feat(eslint): add support for flat configs

ESLint v8.57 has added a new loadESLint() function to load the
correct ESLint class (either one reading legacy configs, either
one reading flat configs).

See:

- https://eslint.org/docs/v8.x/integrate/nodejs-api#loadeslint
- eslint/eslint@1120b9b

Unfortunately @types/eslint is missing v8.57 and has been removed
entirely from the DefinitelyType repository (since upstream now
ships type definitions). As a result we need to manually declare
the type of the loadESLint() function.

Additionally, loadESLint() is async, so we can't call it at plugin
initialization time. Instead, we call it the first time we need it.

Author: emersion

.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;