feat: support flat config (#238)

* feat: support ESLint's flat config

* docs(readme): add new `configType` option to readme

* ci: remove eslint 7x

* test: skip test on node versions lt 20

---------

Co-authored-by: Ricardo Gobbo de Souza <[email protected]>

.github/workflows/nodejs.yml

@@ -60,7 +60,7 @@ jobs:
       matrix:
         os: [ubuntu-latest, windows-latest, macos-latest]
         node-version: [14.x, 16.x, 18.x, 20.x]
-        eslint-version: [7.x, 8.x]
+        eslint-version: [8.x]
         webpack-version: [latest]
 
     runs-on: ${{ matrix.os }}

README.md

@@ -106,6 +106,25 @@ type cacheLocation = string;
 
 Specify the path to the cache location. Can be a file or a directory.
 
+### `configType`
+
+- Type:
+
+```ts
+type configType = "flat" | "eslintrc";
+```
+
+- Default: `eslintrc`
+
+Specify the type of configuration to use with ESLint.
+- `eslintrc` is the classic configuration format available in most ESLint versions.
+- `flat` is the new format introduced in ESLint 8.21.0.
+
+The new configuration format is explained in its [own documentation](https://eslint.org/docs/latest/use/configure/configuration-files-new).
+
+> This configuration format being considered as experimental, it is not exported in the main ESLint module in ESLint 8.
+> You need to set your `eslintPath` to `eslint/use-at-your-own-risk` for this config format to work.
+
 ### `context`
 
 - Type:

package.json

@@ -33,7 +33,7 @@
     "fix:js": "npm run lint:js -- --fix",
     "fix:prettier": "npm run lint:prettier -- --write",
     "fix": "npm-run-all -l fix:js fix:prettier",
-    "test:only": "cross-env NODE_ENV=test jest --testTimeout=60000",
+    "test:only": "cross-env NODE_OPTIONS=--experimental-vm-modules NODE_ENV=test jest --testTimeout=60000",
     "test:watch": "npm run test:only -- --watch",
     "test:coverage": "npm run test:only -- --collectCoverageFrom=\"src/**/*.js\" --coverage",
     "pretest": "npm run lint",

src/getESLint.js

@@ -2,6 +2,8 @@ const { cpus } = require('os');
 
 const { Worker: JestWorker } = require('jest-worker');
 
+// @ts-ignore
+const { setup, lintFiles } = require('./worker');
 const { getESLintOptions } = require('./options');
 const { jsonStringifyReplacerSortKeys } = require('./utils');
 
@@ -13,7 +15,7 @@ const cache = {};
 /** @typedef {import('./options').Options} Options */
 /** @typedef {() => Promise<void>} AsyncTask */
 /** @typedef {(files: string|string[]) => Promise<LintResult[]>} LintTask */
-/** @typedef {{threads: number, ESLint: ESLint, eslint: ESLint, lintFiles: LintTask, cleanup: AsyncTask}} Linter */
+/** @typedef {{threads: number, eslint: ESLint, lintFiles: LintTask, cleanup: AsyncTask}} Linter */
 /** @typedef {JestWorker & {lintFiles: LintTask}} Worker */
 
 /**
@@ -22,24 +24,16 @@ const cache = {};
  */
 function loadESLint(options) {
   const { eslintPath } = options;
-
-  const { ESLint } = require(eslintPath || 'eslint');
-
-  // Filter out loader options before passing the options to ESLint.
-  const eslint = new ESLint(getESLintOptions(options));
+  const eslint = setup({
+    eslintPath,
+    configType: options.configType,
+    eslintOptions: getESLintOptions(options),
+  });
 
   return {
     threads: 1,
-    ESLint,
+    lintFiles,
     eslint,
-    lintFiles: async (files) => {
-      const results = await eslint.lintFiles(files);
-      // istanbul ignore else
-      if (options.fix) {
-        await ESLint.outputFixes(results);
-      }
-      return results;
-    },
     // no-op for non-threaded
     cleanup: async () => {},
   };
@@ -58,7 +52,13 @@ function loadESLintThreaded(key, poolSize, options) {
   const workerOptions = {
     enableWorkerThreads: true,
     numWorkers: poolSize,
-    setupArgs: [{ eslintPath, eslintOptions: getESLintOptions(options) }],
+    setupArgs: [
+      {
+        eslintPath,
+        configType: options.configType,
+        eslintOptions: getESLintOptions(options),
+      },
+    ],
   };
 
   const local = loadESLint(options);

src/options.js

@@ -37,6 +37,7 @@ const schema = require('./options.json');
  * @property {OutputReport=} outputReport
  * @property {number|boolean=} threads
  * @property {RegExp|RegExp[]=} resourceQueryExclude
+ * @property {string=} configType
  */
 
 /** @typedef {PluginOptions & ESLintOptions} Options */
@@ -84,6 +85,11 @@ function getESLintOptions(loaderOptions) {
     delete eslintOptions[option];
   }
 
