Support xo.config.cjs and .xo-config.cjs (#561)

lib/constants.js

@@ -120,7 +120,9 @@ const CONFIG_FILES = [
 	`.${MODULE_NAME}-config`,
 	`.${MODULE_NAME}-config.json`,
 	`.${MODULE_NAME}-config.js`,
-	`${MODULE_NAME}.config.js`
+	`.${MODULE_NAME}-config.cjs`,
+	`${MODULE_NAME}.config.js`,
+	`${MODULE_NAME}.config.cjs`
 ];
 
 const TSCONFIG_DEFFAULTS = {

readme.md

@@ -160,6 +160,14 @@ module.exports = {
 };
 ```
 
+4. For [ECMAScript module (ESM)](https://nodejs.org/api/esm.html) packages with [`"type": "module"`](https://nodejs.org/api/packages.html#packages_type), as a JavaScript module in `.xo-config.cjs` or `xo.config.cjs`:
+
+```js
+module.exports = {
+	space: true
+};
+```
+
 [Globals](https://eslint.org/docs/user-guide/configuring#specifying-globals) and [rules](https://eslint.org/docs/user-guide/configuring#configuring-rules) can be configured inline in files.
 
 ### envs

test/fixtures/config-files/xo-config_cjs/.xo-config.cjs

@@ -0,0 +1,3 @@
+module.exports = {
+	space: true
+};

test/fixtures/config-files/xo-config_cjs/file.js

@@ -0,0 +1,5 @@
+const object = {
+	a: 1
+};
+
+console.log(object.a);

test/fixtures/config-files/xo_config_cjs/file.js

@@ -0,0 +1,5 @@
+const object = {
+	a: 1
+};
+
+console.log(object.a);

test/fixtures/config-files/xo_config_cjs/xo.config.cjs

@@ -0,0 +1,3 @@
+module.exports = {
+	space: true
+};

test/lint-files.js

@@ -278,6 +278,8 @@ async function configType(t, {dir}) {
 configType.title = (_, {type}) => `load config from ${type}`.trim();
 
 test(configType, {type: 'xo.config.js', dir: 'xo-config_js'});
+test(configType, {type: 'xo.config.cjs', dir: 'xo-config_cjs'});
 test(configType, {type: '.xo-config.js', dir: 'xo-config_js'});
+test(configType, {type: '.xo-config.cjs', dir: 'xo-config_cjs'});
 test(configType, {type: '.xo-config.json', dir: 'xo-config_json'});
 test(configType, {type: '.xo-config', dir: 'xo-config'});

test/lint-text.js

@@ -324,6 +324,8 @@ async function configType(t, {dir}) {
 configType.title = (_, {type}) => `load config from ${type}`.trim();
 
 test(configType, {type: 'xo.config.js', dir: 'xo-config_js'});
+test(configType, {type: 'xo.config.cjs', dir: 'xo-config_cjs'});
 test(configType, {type: '.xo-config.js', dir: 'xo-config_js'});
+test(configType, {type: '.xo-config.cjs', dir: 'xo-config_cjs'});
 test(configType, {type: '.xo-config.json', dir: 'xo-config_json'});
 test(configType, {type: '.xo-config', dir: 'xo-config'});