feat: Enable eslint.config.mjs and eslint.config.cjs (#18066)

* feat: Enable eslint.config.mjs and eslint.config.cjs

* Update CLI description

(cherry picked from commit 8792464)

* docs: update documentation for eslint.config.mjs & eslint.config.cjs support

---------

Co-authored-by: Nicholas C. Zakas <nicholas@humanwhocodes.com>

docs/src/use/configure/configuration-files-new.md

@@ -16,7 +16,13 @@ You can put your ESLint project configuration in a configuration file. You can i
 
 ## Configuration File
 
-The ESLint configuration file is named `eslint.config.js`. It should be placed in the root directory of your project and export an array of [configuration objects](#configuration-objects). Here's an example:
+The ESLint configuration file may be named any of the following:
+
+* `eslint.config.js`
+* `eslint.config.mjs`
+* `eslint.config.cjs`
+
+It should be placed in the root directory of your project and export an array of [configuration objects](#configuration-objects). Here's an example:
 
 ```js
 export default [
@@ -44,24 +50,6 @@ module.exports = [
 ];
 ```
 
-The configuration file can also export a promise that resolves to the configuration array. This can be useful for using ESM dependencies in CommonJS configuration files, as in this example:
-
-```js
-module.exports = (async () => {
-
-    const someDependency = await import("some-esm-dependency");
-
-    return [
-        // ... use `someDependency` here
-    ];
-
-})();
-```
-
-::: warning
-ESLint only automatically looks for a config file named `eslint.config.js` and does not look for `eslint.config.cjs` or `eslint.config.mjs`. If you'd like to specify a different config filename than the default, use the `--config` command line option.
-:::
-
 ## Configuration Objects
 
 Each configuration object contains all of the information ESLint needs to execute on a set of files. Each configuration object is made up of these properties:
@@ -668,7 +656,7 @@ export default [
 
 ## Configuration File Resolution
 
-When ESLint is run on the command line, it first checks the current working directory for `eslint.config.js`. If the file is not found, it looks to the next parent directory for the file. This search continues until either the file is found or the root directory is reached.
+When ESLint is run on the command line, it first checks the current working directory for `eslint.config.js`. If that file is found, then the search stops, otherwise it checks for `eslint.config.mjs`. If that file is found, then the search stops, otherwise it checks for `eslint.config.cjs`. If none of the files are not found, it checks the parent directory for each file. This search continues until either a config file is found or the root directory is reached.
 
 You can prevent this search for `eslint.config.js` by setting the `ESLINT_USE_FLAT_CONFIG` environment variable to `true` and using the `-c` or `--config` option on the command line to specify an alternate configuration file, such as:
 

lib/eslint/flat-eslint.js

@@ -91,7 +91,11 @@ const LintResultCache = require("../cli-engine/lint-result-cache");
 // Helpers
 //------------------------------------------------------------------------------
 
-const FLAT_CONFIG_FILENAME = "eslint.config.js";
+const FLAT_CONFIG_FILENAMES = [
+    "eslint.config.js",
+    "eslint.config.mjs",
+    "eslint.config.cjs"
+];
 const debug = require("debug")("eslint:flat-eslint");
 const removedFormatters = new Set(["table", "codeframe"]);
 const privateMembers = new WeakMap();
@@ -248,7 +252,7 @@ function compareResultsByFilePath(a, b) {
  */
 function findFlatConfigFile(cwd) {
     return findUp(
-        FLAT_CONFIG_FILENAME,
+        FLAT_CONFIG_FILENAMES,
         { cwd }
     );
 }

lib/options.js

@@ -168,7 +168,7 @@ module.exports = function(usingFlatConfig) {
                 alias: "c",
                 type: "path::String",
                 description: usingFlatConfig
-                    ? "Use this configuration instead of eslint.config.js"
+                    ? "Use this configuration instead of eslint.config.js, eslint.config.mjs, or eslint.config.cjs"
                     : "Use this configuration, overriding .eslintrc.* config options if present"
             },
             envFlag,

tests/fixtures/cjs-config/eslint.config.cjs

@@ -0,0 +1,5 @@
+module.exports = {
+    rules: {
+        "no-undef": "warn"
+    }
+};

tests/fixtures/cjs-config/foo.js

@@ -0,0 +1 @@
+foo;

tests/fixtures/js-mjs-cjs-config/eslint.config.cjs

@@ -0,0 +1,5 @@
+module.exports= {
+    rules: {
+        "no-undef": "warn"
+    }
+};

tests/fixtures/js-mjs-cjs-config/eslint.config.js

@@ -0,0 +1,5 @@
+module.exports = {
+    rules: {
+        "no-undef": "off"
+    }
+};

tests/fixtures/js-mjs-cjs-config/eslint.config.mjs

@@ -0,0 +1,5 @@
+export default {
+    rules: {
+        "no-undef": "error"
+    }
+};

tests/fixtures/js-mjs-cjs-config/foo.js

@@ -0,0 +1 @@
+foo;

tests/fixtures/mjs-cjs-config/eslint.config.cjs

@@ -0,0 +1,5 @@
+module.exports = {
+    rules: {
+        "no-undef": "warn"
+    }
+};

tests/fixtures/mjs-cjs-config/eslint.config.mjs

@@ -0,0 +1,5 @@
+export default {
+    rules: {
+        "no-undef": "error"
+    }
+};

