fix: don't create .eslintrc if package.json contains eslintConfig

packages/next/lib/eslint/writeDefaultConfig.ts

@@ -49,25 +49,25 @@ export async function writeDefaultConfig(
         )
       )
     }
-  } else if (
-    packageJsonConfig?.eslintConfig &&
-    Object.entries(packageJsonConfig?.eslintConfig).length === 0
-  ) {
-    packageJsonConfig.eslintConfig = defaultConfig
+  } else if (packageJsonConfig?.eslintConfig) {
+    // Creates .eslintrc only if package.json's eslintConfig field is empty
+    if (Object.entries(packageJsonConfig?.eslintConfig).length === 0) {
+      packageJsonConfig.eslintConfig = defaultConfig
 
-    if (pkgJsonPath)
-      await fs.writeFile(
-        pkgJsonPath,
-        CommentJson.stringify(packageJsonConfig, null, 2) + os.EOL
-      )
+      if (pkgJsonPath)
+        await fs.writeFile(
+          pkgJsonPath,
+          CommentJson.stringify(packageJsonConfig, null, 2) + os.EOL
+        )
 
-    console.log(
-      chalk.green(
-        `We detected an empty ${chalk.bold(
-          'eslintConfig'
-        )} field in package.json and updated it for you to include the base Next.js ESLint configuration.`
+      console.log(
+        chalk.green(
+          `We detected an empty ${chalk.bold(
+            'eslintConfig'
+          )} field in package.json and updated it for you to include the base Next.js ESLint configuration.`
+        )
       )
-    )
+    }
   } else {
     await fs.writeFile(
       '.eslintrc',

test/integration/eslint/config-in-package-json/package.json

@@ -0,0 +1,6 @@
+{
+  "eslintConfig": {
+    "extends": "next",
+    "root": true
+  }
+}

test/integration/eslint/config-in-package-json/pages/index.js

@@ -0,0 +1,9 @@
+export default class Test {
+  render() {
+    return (
+      <div>
+        <h1>Hello title</h1>
+      </div>
+    )
+  }
+}

test/integration/eslint/test/index.test.js

@@ -1,4 +1,6 @@
+import fs from 'fs-extra'
 import { join } from 'path'
+import findUp from 'next/dist/compiled/find-up'
 import { nextBuild, nextLint } from 'next-test-utils'
 import { writeFile, readFile } from 'fs-extra'
 
@@ -8,6 +10,7 @@ const dirFirstTimeSetup = join(__dirname, '../first-time-setup')
 const dirCustomConfig = join(__dirname, '../custom-config')
 const dirIgnoreDuringBuilds = join(__dirname, '../ignore-during-builds')
 const dirCustomDirectories = join(__dirname, '../custom-directories')
+const dirConfigInPackageJson = join(__dirname, '../config-in-package-json')
 
 describe('ESLint', () => {
   describe('Next Build', () => {
@@ -113,5 +116,43 @@ describe('ESLint', () => {
       const output = stdout + stderr
       expect(output).toContain('No ESLint warnings or errors')
     })
+
+    test("don't create .eslintrc file if package.json has eslintConfig field", async () => {
+      const eslintrcFile =
+        (await findUp(
+          [
+            '.eslintrc.js',
+            '.eslintrc.yaml',
+            '.eslintrc.yml',
+            '.eslintrc.json',
+            '.eslintrc',
+          ],
+          {
+            cwd: '.',
+          }
+        )) ?? null
+
+      try {
+        // If we found a .eslintrc file, it's probably config from root Next.js directory. Rename it during the test
+        if (eslintrcFile) {
+          await fs.move(eslintrcFile, `${eslintrcFile}.original`)
+        }
+
+        const { stdout, stderr } = await nextLint(dirConfigInPackageJson, [], {
+          stdout: true,
+          stderr: true,
+        })
+
+        const output = stdout + stderr
+        expect(output).not.toContain(
+          'We created the .eslintrc file for you and included the base Next.js ESLint configuration'
+        )
+      } finally {
+        // Restore original .eslintrc file
+        if (eslintrcFile) {
+          await fs.move(`${eslintrcFile}.original`, eslintrcFile)
+        }
+      }
+    })
   })
 })