feat(eslint): add support for flat configs

.eslintignore

@@ -4,6 +4,7 @@
 **/test/**/output
 packages/commonjs/test/fixtures
 packages/typescript/test/fixtures/syntax-error
+packages/eslint/test/fixtures/flat-config
 
 # temporary workaround for eslint bug where package.json is a directory
 packages/node-resolve/test/fixtures/package-json-in-path

packages/eslint/.eslintignore

@@ -1 +1 @@
-test/fixtures/ignored.js
+test/fixtures/legacy-config/ignored.js

packages/eslint/package.json

@@ -63,12 +63,12 @@
   },
   "dependencies": {
     "@rollup/pluginutils": "^5.0.1",
-    "eslint": "^8.24.0"
+    "eslint": "^8.57.1"
   },
   "devDependencies": {
     "@rollup/plugin-node-resolve": "^15.0.0",
     "@rollup/plugin-typescript": "^9.0.1",
-    "@types/eslint": "^8.4.6",
+    "@types/eslint": "^8.56.12",
     "rollup": "^4.0.0-24",
     "typescript": "^4.8.3"
   },

packages/eslint/src/index.ts

@@ -2,10 +2,15 @@ import { relative, resolve, sep } from 'path';
 
 import type { Plugin } from 'rollup';
 import { createFilter } from '@rollup/pluginutils';
-import { ESLint } from 'eslint';
+import { ESLint, loadESLint } from 'eslint';
 
 import type { RollupEslintOptions } from '../types';
 
+// New API introduced in v8.57 missing from @types/eslint
+declare module 'eslint' {
+  function loadESLint(): Promise<typeof ESLint>;
+}
+
 function normalizePath(id: string) {
   return relative(process.cwd(), id).split(sep).join('/');
 }
@@ -29,12 +34,17 @@ export default function eslint(options = {} as RollupEslintOptions): Plugin {
     ...eslintOptions
   } = options;
 
-  const eslintInstance = new ESLint(eslintOptions);
+  let eslintInstance: ESLint | null = null;
   const filter = createFilter(include, exclude);
 
   return {
     name: 'eslint',
     async transform(_, id: string) {
+      if (!eslintInstance) {
+        const DefaultESLint = await loadESLint();
+        eslintInstance = new DefaultESLint(eslintOptions);
+      }
+
       const file = normalizePath(id);
       if (!filter(id) || (await eslintInstance.isPathIgnored(file))) {
         return null;

packages/eslint/test/fixtures/flat-config/eslint.config.js

@@ -0,0 +1,34 @@
+export default [
+  {
+    files: ["**/*.js"],
+    rules: {
+      "no-alert": 2,
+      "no-bitwise": 1,
+      "camelcase": 1,
+      "curly": 1,
+      "eqeqeq": 0,
+      "no-eq-null": 0,
+      "guard-for-in": 1,
+      "no-empty": 1,
+      "no-use-before-define": 0,
+      "object-curly-spacing": 0,
+      "no-obj-calls": 2,
+      "no-unused-vars": 0,
+      "new-cap": 1,
+      "no-shadow": 0,
+      "strict": 2,
+      "global-strict": 0,
+      "no-invalid-regexp": 2,
+      "comma-dangle": 2,
+      "no-undef": 1,
+      "no-new": 1,
+      "no-extra-semi": 1,
+      "no-debugger": 2,
+      "no-caller": 1,
+      "semi": 1,
+      "quotes": 0,
+      "no-unreachable": 2,
+      "eol-last": 0,
+    },
+  },
+];

packages/eslint/test/fixtures/flat-config/undeclared.js

@@ -0,0 +1 @@
+x = 0;

packages/eslint/test/test.mjs

@@ -11,7 +11,7 @@ import eslint from 'current-package';
 test('should lint files', async (t) => {
   let count = 0;
   await rollup({
-    input: './test/fixtures/undeclared.js',
+    input: './test/fixtures/legacy-config/undeclared.js',
     plugins: [
       eslint({
         formatter: (results) => {
@@ -29,7 +29,7 @@ test('should lint files', async (t) => {
 
 test('should not fail with default options', async (t) => {
   await rollup({
-    input: './test/fixtures/undeclared.js',
+    input: './test/fixtures/legacy-config/undeclared.js',
     plugins: [eslint()]
   });
 
