Update: support .eslintrc.cjs (refs eslint/rfcs#43) (#12321)

* Fix: ES module compatibility (fixes #12319)

In ES module packages w/ "type": "module" defined treat all .js files as ES modules. CommonJS files contained in an ES module package should use the .cjs extension.

* Fix Documentation

* Add Tests

* Fix Lint Error

docs/user-guide/configuring.md

@@ -664,6 +664,7 @@ In each case, the settings in the configuration file override default settings.
 ESLint supports configuration files in several formats:
 
 * **JavaScript** - use `.eslintrc.js` and export an object containing your configuration.
+* **JavaScript (ESM)** - use `.eslintrc.cjs` when running ESLint in JavaScript packages that specify `"type":"module"` in their `package.json`. Note that ESLint does not support ESM configuration at this time.
 * **YAML** - use `.eslintrc.yaml` or `.eslintrc.yml` to define the configuration structure.
 * **JSON** - use `.eslintrc.json` to define the configuration structure. ESLint's JSON files also allow JavaScript-style comments.
 * **Deprecated** - use `.eslintrc`, which can be either JSON or YAML.
@@ -672,13 +673,13 @@ ESLint supports configuration files in several formats:
 If there are multiple configuration files in the same directory, ESLint will only use one. The priority order is:
 
 1. `.eslintrc.js`
+1. `.eslintrc.cjs`
 1. `.eslintrc.yaml`
 1. `.eslintrc.yml`
 1. `.eslintrc.json`
 1. `.eslintrc`
 1. `package.json`
 
-
 ## Configuration Cascading and Hierarchy
 
 When using `.eslintrc.*` and `package.json` files for configuration, you can take advantage of configuration cascading. For instance, suppose you have the following structure:

lib/cli-engine/config-array-factory.js

@@ -62,6 +62,7 @@ const eslintRecommendedPath = path.resolve(__dirname, "../../conf/eslint-recomme
 const eslintAllPath = path.resolve(__dirname, "../../conf/eslint-all.js");
 const configFilenames = [
     ".eslintrc.js",
+    ".eslintrc.cjs",
     ".eslintrc.yaml",
     ".eslintrc.yml",
     ".eslintrc.json",
@@ -279,6 +280,7 @@ function configMissingError(configName, importerName) {
 function loadConfigFile(filePath) {
     switch (path.extname(filePath)) {
         case ".js":
+        case ".cjs":
             return loadJSConfigFile(filePath);
 
         case ".json":

tests/fixtures/config-file/cjs/.eslintrc.cjs

@@ -0,0 +1,5 @@
+module.exports = {
+    rules: {
+        semi: [2, "always"]
+    }
+};

tests/lib/cli-engine/config-array-factory.js

@@ -171,6 +171,7 @@ describe("ConfigArrayFactory", () => {
     describe("'loadFile(filePath, options)' method should load a config file.", () => {
         const basicFiles = {
             "js/.eslintrc.js": "exports.settings = { name: 'js/.eslintrc.js' }",
+            "cjs/.eslintrc.cjs": "exports.settings = { name: 'cjs/.eslintrc.cjs' }",
             "json/.eslintrc.json": "{ \"settings\": { \"name\": \"json/.eslintrc.json\" } }",
             "legacy-json/.eslintrc": "{ \"settings\": { \"name\": \"legacy-json/.eslintrc\" } }",
             "legacy-yml/.eslintrc": "settings:\n  name: legacy-yml/.eslintrc",
@@ -288,6 +289,7 @@ describe("ConfigArrayFactory", () => {
     describe("'loadInDirectory(directoryPath, options)' method should load the config file of a directory.", () => {
         const basicFiles = {
             "js/.eslintrc.js": "exports.settings = { name: 'js/.eslintrc.js' }",
+            "cjs/.eslintrc.cjs": "exports.settings = { name: 'cjs/.eslintrc.cjs' }",
             "json/.eslintrc.json": "{ \"settings\": { \"name\": \"json/.eslintrc.json\" } }",
             "legacy-json/.eslintrc": "{ \"settings\": { \"name\": \"legacy-json/.eslintrc\" } }",
             "legacy-yml/.eslintrc": "settings:\n  name: legacy-yml/.eslintrc",
@@ -1497,6 +1499,7 @@ describe("ConfigArrayFactory", () => {
             "node_modules/eslint-plugin-invalid-parser/index.js": "exports.configs = { foo: { parser: 'nonexistent-parser' } }",
             "node_modules/eslint-plugin-invalid-config/index.js": "exports.configs = { foo: {} }",
             "js/.eslintrc.js": "module.exports = { rules: { semi: [2, 'always'] } };",
+            "cjs/.eslintrc.cjs": "module.exports = { rules: { semi: [2, 'always'] } };",
             "json/.eslintrc.json": "{ \"rules\": { \"quotes\": [2, \"double\"] } }",
             "package-json/package.json": "{ \"eslintConfig\": { \"env\": { \"es6\": true } } }",
             "yaml/.eslintrc.yaml": "env:\n    browser: true"
@@ -1747,6 +1750,22 @@ describe("ConfigArrayFactory", () => {
             });
         });
 
+        it("should load information from a JavaScript file with a .cjs extension", () => {
+            const { ConfigArrayFactory } = defineConfigArrayFactoryWithInMemoryFileSystem({
+                files: {
+                    "cjs/.eslintrc.cjs": "module.exports = { rules: { semi: [2, 'always'] } };"
+                }
+            });
+            const factory = new ConfigArrayFactory();
+            const config = load(factory, "cjs/.eslintrc.cjs");
+
+            assertConfig(config, {
+                rules: {
+                    semi: [2, "always"]
+                }
+            });
+        });
+
         it("should throw error when loading invalid JavaScript file", () => {
             const { ConfigArrayFactory } = defineConfigArrayFactoryWithInMemoryFileSystem({
                 files: {