next lint + ESLint in Create Next App (#25064)

Co-authored-by: Tim Neutkens <tim@timneutkens.nl>
Co-authored-by: Tim Neutkens <timneutkens@me.com>

docs/api-reference/cli.md

@@ -21,7 +21,7 @@ Usage
   $ next <command>
 
 Available commands
-  build, start, export, dev, telemetry
+  build, start, export, dev, lint, telemetry
 
 Options
   --version, -v   Version number
@@ -84,6 +84,16 @@ The application will start at `http://localhost:3000` by default. The default po
 npx next start -p 4000
 ```
 
+## Lint
+
+`next lint` runs ESLint for all files in the `pages` directory and provides a guided setup to install any required dependencies if ESLint is not already configured in your application.
+
+You can also run ESLint on other directories with the `--dir` flag:
+
+```bash
+next lint --dir components
+```
+
 ## Telemetry
 
 Next.js collects **completely anonymous** telemetry data about general usage.

docs/basic-features/eslint.md

@@ -0,0 +1,119 @@
+---
+description: Next.js supports ESLint by default. You can get started with ESLint in Next.js here.
+---
+
+# ESLint
+
+Since version **11.0.0**, Next.js provides an integrated [ESLint](https://eslint.org/) experience out of the box. To get started, run `next lint`:
+
+```bash
+next lint
+```
+
+If you don't already have ESLint configured in your application, you will be guided through the installation of any required packages.
+
+```bash
+next lint
+
+# You'll see instructions like these:
+#
+# Please install eslint and eslint-config-next by running:
+#
+#         yarn add --dev eslint eslint-config-next
+#
+# ...
+```
+
+If no ESLint configuration is present, Next.js will create an `.eslintrc` file in the root of your project and automatically configure it with the base configuration:
+
+```
+{
+  "extends": "next"
+}
+```
+
+Now you can run `next lint` every time you want to run ESLint to catch errors
+
+> The default base configuration (`"extends": "next"`) can be updated at any time and will only be included if no ESLint configuration is present.
+
+We recommend using an appropriate [integration](https://eslint.org/docs/user-guide/integrations#editors) to view warnings and errors directly in your code editor during development.
+
+## Linting During Builds
+
+Once ESLint has been set up, it will automatically run during every build (`next build`). Errors will fail the build while warnings will not.
+
+If you do not want ESLint to run as a build step, it can be disabled using the `--no-lint` flag:
+
+```bash
+next build --no-lint
+```
+
+This is not recommended unless you have configured ESLint to run in a separate part of your workflow (for example, in CI or a pre-commit hook).
+
+## Linting Custom Directories
+
+By default, Next.js will only run ESLint for all files in the `pages/` directory. However, you can specify other custom directories to run by using the `--dir` flag in `next lint`:
+
+```bash
+next lint --dir components --dir lib
+```
+
+## ESLint Plugin
+
+Next.js provides an ESLint plugin, [`eslint-plugin-next`](https://www.npmjs.com/package/@next/eslint-plugin-next), that makes it easier to catch common issues and problems in a Next.js application. The full set of rules can be found in the [package repository](https://github.com/vercel/next.js/tree/master/packages/eslint-plugin-next/lib/rules).
+
+## Base Configuration
+
+The Next.js base ESLint configuration is automatically generated when `next lint` is run for the first time:
+
+```
+{
+  "extends": "next"
+}
+```
+
+This configuration extends recommended rule sets from various Eslint plugins:
+
+- [`eslint-plugin-react`](https://www.npmjs.com/package/eslint-plugin-react)
+- [`eslint-plugin-react-hooks`](https://www.npmjs.com/package/eslint-plugin-react-hooks)
+- [`eslint-plugin-next`](https://www.npmjs.com/package/@next/eslint-plugin-next)
+
+You can see the full details of the shareable configuration in the [`eslint-config-next`](https://www.npmjs.com/package/eslint-config-next) package.
+
+If you would like to modify any rules provided by the supported plugins (`react`, `react-hooks`, `next`), you can directly modify them using the `rules` property:
+
+```
+{
+  "extends": "next",
+  "rules": {
+    "react/no-unescaped-entities": "off",
+    "@next/next/no-page-custom-font": "error",
+  }
+}
+```
+
+> **Note**: If you need to also include a separate, custom ESLint configuration, it is highly recommended that `eslint-config-next` is extended last after other configurations. For example:
+>
+> ```
+> {
+>   "extends": ["eslint:recommended", "next"]
+> }
+> ```
+>
+> The `next` configuration already handles setting default values for the `parser`, `plugins` and `settings` properties.
+> There is no need to manually re-declare any of these properties unless you need a different configuration for your use case.
+> If you include any other shareable configurations, you will need to make sure that these properties are not overwritten or modified.
+
+### Core Web Vitals
+
+A stricter `next/core-web-vitals` entrypoint can also be specified in `.eslintrc`:
+
+```
+{
+  "extends": ["next", "next/core-web-vitals"]
+}
+```
+
+`next/core-web-vitals` updates `eslint-plugin-next` to error on a number of rules that are warnings by default if they affect [Core Web Vitals](https://web.dev/vitals/).
+
+> Both `next` and `next/core-web-vitals` entry points are automatically included for new applications built with [Create Next App](/docs/api-reference/create-next-app.md).

docs/getting-started.md

@@ -55,7 +55,8 @@ Open `package.json` and add the following `scripts`:
 "scripts": {
   "dev": "next dev",
   "build": "next build",
-  "start": "next start"
+  "start": "next start",
+  "lint": "next lint"
 }
 ```
 