@@ -39,11 +39,11 @@ test('should not fail with default options', async (t) => {
 test('should ignore node_modules with exclude option', async (t) => {
   let count = 0;
   await rollup({
-    input: './test/fixtures/modules.js',
+    input: './test/fixtures/legacy-config/modules.js',
     plugins: [
       nodeResolve({ jsnext: true }),
       eslint({
-        overrideConfigFile: './test/fixtures/.eslintrc-babel',
+        overrideConfigFile: './test/fixtures/legacy-config/.eslintrc-babel',
         formatter: () => {
           count += 1;
         }
@@ -57,7 +57,7 @@ test('should ignore node_modules with exclude option', async (t) => {
 test('should ignore files according .eslintignore', async (t) => {
   let count = 0;
   await rollup({
-    input: './test/fixtures/ignored.js',
+    input: './test/fixtures/legacy-config/ignored.js',
     plugins: [
       eslint({
         formatter: () => {
@@ -74,7 +74,7 @@ test('should fail with enabled throwOnWarning and throwOnError options', async (
   await t.throwsAsync(
     async () => {
       await rollup({
-        input: './test/fixtures/use-strict.js',
+        input: './test/fixtures/legacy-config/use-strict.js',
         plugins: [
           eslint({
             throwOnWarning: true,
@@ -92,7 +92,7 @@ test('should fail with enabled throwOnError option', async (t) => {
   await t.throwsAsync(
     async () => {
       await rollup({
-        input: './test/fixtures/use-strict.js',
+        input: './test/fixtures/legacy-config/use-strict.js',
         plugins: [
           eslint({
             throwOnError: true,
@@ -109,7 +109,7 @@ test('should fail with enabled throwOnWarning option', async (t) => {
   await t.throwsAsync(
     async () => {
       await rollup({
-        input: './test/fixtures/use-strict.js',
+        input: './test/fixtures/legacy-config/use-strict.js',
         plugins: [
           eslint({
             throwOnWarning: true,
@@ -124,7 +124,7 @@ test('should fail with enabled throwOnWarning option', async (t) => {
 
 test('should not fail with throwOnError and throwOnWarning disabled', async (t) => {
   await rollup({
-    input: './test/fixtures/use-strict.js',
+    input: './test/fixtures/legacy-config/use-strict.js',
     plugins: [
       eslint({
         throwOnError: false,
@@ -141,7 +141,7 @@ test('should fail with not found formatter', async (t) => {
   await t.throwsAsync(
     async () => {
       await rollup({
-        input: './test/fixtures/use-strict.js',
+        input: './test/fixtures/legacy-config/use-strict.js',
         plugins: [
           eslint({
             formatter: 'not-found-formatter'
@@ -155,7 +155,7 @@ test('should fail with not found formatter', async (t) => {
 
 test('should not fail with found formatter', async (t) => {
   rollup({
-    input: './test/fixtures/use-strict.js',
+    input: './test/fixtures/legacy-config/use-strict.js',
     plugins: [
       eslint({
         formatter: 'stylish'
@@ -168,7 +168,7 @@ test('should not fail with found formatter', async (t) => {
 
 test('should not fail with asynchronous formatter function', async (t) => {
   await rollup({
-    input: './test/fixtures/use-strict.js',
+    input: './test/fixtures/legacy-config/use-strict.js',
     plugins: [
       eslint({
         formatter: async () => 'json'
@@ -181,12 +181,12 @@ test('should not fail with asynchronous formatter function', async (t) => {
 
 test('should fix source code', async (t) => {
   fs.writeFileSync(
-    './test/fixtures/fixable-clone.js',
-    fs.readFileSync('./test/fixtures/fixable.js')
+    './test/fixtures/legacy-config/fixable-clone.js',
+    fs.readFileSync('./test/fixtures/legacy-config/fixable.js')
   );
 
   await rollup({
-    input: './test/fixtures/fixable-clone.js',
+    input: './test/fixtures/legacy-config/fixable-clone.js',
     plugins: [
       eslint({
         fix: true
@@ -195,19 +195,19 @@ test('should fix source code', async (t) => {
   });
 
   t.is(
-    fs.readFileSync('./test/fixtures/fixable-clone.js').toString(),
-    fs.readFileSync('./test/fixtures/fixed.js').toString()
+    fs.readFileSync('./test/fixtures/legacy-config/fixable-clone.js').toString(),
+    fs.readFileSync('./test/fixtures/legacy-config/fixed.js').toString()
   );
 
-  fs.unlinkSync('./test/fixtures/fixable-clone.js');
+  fs.unlinkSync('./test/fixtures/legacy-config/fixable-clone.js');
 });
 
 test('works with cjs plugin', async (t) => {
   const require = createRequire(import.meta.url);
   const eslintPluginCjs = require('current-package');
   let count = 0;
   await rollup({
-    input: './test/fixtures/undeclared.js',
+    input: './test/fixtures/legacy-config/undeclared.js',
     plugins: [
       eslintPluginCjs({
         formatter: (results) => {
@@ -222,3 +222,22 @@ test('works with cjs plugin', async (t) => {
 
   t.is(count, 1);
 });
+
+test('works with flat config', async (t) => {
+  let count = 0;
+  await rollup({
+    input: './test/fixtures/flat-config/undeclared.js',
+    plugins: [
+      eslint({
+        formatter: (results) => {
+          count += results[0].messages.length;
+          // eslint-disable-next-line prefer-destructuring
+          const { message } = results[0].messages[0];
+          t.is(message, "'x' is not defined.");
+        }
+      })
+    ]
+  });
+
+  t.is(count, 1);
+});