fix(linter): only set flat config env for eslint v9+ (#25189)

<!-- Please make sure you have read the submission guidelines before
posting an PR -->
<!--
https://github.com/nrwl/nx/blob/master/CONTRIBUTING.md#-submitting-a-pr
-->

<!-- Please make sure that your commit message follows our format -->
<!-- Example: `fix(nx): must begin with lowercase` -->

## Current Behavior
<!-- This is the behavior we have today -->

The `ESLINT_USE_FLAT_CONFIG` env flag was set unnecessarily even if the
installed version of ESLint is < 9.

## Expected Behavior
<!-- This is the behavior we should expect with the changes in this PR
-->

The `ESLINT_USE_FLAT_CONFIG` env flag is only set if the value is
different than the default in the installed version of ESLint.

## Related Issue(s)
<!-- Please link the issue being fixed so it gets closed when this is
merged. -->

Fixes #

packages/eslint/package.json

@@ -36,6 +36,7 @@
     "@nx/devkit": "file:../devkit",
     "@nx/js": "file:../js",
     "eslint": "^8.0.0 || ^9.0.0",
+    "semver": "^7.5.3",
     "tslib": "^2.3.0",
     "typescript": "~5.4.2"
   },

packages/eslint/src/plugins/plugin.spec.ts

@@ -108,9 +108,6 @@ describe('@nx/eslint/plugin', () => {
                   ],
                   "options": {
                     "cwd": ".",
-                    "env": {
-                      "ESLINT_USE_FLAT_CONFIG": "false",
-                    },
                   },
                   "outputs": [
                     "{options.outputFile}",
@@ -183,9 +180,6 @@ describe('@nx/eslint/plugin', () => {
                   ],
                   "options": {
                     "cwd": "apps/my-app",
-                    "env": {
-                      "ESLINT_USE_FLAT_CONFIG": "false",
-                    },
                   },
                   "outputs": [
                     "{options.outputFile}",
@@ -227,9 +221,6 @@ describe('@nx/eslint/plugin', () => {
                   ],
                   "options": {
                     "cwd": "apps/my-app",
-                    "env": {
-                      "ESLINT_USE_FLAT_CONFIG": "false",
-                    },
                   },
                   "outputs": [
                     "{options.outputFile}",
@@ -343,9 +334,6 @@ describe('@nx/eslint/plugin', () => {
                   ],
                   "options": {
                     "cwd": "apps/my-app",
-                    "env": {
-                      "ESLINT_USE_FLAT_CONFIG": "false",
-                    },
                   },
                   "outputs": [
                     "{options.outputFile}",
@@ -371,9 +359,6 @@ describe('@nx/eslint/plugin', () => {
                   ],
                   "options": {
                     "cwd": "libs/my-lib",
-                    "env": {
-                      "ESLINT_USE_FLAT_CONFIG": "false",
-                    },
                   },
                   "outputs": [
                     "{options.outputFile}",
@@ -459,9 +444,6 @@ describe('@nx/eslint/plugin', () => {
                   ],
                   "options": {
                     "cwd": "apps/my-app",
-                    "env": {
-                      "ESLINT_USE_FLAT_CONFIG": "false",
-                    },
                   },
                   "outputs": [
                     "{options.outputFile}",
@@ -488,9 +470,6 @@ describe('@nx/eslint/plugin', () => {
                   ],
                   "options": {
                     "cwd": "libs/my-lib",
-                    "env": {
-                      "ESLINT_USE_FLAT_CONFIG": "false",
-                    },
                   },
                   "outputs": [
                     "{options.outputFile}",
@@ -534,9 +513,6 @@ describe('@nx/eslint/plugin', () => {
                   ],
                   "options": {
                     "cwd": "apps/myapp",
-                    "env": {
-                      "ESLINT_USE_FLAT_CONFIG": "false",
-                    },
                   },
                   "outputs": [
                     "{options.outputFile}",
@@ -585,9 +561,6 @@ describe('@nx/eslint/plugin', () => {
                   ],
                   "options": {
                     "cwd": "apps/myapp/nested/mylib",
-                    "env": {
-                      "ESLINT_USE_FLAT_CONFIG": "false",
-                    },
                   },
                   "outputs": [
                     "{options.outputFile}",

packages/eslint/src/plugins/plugin.ts

@@ -15,6 +15,7 @@ import {
   isFlatConfig,
 } from '../utils/config-file';
 import { resolveESLintClass } from '../utils/resolve-eslint-class';
+import { gte } from 'semver';
 
 export interface EslintPluginOptions {
   targetName?: string;
@@ -67,6 +68,7 @@ export const createNodes: CreateNodes<EslintPluginOptions> = [
     const excludePatterns = dedupedProjectRoots.map((root) => `${root}/**/*`);
 
     const ESLint = await resolveESLintClass(isFlatConfig(configFilePath));
+    const eslintVersion = ESLint.version;
     const childProjectRoots = new Set<string>();
 
     await Promise.all(
@@ -101,6 +103,7 @@ export const createNodes: CreateNodes<EslintPluginOptions> = [
       projects: getProjectsUsingESLintConfig(
         configFilePath,
         uniqueChildProjectRoots,
+        eslintVersion,
         options,
         context
       ),
@@ -111,6 +114,7 @@ export const createNodes: CreateNodes<EslintPluginOptions> = [
 function getProjectsUsingESLintConfig(
   configFilePath: string,
   childProjectRoots: string[],
+  eslintVersion: string,
   options: EslintPluginOptions,
   context: CreateNodesContext
 ): CreateNodesResult['projects'] {
@@ -142,6 +146,7 @@ function getProjectsUsingESLintConfig(
     projects[projectRoot] = {
       targets: buildEslintTargets(
         eslintConfigs,
+        eslintVersion,
         projectRoot,
         context.workspaceRoot,
         options,
@@ -155,6 +160,7 @@ function getProjectsUsingESLintConfig(
 
 function buildEslintTargets(
   eslintConfigs: string[],
+  eslintVersion: string,
   projectRoot: string,
   workspaceRoot: string,
   options: EslintPluginOptions,
@@ -191,9 +197,13 @@ function buildEslintTargets(
 
   // Always set the environment variable to ensure that the ESLint CLI can run on eslint v8 and v9
   const useFlatConfig = eslintConfigs.some((config) => isFlatConfig(config));
-  targetConfig.options.env = {
-    ESLINT_USE_FLAT_CONFIG: useFlatConfig ? 'true' : 'false',
-  };
+  // Flat config is default for 9.0.0+
+  const defaultSetting = gte(eslintVersion, '9.0.0');
+  if (useFlatConfig !== defaultSetting) {
+    targetConfig.options.env = {
+      ESLINT_USE_FLAT_CONFIG: useFlatConfig ? 'true' : 'false',
+    };
+  }
 
   targets[options.targetName] = targetConfig;