@@ -64,6 +65,7 @@ These scripts refer to the different stages of developing an application:
 - `dev` - Runs [`next dev`](/docs/api-reference/cli.md#development) which starts Next.js in development mode
 - `build` - Runs [`next build`](/docs/api-reference/cli.md#build) which builds the application for production usage
 - `start` - Runs [`next start`](/docs/api-reference/cli.md#production) which starts a Next.js production server
+- `lint` - Runs [`next lint`](/docs/api-reference/cli.md#lint) which sets up Next.js' built-in ESLint configuration
 
 Next.js is built around the concept of [pages](/docs/basic-features/pages.md). A page is a [React Component](https://reactjs.org/docs/components-and-props.html) exported from a `.js`, `.jsx`, `.ts`, or `.tsx` file in the `pages` directory.
 

docs/manifest.json

@@ -37,6 +37,10 @@
               "title": "Fast Refresh",
               "path": "/docs/basic-features/fast-refresh.md"
             },
+            {
+              "title": "ESLint",
+              "path": "/docs/basic-features/eslint.md"
+            },
             {
               "title": "TypeScript",
               "path": "/docs/basic-features/typescript.md"

packages/create-next-app/create-app.ts

@@ -187,6 +187,7 @@ export async function createApp({
         dev: 'next dev',
         build: 'next build',
         start: 'next start',
+        lint: 'next lint',
       },
     }
     /**
@@ -207,7 +208,7 @@ export async function createApp({
     /**
      * Default devDependencies.
      */
-    const devDependencies = []
+    const devDependencies = ['eslint', 'eslint-config-next']
     /**
      * TypeScript projects will have type definitions and other devDependencies.
      */
@@ -250,7 +251,8 @@ export async function createApp({
       cwd: path.join(__dirname, 'templates', template),
       rename: (name) => {
         switch (name) {
-          case 'gitignore': {
+          case 'gitignore':
+          case 'eslintrc': {
             return '.'.concat(name)
           }
           // README.md is ignored by webpack-asset-relocator-loader used by ncc:

packages/create-next-app/templates/default/eslintrc

@@ -0,0 +1,3 @@
+{
+  "extends": ["next", "next/core-web-vitals"]
+}

packages/create-next-app/templates/default/pages/api/hello.js

@@ -1,5 +1,5 @@
 // Next.js API route support: https://nextjs.org/docs/api-routes/introduction
 
