support all config file extensions (.js,.mjs,...)

cli/run/loadConfigFile.ts

@@ -39,14 +39,18 @@ export default function loadConfigFile(
 		})
 		.then(({ output: [{ code }] }: RollupOutput) => {
 			// temporarily override require
-			const defaultLoader = require.extensions['.js'];
-			require.extensions['.js'] = (module: NodeModule, filename: string) => {
-				if (filename === configFile) {
-					(module as NodeModuleWithCompile)._compile(code, filename);
-				} else {
-					defaultLoader(module, filename);
-				}
-			};
+			const extensions = ['.js','.mjs'];
+			const defaultLoaders = extensions.map(e => {
+				const defaultLoader = require.extensions[e];
+				require.extensions[e] = (module: NodeModule, filename: string) => {
+					if (filename === configFile) {
+						(module as NodeModuleWithCompile)._compile(code, filename);
+					} else {
+						defaultLoader(module, filename);
+					}
+				};	
+				return defaultLoader;
+			});
 
 			delete require.cache[configFile];
 
@@ -67,7 +71,7 @@ export default function loadConfigFile(
 						});
 					}
 
-					require.extensions['.js'] = defaultLoader;
+					extensions.forEach((extension,index) => require.extensions[extension] = defaultLoaders[index]);
 
 					return Array.isArray(configs) ? configs : [configs];
 				});

docs/01-command-line-reference.md

@@ -164,6 +164,9 @@ $ rollup --config
 
 # alternatively, specify a custom config file location
 $ rollup --config my.config.js
+
+# .js and .mjs are supported
+$ rollup --config my.config.mjs
 ```
 
 You can also export a function that returns any of the above configuration formats. This function will be passed the current command line arguments so that you can dynamically adapt your configuration to respect e.g. [`--silent`](guide/en/#--silent). You can even define your own command line options if you prefix them with `config`:

test/cli/samples/config-mjs/_config.js

@@ -0,0 +1,5 @@
+module.exports = {
+	description: 'uses config file (.mjs)',
+	command: 'rollup --config rollup.config.mjs',
+	execute: true
+};

test/cli/samples/config-mjs/main.js

@@ -0,0 +1 @@
+assert.equal( ANSWER, 42 );

test/cli/samples/config-mjs/rollup.config.mjs

@@ -0,0 +1,11 @@
+import replace from 'rollup-plugin-replace';
+
+export default {
+	input: 'main.js',
+	output: {
+		format: 'cjs'
+	},
+	plugins: [
+		replace( { 'ANSWER': 42 } )
+	]
+};