 
 export function load ( id ) {
 	return readFileSync( id, 'utf-8' );
 }
 
+function addExt ( id ) {
+	if ( !extname( id ) ) id += '.js';
+	return id;
+}
+
 export function resolveId ( importee, importer ) {
 	// absolute paths are left untouched
-	if ( isAbsolute( importee ) ) return importee;
+	if ( isAbsolute( importee ) ) return addExt( importee );
 
 	// if this is the entry point, resolve against cwd
-	if ( importer === undefined ) return resolve( process.cwd(), importee );
+	if ( importer === undefined ) return resolve( process.cwd(), addExt( importee ) );
 
 	// external modules are skipped at this stage
 	if ( importee[0] !== '.' ) return null;
 
-	return resolve( dirname( importer ), importee ).replace( /\.js$/, '' ) + '.js';
+	return resolve( dirname( importer ), addExt( importee ) );
 }
 
 export function onwarn ( msg ) {

src/utils/path.js

@@ -21,7 +21,7 @@ export function dirname ( path ) {
 }
 
 export function extname ( path ) {
-	const match = /\.[^\.]+$/.exec( path );
+	const match = /\.[^\.]+$/.exec( basename( path ) );
 	if ( !match ) return '';
 	return match[0];
 }

test/function/non-js-extensions/_config.js

@@ -0,0 +1,14 @@
+var path = require( 'path' );
+
+module.exports = {
+	description: 'non .js extensions are preserved',
+	options: {
+		plugins: [{
+			transform: function ( code, id ) {
+				if ( path.extname( id ) === '.json' ) {
+					return 'export default ' + code;
+				}
+			}
+		}]
+	}
+};

test/function/non-js-extensions/info.json

@@ -0,0 +1,3 @@
+{
+	"answer": 42
+}

test/function/non-js-extensions/main.js

@@ -0,0 +1,3 @@
+import info from './info.json';
+
+assert.equal( info.answer, 42 );