-export default (req, res) => {
+export default function handler(req, res) {
   res.status(200).json({ name: 'John Doe' })
 }

packages/create-next-app/templates/typescript/eslintrc

@@ -0,0 +1,3 @@
+{
+  "extends": ["next", "next/core-web-vitals"]
+}

packages/create-next-app/templates/typescript/pages/api/hello.ts

@@ -5,6 +5,9 @@ type Data = {
   name: string
 }
 
-export default (req: NextApiRequest, res: NextApiResponse<Data>) => {
+export default function handler(
+  req: NextApiRequest,
+  res: NextApiResponse<Data>
+) {
   res.status(200).json({ name: 'John Doe' })
 }

packages/eslint-config-next/core-web-vitals.js

@@ -0,0 +1,8 @@
+module.exports = {
+  extends: ['.'].map(require.resolve),
+  rules: {
+    '@next/next/no-sync-scripts': 2,
+    '@next/next/no-html-link-for-pages': 2,
+    '@next/next/no-img-element': 2,
+  },
+}

packages/eslint-config-next/index.js

@@ -16,6 +16,7 @@ module.exports = {
   rules: {
     'import/no-anonymous-default-export': 'warn',
     'react/react-in-jsx-scope': 'off',
+    'react/prop-types': 'off',
     'jsx-a11y/alt-text': [
       'warn',
       {

packages/eslint-plugin-next/lib/rules/no-sync-scripts.js

@@ -18,7 +18,7 @@ module.exports = function (context) {
         context.report({
           node,
           message:
-            'Synchronous scripts are forbidden. See: https://nextjs.org/docs/messages/no-sync-scripts.',
+            'External synchronous scripts are forbidden. See: https://nextjs.org/docs/messages/no-sync-scripts.',
         })
       }
     },

packages/next/bin/next.ts

@@ -20,6 +20,7 @@ const commands: { [command: string]: () => Promise<cliCommand> } = {
   start: () => import('../cli/next-start').then((i) => i.nextStart),
   export: () => import('../cli/next-export').then((i) => i.nextExport),
   dev: () => import('../cli/next-dev').then((i) => i.nextDev),
+  lint: () => import('../cli/next-lint').then((i) => i.nextLint),
   telemetry: () => import('../cli/next-telemetry').then((i) => i.nextTelemetry),
 }
 

packages/next/build/index.ts

@@ -119,7 +119,8 @@ export default async function build(
   dir: string,
   conf = null,
   reactProductionProfiling = false,
-  debugOutput = false
+  debugOutput = false,
+  runLint = true
 ): Promise<void> {
   const nextBuildSpan = trace('next-build')
 
@@ -212,13 +213,12 @@ export default async function build(
       typeCheckingSpinner.stopAndPersist()
     }
 
-    if (config.experimental.eslint) {
+    if (runLint) {
       await nextBuildSpan
         .traceChild('verify-and-lint')
         .traceAsyncFn(async () => {
           await verifyAndLint(
             dir,
-            pagesDir,
             config.experimental.cpus,
             config.experimental.workerThreads
           )

packages/next/cli/next-build.ts

@@ -13,6 +13,7 @@ const nextBuild: cliCommand = (argv) => {
     '--help': Boolean,
     '--profile': Boolean,
     '--debug': Boolean,
+    '--no-lint': Boolean,
     // Aliases
     '-h': '--help',
     '-d': '--debug',
@@ -41,6 +42,7 @@ const nextBuild: cliCommand = (argv) => {
 
       Options
       --profile     Can be used to enable React Production Profiling
+      --no-lint     Disable linting
     `,
       0
     )
@@ -48,6 +50,9 @@ const nextBuild: cliCommand = (argv) => {
   if (args['--profile']) {
     Log.warn('Profiling is enabled. Note: This may affect performance')
   }
+  if (args['--no-lint']) {
+    Log.warn('Linting is disabled')
+  }
   const dir = resolve(args._[0] || '.')
 
   // Check if the provided directory exists
@@ -93,7 +98,9 @@ const nextBuild: cliCommand = (argv) => {
   }
 
   return preflight()
-    .then(() => build(dir, null, args['--profile'], args['--debug']))
+    .then(() =>
+      build(dir, null, args['--profile'], args['--debug'], !args['--no-lint'])
+    )
     .catch((err) => {
       console.error('')
       console.error('> Build error occurred')

