fix(linter): eslint flat config not working correctly (#18379)

packages/linter/src/executors/eslint/lint.impl.spec.ts

@@ -43,6 +43,7 @@ jest.mock('./utility/eslint-utils', () => {
   };
 });
 import lintExecutor from './lint.impl';
+import { resolve } from 'path';
 
 let mockChdir = jest.fn().mockImplementation(() => {});
 
@@ -150,7 +151,7 @@ describe('Linter Builder', () => {
       mockContext
     );
     expect(mockResolveAndInstantiateESLint).toHaveBeenCalledWith(
-      '/root/.eslintrc.json',
+      resolve('/root', '.eslintrc.json'),
       {
         lintFilePatterns: [],
         eslintConfig: './.eslintrc.json',
@@ -649,4 +650,33 @@ Please see https://nx.dev/guides/eslint for full guidance on how to resolve this
     expect(console.log).toHaveBeenCalledWith('{\n "file": "test-source.ts"\n}');
     expect(result).toEqual({ success: true });
   });
+
+  it('should pass path to eslint.config.js to resolveAndInstantiateESLint if it is unspecified and we are using flag configuration', async () => {
+    setupMocks();
+    jest.spyOn(fs, 'existsSync').mockReturnValue(true);
+    await lintExecutor(createValidRunBuilderOptions(), mockContext);
+    expect(mockResolveAndInstantiateESLint).toHaveBeenCalledWith(
+      'apps/proj/eslint.config.js',
+      {
+        lintFilePatterns: [],
+        eslintConfig: null,
+        fix: true,
+        cache: true,
+        cacheLocation: 'cacheLocation1/proj',
+        cacheStrategy: 'content',
+        format: 'stylish',
+        force: false,
+        silent: false,
+        ignorePath: null,
+        maxWarnings: -1,
+        outputFile: null,
+        quiet: false,
+        noEslintrc: false,
+        rulesdir: [],
+        resolvePluginsRelativeTo: null,
+        reportUnusedDisableDirectives: null,
+      },
+      true
+    );
+  });
 });

packages/linter/src/executors/eslint/lint.impl.ts

@@ -1,7 +1,7 @@
 import { ExecutorContext, joinPathFragments } from '@nx/devkit';
 import { ESLint } from 'eslint';
 import { existsSync, mkdirSync, writeFileSync } from 'fs';
-import { dirname, join, resolve } from 'path';
+import { dirname, resolve } from 'path';
 
 import type { Schema } from './schema';
 import { resolveAndInstantiateESLint } from './utility/eslint-utils';
@@ -31,12 +31,12 @@ export default async function run(
    * We want users to have the option of not specifying the config path, and let
    * eslint automatically resolve the `.eslintrc.json` files in each folder.
    */
-  const eslintConfigPath = options.eslintConfig
+  let eslintConfigPath = options.eslintConfig
     ? resolve(systemRoot, options.eslintConfig)
     : undefined;
 
   options.cacheLocation = options.cacheLocation
-    ? join(options.cacheLocation, projectName)
+    ? joinPathFragments(options.cacheLocation, projectName)
     : undefined;
 
   const { printConfig, ...normalizedOptions } = options;
@@ -49,6 +49,13 @@ export default async function run(
   const useFlatConfig = existsSync(
     joinPathFragments(systemRoot, 'eslint.config.js')
   );
+
+  if (!eslintConfigPath && useFlatConfig) {
+    const projectRoot =
+      context.projectsConfigurations.projects[context.projectName].root;
+    eslintConfigPath = joinPathFragments(projectRoot, 'eslint.config.js');
+  }
+
   const { eslint, ESLint } = await resolveAndInstantiateESLint(
     eslintConfigPath,
     normalizedOptions,
@@ -158,7 +165,10 @@ Please see https://nx.dev/guides/eslint for full guidance on how to resolve this
   const formattedResults = await formatter.format(lintResults);
 
   if (normalizedOptions.outputFile) {
-    const pathToOutputFile = join(context.root, normalizedOptions.outputFile);
+    const pathToOutputFile = joinPathFragments(
+      context.root,
+      normalizedOptions.outputFile
+    );
     mkdirSync(dirname(pathToOutputFile), { recursive: true });
     writeFileSync(pathToOutputFile, formattedResults);
   } else {