tests/fixtures/mjs-cjs-config/foo.js

@@ -0,0 +1 @@
+foo;

tests/fixtures/mjs-config/eslint.config.mjs

@@ -0,0 +1,5 @@
+export default {
+    rules: {
+        "no-undef": "error"
+    }
+};

tests/fixtures/mjs-config/foo.js

@@ -0,0 +1 @@
+foo;

tests/lib/eslint/flat-eslint.js

@@ -821,6 +821,73 @@ describe("FlatESLint", () => {
             assert.strictEqual(results[0].messages[0].severity, 2);
             assert.strictEqual(results[0].messages[0].ruleId, "quotes");
         });
+
+        describe("Alternate config files", () => {
+
+            it("should find eslint.config.mjs when present", async () => {
+
+                const cwd = getFixturePath("mjs-config");
+
+                eslint = new FlatESLint({
+                    cwd
+                });
+
+                const results = await eslint.lintText("foo");
+
+                assert.strictEqual(results.length, 1);
+                assert.strictEqual(results[0].messages.length, 1);
+                assert.strictEqual(results[0].messages[0].severity, 2);
+                assert.strictEqual(results[0].messages[0].ruleId, "no-undef");
+
+            });
+
+            it("should find eslint.config.cjs when present", async () => {
+
+                const cwd = getFixturePath("cjs-config");
+
+                eslint = new FlatESLint({
+                    cwd
+                });
+
+                const results = await eslint.lintText("foo");
+
+                assert.strictEqual(results.length, 1);
+                assert.strictEqual(results[0].messages.length, 1);
+                assert.strictEqual(results[0].messages[0].severity, 1);
+                assert.strictEqual(results[0].messages[0].ruleId, "no-undef");
+
+            });
+
+            it("should favor eslint.config.js when eslint.config.mjs and eslint.config.cjs are present", async () => {
+
+                const cwd = getFixturePath("js-mjs-cjs-config");
+
+                eslint = new FlatESLint({
+                    cwd
+                });
+
+                const results = await eslint.lintText("foo");
+
+                assert.strictEqual(results.length, 1);
+                assert.strictEqual(results[0].messages.length, 0);
+            });
+
+            it("should favor eslint.config.mjs when eslint.config.cjs is present", async () => {
+
+                const cwd = getFixturePath("mjs-cjs-config");
+
+                eslint = new FlatESLint({
+                    cwd
+                });
+
+                const results = await eslint.lintText("foo");
+
+                assert.strictEqual(results.length, 1);
+                assert.strictEqual(results[0].messages.length, 1);
+                assert.strictEqual(results[0].messages[0].severity, 2);
+                assert.strictEqual(results[0].messages[0].ruleId, "no-undef");
+            });
+        });
     });
 
     describe("lintFiles()", () => {
@@ -4110,6 +4177,75 @@ describe("FlatESLint", () => {
             await assert.rejects(() => eslint.lintFiles(777), /'patterns' must be a non-empty string or an array of non-empty strings/u);
             await assert.rejects(() => eslint.lintFiles([null]), /'patterns' must be a non-empty string or an array of non-empty strings/u);
         });
+
+        describe("Alternate config files", () => {
+
+            it("should find eslint.config.mjs when present", async () => {
+
+                const cwd = getFixturePath("mjs-config");
+
+                eslint = new FlatESLint({
+                    cwd
+                });
+
+                const results = await eslint.lintFiles("foo.js");
+
+                assert.strictEqual(results.length, 1);
+                assert.strictEqual(results[0].messages.length, 1);
+                assert.strictEqual(results[0].messages[0].severity, 2);
+                assert.strictEqual(results[0].messages[0].ruleId, "no-undef");
+
+            });
+
+            it("should find eslint.config.cjs when present", async () => {
+
+                const cwd = getFixturePath("cjs-config");
+
+                eslint = new FlatESLint({
+                    cwd
+                });
+
+                const results = await eslint.lintFiles("foo.js");
+
+                assert.strictEqual(results.length, 1);
+                assert.strictEqual(results[0].messages.length, 1);
+                assert.strictEqual(results[0].messages[0].severity, 1);
+                assert.strictEqual(results[0].messages[0].ruleId, "no-undef");
+
+            });
+
+            it("should favor eslint.config.js when eslint.config.mjs and eslint.config.cjs are present", async () => {
+
+                const cwd = getFixturePath("js-mjs-cjs-config");
+
+                eslint = new FlatESLint({
+                    cwd
+                });
+
+                const results = await eslint.lintFiles("foo.js");
+
+                assert.strictEqual(results.length, 1);
+                assert.strictEqual(results[0].messages.length, 0);
+            });
+
+            it("should favor eslint.config.mjs when eslint.config.cjs is present", async () => {
+
+                const cwd = getFixturePath("mjs-cjs-config");
+
+                eslint = new FlatESLint({
+                    cwd
+                });
+
+                const results = await eslint.lintFiles("foo.js");
+
+                assert.strictEqual(results.length, 1);
+                assert.strictEqual(results[0].messages.length, 1);
+                assert.strictEqual(results[0].messages[0].severity, 2);
+                assert.strictEqual(results[0].messages[0].ruleId, "no-undef");
+            });
+        });
+
+
     });
 
     describe("Fix Types", () => {