packages/next/cli/next-lint.ts

@@ -0,0 +1,76 @@
+#!/usr/bin/env node
+import { existsSync } from 'fs'
+import arg from 'next/dist/compiled/arg/index.js'
+import { resolve, join } from 'path'
+import { cliCommand } from '../bin/next'
+import { runLintCheck } from '../lib/eslint/runLintCheck'
+import { printAndExit } from '../server/lib/utils'
+
+const nextLint: cliCommand = (argv) => {
+  const validArgs: arg.Spec = {
+    // Types
+    '--help': Boolean,
+    '--dir': [String],
+
+    // Aliases
+    '-h': '--help',
+    '-d': '--dir',
+  }
+
+  let args: arg.Result<arg.Spec>
+  try {
+    args = arg(validArgs, { argv })
+  } catch (error) {
+    if (error.code === 'ARG_UNKNOWN_OPTION') {
+      return printAndExit(error.message, 1)
+    }
+    throw error
+  }
+  if (args['--help']) {
+    printAndExit(
+      `
+      Description
+        Run ESLint on every file in specified directories. 
+        If not configured, ESLint will be set up for the first time.
+
+      Usage
+        $ next lint <baseDir> [options]
+
+      <baseDir> represents the directory of the Next.js application.
+      If no directory is provided, the current directory will be used.
+
+      Options
+      -h - list this help
+      -d - set directory, or directories, to run ESLint (defaults to only 'pages')
+    `,
+      0
+    )
+  }
+
+  const baseDir = resolve(args._[0] || '.')
+
+  // Check if the provided directory exists
+  if (!existsSync(baseDir)) {
+    printAndExit(`> No such directory exists as the project root: ${baseDir}`)
+  }
+
+  const dirs: string[] = args['--dir']
+  const lintDirs = dirs
+    ? dirs.reduce((res: string[], d: string) => {
+        const currDir = join(baseDir, d)
+        if (!existsSync(currDir)) return res
+        res.push(currDir)
+        return res
+      }, [])
+    : null
+
+  runLintCheck(baseDir, lintDirs)
+    .then((results) => {
+      if (results) console.log(results)
+    })
+    .catch((err) => {
+      printAndExit(err.message)
+    })
+}
+
+export { nextLint }

packages/next/lib/eslint/getLintIntent.ts

@@ -1,32 +0,0 @@
-import { promises as fs } from 'fs'
-
-import * as CommentJson from 'next/dist/compiled/comment-json'
-
-export type LintIntent = { firstTimeSetup: boolean }
-
-export async function getLintIntent(
-  eslintrcFile: string | null,
-  pkgJsonEslintConfig: string | null
-): Promise<LintIntent | false> {
-  if (eslintrcFile) {
-    const content = await fs.readFile(eslintrcFile, { encoding: 'utf8' }).then(
-      (txt) => txt.trim().replace(/\n/g, ''),
-      () => null
-    )
-
-    // User is setting up ESLint for the first time setup if eslint config exists but is empty
-    return {
-      firstTimeSetup:
-        content === '' ||
-        content === '{}' ||
-        content === '---' ||
-        content === 'module.exports = {}',
-    }
-  } else if (pkgJsonEslintConfig) {
-    return {
-      firstTimeSetup: CommentJson.stringify(pkgJsonEslintConfig) === '{}',
-    }
-  }
-
-  return false
-}

packages/next/lib/eslint/runLintCheck.ts

@@ -1,13 +1,14 @@
-import { promises } from 'fs'
-import { extname } from 'path'
+import { promises as fs } from 'fs'
+import chalk from 'chalk'
 
 import findUp from 'next/dist/compiled/find-up'
 import semver from 'next/dist/compiled/semver'
+import * as CommentJson from 'next/dist/compiled/comment-json'
 
 import { formatResults } from './customFormatter'
-import { getLintIntent } from './getLintIntent'
 import { writeDefaultConfig } from './writeDefaultConfig'
 import { getPackageVersion } from '../get-package-version'
+import { findPagesDir } from '../find-pages-dir'
 
 import { CompileError } from '../compile-error'
 import {
@@ -22,12 +23,14 @@ type Config = {
   rules: { [key: string]: Array<number | string> }
 }
 