+  // Some options aren't available in flat mode
+  if (loaderOptions.configType === 'flat') {
+    delete eslintOptions.extensions;
+  }
+
   return eslintOptions;
 }
 

src/options.json

@@ -2,6 +2,10 @@
   "type": "object",
   "additionalProperties": true,
   "properties": {
+    "configType": {
+      "description": "Enable flat config by setting this value to `flat`.",
+      "type": "string"
+    },
     "context": {
       "description": "A string indicating the root of your files.",
       "type": "string"

src/worker.js

@@ -1,12 +1,13 @@
 /** @typedef {import('eslint').ESLint} ESLint */
 /** @typedef {import('eslint').ESLint.Options} ESLintOptions */
+/** @typedef {import('eslint').ESLint.LintResult} LintResult */
 
 Object.assign(module.exports, {
   lintFiles,
   setup,
 });
 
-/** @type {{ new (arg0: import("eslint").ESLint.Options): import("eslint").ESLint; outputFixes: (arg0: import("eslint").ESLint.LintResult[]) => any; }} */
+/** @type {{ new (arg0: ESLintOptions): ESLint; outputFixes: (arg0: LintResult[]) => any; }} */
 let ESLint;
 
 /** @type {ESLint} */
@@ -18,20 +19,44 @@ let fix;
 /**
  * @typedef {object} setupOptions
  * @property {string=} eslintPath - import path of eslint
- * @property {ESLintOptions=} eslintOptions - linter options
+ * @property {string=} configType
+ * @property {ESLintOptions} eslintOptions - linter options
  *
  * @param {setupOptions} arg0 - setup worker
  */
-function setup({ eslintPath, eslintOptions = {} }) {
+function setup({ eslintPath, configType, eslintOptions }) {
   fix = !!(eslintOptions && eslintOptions.fix);
-  ({ ESLint } = require(eslintPath || 'eslint'));
-  eslint = new ESLint(eslintOptions);
+  const eslintModule = require(eslintPath || 'eslint');
+
+  let FlatESLint;
+
+  if (eslintModule.LegacyESLint) {
+    ESLint = eslintModule.LegacyESLint;
+    ({ FlatESLint } = eslintModule);
+  } else {
+    ({ ESLint } = eslintModule);
+
+    if (configType === 'flat') {
+      throw new Error(
+        "Couldn't find FlatESLint, you might need to set eslintPath to 'eslint/use-at-your-own-risk'",
+      );
+    }
+  }
+
+  if (configType === 'flat') {
+    eslint = new FlatESLint(eslintOptions);
+  } else {
+    eslint = new ESLint(eslintOptions);
+  }
+
+  return eslint;
 }
 
 /**
  * @param {string | string[]} files
  */
 async function lintFiles(files) {
+  /** @type {LintResult[]} */
   const result = await eslint.lintFiles(files);
   // if enabled, use eslint autofixing where possible
   if (fix) {

test/fixtures/flat-config.js

@@ -0,0 +1,7 @@
+
+module.exports = [
+    {
+        files: ["*.js"],
+        rules: {}
+    }
+];

test/flat-config.test.js

@@ -0,0 +1,50 @@
+import { join } from 'path';
+
+import pack from './utils/pack';
+
+describe('succeed on flat-configuration', () => {
+  it('cannot load FlatESLint class on default ESLint module', (done) => {
+    const overrideConfigFile = join(__dirname, 'fixtures', 'flat-config.js');
+    const compiler = pack('full-of-problems', {
+      configType: 'flat',
+      overrideConfigFile,
+      threads: 1,
+    });
+
+    compiler.run((err, stats) => {
+      expect(err).toBeNull();
+      const { errors } = stats.compilation;
+
+      expect(stats.hasErrors()).toBe(true);
+      expect(errors).toHaveLength(1);
+      expect(errors[0].message).toMatch(
+        /Couldn't find FlatESLint, you might need to set eslintPath to 'eslint\/use-at-your-own-risk'/i,
+      );
+      done();
+    });
+  });
+
+  (process.version.match(/^v(\d+\.\d+)/)[1] >= 20 ? it : it.skip)('finds errors on files', (done) => {
+    const overrideConfigFile = join(__dirname, 'fixtures', 'flat-config.js');
+    const compiler = pack('full-of-problems', {
+      configType: 'flat',
+      // needed for now
+      eslintPath: 'eslint/use-at-your-own-risk',
+      overrideConfigFile,
+      threads: 1,
+    });
+
+    compiler.run((err, stats) => {
+      expect(err).toBeNull();
+      const { errors } = stats.compilation;
+
+      expect(stats.hasErrors()).toBe(true);
+      expect(errors).toHaveLength(1);
+      expect(errors[0].message).toMatch(
+        /full-of-problems\.js/i,
+      );
+      expect(stats.hasWarnings()).toBe(true);
+      done();
+    });
+  });
+});