-const linteableFileTypes = ['jsx', 'js', 'ts', 'tsx']
+const linteableFiles = (dir: string) => {
+  return `${dir}/**/*.{${['jsx', 'js', 'ts', 'tsx'].join(',')}}`
+}
 
 async function lint(
   deps: NecessaryDependencies,
   baseDir: string,
-  pagesDir: string,
+  lintDirs: string[] | null,
   eslintrcFile: string | null,
   pkgJsonPath: string | null
 ): Promise<string | null> {
@@ -41,8 +44,8 @@ async function lint(
     })
 
     if (eslintVersion && semver.lt(eslintVersion, '7.0.0')) {
-      Log.warn(
-        `Your project has an older version of ESLint installed (${eslintVersion}). Please upgrade to v7 or later to run ESLint during the build process.`
+      Log.error(
+        `Your project has an older version of ESLint installed (${eslintVersion}). Please upgrade to v7 or later`
       )
     }
     return null
@@ -70,6 +73,8 @@ async function lint(
     }
   }
 
+  const pagesDir = findPagesDir(baseDir)
+
   if (nextEslintPluginIsEnabled) {
     let updatedPagesDir = false
 
@@ -93,9 +98,12 @@ async function lint(
     }
   }
 
-  const results = await eslint.lintFiles([
-    `${pagesDir}/**/*.{${linteableFileTypes.join(',')}}`,
-  ])
+  // If no directories to lint are provided, only the pages directory will be linted
+  const filesToLint = lintDirs
+    ? lintDirs.map(linteableFiles)
+    : linteableFiles(pagesDir)
+
+  const results = await eslint.lintFiles(filesToLint)
 
   if (ESLint.getErrorResults(results)?.length > 0) {
     throw new CompileError(await formatResults(baseDir, results))
@@ -105,19 +113,10 @@ async function lint(
 
 export async function runLintCheck(
   baseDir: string,
-  pagesDir: string
+  lintDirs: string[] | null,
+  lintDuringBuild: boolean = false
 ): Promise<string | null> {
   try {
-    // Check if any pages exist that can be linted
-    const pages = await promises.readdir(pagesDir)
-    if (
-      !pages.some((page) =>
-        linteableFileTypes.includes(extname(page).replace('.', ''))
-      )
-    ) {
-      return null
-    }
-
     // Find user's .eslintrc file
     const eslintrcFile =
       (await findUp(
@@ -134,33 +133,39 @@ export async function runLintCheck(
       )) ?? null
 
     const pkgJsonPath = (await findUp('package.json', { cwd: baseDir })) ?? null
-
-    const { eslintConfig: pkgJsonEslintConfig = null } = !!pkgJsonPath
-      ? await import(pkgJsonPath!)
-      : {}
-
-    // Check if the project uses ESLint
-    const eslintIntent = await getLintIntent(eslintrcFile, pkgJsonEslintConfig)
-
-    if (!eslintIntent) {
-      return null
+    let packageJsonConfig = null
+    if (pkgJsonPath) {
+      const pkgJsonContent = await fs.readFile(pkgJsonPath, {
+        encoding: 'utf8',
+      })
+      packageJsonConfig = CommentJson.parse(pkgJsonContent)
     }
 
-    const firstTimeSetup = eslintIntent.firstTimeSetup
+    // Warning displayed if no ESLint configuration is present during build
+    if (lintDuringBuild && !eslintrcFile && !packageJsonConfig.eslintConfig) {
+      Log.warn(
+        `No ESLint configuration detected. Run ${chalk.bold.cyan(
+          'next lint'
+        )} to begin setup`
+      )
+      return null
+    }
 
     // Ensure ESLint and necessary plugins and configs are installed:
     const deps: NecessaryDependencies = await hasNecessaryDependencies(
       baseDir,
       false,
-      !!eslintIntent,
-      eslintrcFile
+      true,
+      eslintrcFile ?? '',
+      !!packageJsonConfig.eslintConfig,
+      lintDuringBuild
     )
 
-    // Create the user's eslintrc config for them
-    if (firstTimeSetup) await writeDefaultConfig(eslintrcFile, pkgJsonPath)
+    // Write default ESLint config if none is present
+    await writeDefaultConfig(eslintrcFile, pkgJsonPath, packageJsonConfig)
 
     // Run ESLint
-    return await lint(deps, baseDir, pagesDir, eslintrcFile, pkgJsonPath)
+    return await lint(deps, baseDir, lintDirs, eslintrcFile, pkgJsonPath)
   } catch (err) {
     throw err
   }

packages/next/lib/eslint/writeDefaultConfig.ts

@@ -7,58 +7,79 @@ import * as CommentJson from 'next/dist/compiled/comment-json'
 
 export async function writeDefaultConfig(
   eslintrcFile: string | null,
-  pkgJsonPath: string | null
+  pkgJsonPath: string | null,
+  packageJsonConfig: { eslintConfig: any } | null
 ) {
   const defaultConfig = {
     extends: 'next',
   }
 
   if (eslintrcFile) {
+    const content = await fs.readFile(eslintrcFile, { encoding: 'utf8' }).then(
+      (txt) => txt.trim().replace(/\n/g, ''),
+      () => null
+    )
+
+    if (
+      content === '' ||
+      content === '{}' ||
+      content === '---' ||
+      content === 'module.exports = {}'
+    ) {
       const ext = path.extname(eslintrcFile)
 
-    let fileContent
+      let newFileContent
       if (ext === '.yaml' || ext === '.yml') {
-      fileContent = "extends: 'next'"
+        newFileContent = "extends: 'next'"
       } else {
-      fileContent = CommentJson.stringify(defaultConfig, null, 2)
+        newFileContent = CommentJson.stringify(defaultConfig, null, 2)
 
         if (ext === '.js') {
-        fileContent = 'module.exports = ' + fileContent
+          newFileContent = 'module.exports = ' + newFileContent
         }
       }
 
-    await fs.writeFile(eslintrcFile, fileContent + os.EOL)
+      await fs.writeFile(eslintrcFile, newFileContent + os.EOL)
 
       console.log(
-      '\n' +
         chalk.green(
-          `We detected ESLint in your project and updated the ${chalk.bold(
+          `We detected an empty ESLint configuration file (${chalk.bold(
             path.basename(eslintrcFile)
-          )} file for you.`
-        ) +
-        '\n'
+          )}) and updated it for you to include the base Next.js ESLint configuration.`
         )
-  } else if (pkgJsonPath) {
-    const pkgJsonContent = await fs.readFile(pkgJsonPath, {
-      encoding: 'utf8',
-    })
-    let packageJsonConfig = CommentJson.parse(pkgJsonContent)
-
+      )
+    }
+  } else if (
+    packageJsonConfig?.eslintConfig &&
+    Object.entries(packageJsonConfig?.eslintConfig).length === 0
+  ) {
     packageJsonConfig.eslintConfig = defaultConfig
 
+    if (pkgJsonPath)
       await fs.writeFile(
         pkgJsonPath,
         CommentJson.stringify(packageJsonConfig, null, 2) + os.EOL
       )
 
     console.log(
-      '\n' +
       chalk.green(
-          `We detected ESLint in your project and updated the ${chalk.bold(
+        `We detected an empty ${chalk.bold(
           'eslintConfig'
-          )} field for you in package.json...`
-        ) +
-        '\n'
+        )} field in package.json and updated it for you to include the base Next.js ESLint configuration.`
+      )
+    )
+  } else {
+    await fs.writeFile(
+      '.eslintrc',
+      CommentJson.stringify(defaultConfig, null, 2) + os.EOL
+    )
+
+    console.log(
+      chalk.green(
+        `We created the ${chalk.bold(
+          '.eslintrc'
+        )} file for you and included the base Next.js ESLint configuration.`
+      )
     )
   }
 }

packages/next/lib/has-necessary-dependencies.ts

@@ -1,5 +1,5 @@
 import chalk from 'chalk'
-import path from 'path'
+import { basename, join } from 'path'
 
 import { fileExists } from './file-exists'
 import { getOxfordCommaList } from './oxford-comma-list'
@@ -24,7 +24,9 @@ export async function hasNecessaryDependencies(
   baseDir: string,
   checkTSDeps: boolean,
   checkESLintDeps: boolean,
-  eslintrcFile: string | null = null
+  eslintrcFile: string = '',
+  pkgJsonEslintConfig: boolean = false,
+  lintDuringBuild: boolean = false
 ): Promise<NecessaryDependencies> {
   if (!checkTSDeps && !checkESLintDeps) {
     return { resolved: undefined! }
@@ -55,28 +57,39 @@ export async function hasNecessaryDependencies(
   const packagesHuman = getOxfordCommaList(missingPackages.map((p) => p.pkg))
   const packagesCli = missingPackages.map((p) => p.pkg).join(' ')
 
-  const yarnLockFile = path.join(baseDir, 'yarn.lock')
+  const yarnLockFile = join(baseDir, 'yarn.lock')
   const isYarn = await fileExists(yarnLockFile).catch(() => false)
-  const removalMsg = checkTSDeps
-    ? chalk.bold(
+
+  const removalTSMsg =
+    '\n\n' +
+    chalk.bold(
       'If you are not trying to use TypeScript, please remove the ' +
         chalk.cyan('tsconfig.json') +
         ' file from your package root (and any TypeScript files in your pages directory).'
     )
-    : chalk.bold(
-        `If you are not trying to use ESLint, please remove the ${
-          eslintrcFile
-            ? chalk.cyan(path.basename(eslintrcFile)) +
-              ' file from your application'
-            : chalk.cyan('eslintConfig') + ' field from your package.json file'
-        }.`
-      )
+  const removalLintMsg =
+    `\n\n` +
+    (lintDuringBuild
+      ? `If you do not want to run ESLint during builds, run ${chalk.bold.cyan(
+          'next build --no-lint'
+        )}` +
+        (!!eslintrcFile
+          ? ` or remove the ${chalk.bold(
+              basename(eslintrcFile)
+            )} file from your package root.`
+          : pkgJsonEslintConfig
+          ? ` or remove the ${chalk.bold(
+              'eslintConfig'
+            )} field from package.json.`
+          : '')
+      : `Once installed, run ${chalk.bold.cyan('next lint')} again.`)
+  const removalMsg = checkTSDeps ? removalTSMsg : removalLintMsg
 
   throw new FatalError(
     chalk.bold.red(
-      `It looks like you're trying to use ${
-        checkTSDeps ? 'TypeScript' : 'ESLint'
-      } but do not have the required package(s) installed.`
+      checkTSDeps
+        ? `It looks like you're trying to use TypeScript but do not have the required package(s) installed.`
+        : `To use ESLint, additional required package(s) must be installed.`
     ) +
       '\n\n' +
       chalk.bold(`Please install ${chalk.bold(packagesHuman)} by running:`) +
@@ -86,7 +99,6 @@ export async function hasNecessaryDependencies(
           ' ' +
           packagesCli
       )}` +
-      '\n\n' +
       removalMsg +
       '\n'
   )

packages/next/lib/verifyAndLint.ts

@@ -3,7 +3,6 @@ import { Worker } from 'jest-worker'
 
 export async function verifyAndLint(
   dir: string,
-  pagesDir: string,
   numWorkers: number | undefined,
   enableWorkerThreads: boolean | undefined
 ): Promise<void> {
@@ -18,7 +17,7 @@ export async function verifyAndLint(
     lintWorkers.getStdout().pipe(process.stdout)
     lintWorkers.getStderr().pipe(process.stderr)
 
-    const lintResults = await lintWorkers.runLintCheck(dir, pagesDir)
+    const lintResults = await lintWorkers.runLintCheck(dir, null, true)
     if (lintResults) {
       console.log(lintResults)
     }

packages/next/next-server/server/config-shared.ts

@@ -35,7 +35,6 @@ export type NextConfig = { [key: string]: any } & {
     excludeDefaultMomentLocales?: boolean
     webpack5?: boolean
   }
-
   experimental: {
     cpus?: number
     plugins?: boolean
@@ -55,7 +54,6 @@ export type NextConfig = { [key: string]: any } & {
       validator?: string
       skipValidation?: boolean
     }
-    eslint?: boolean
     reactRoot?: boolean
     enableBlurryPlaceholder?: boolean
     disableOptimizedLoading?: boolean
@@ -111,7 +109,6 @@ export const defaultConfig: NextConfig = {
     scrollRestoration: false,
     stats: false,
     externalDir: false,
-    eslint: false,
     reactRoot: Number(process.env.NEXT_PRIVATE_REACT_ROOT) > 0,
     enableBlurryPlaceholder: false,
     disableOptimizedLoading: true,

test-pnp.sh

@@ -39,5 +39,5 @@ do
   yarn config set enableGlobalCache true
   yarn link --all --private -r ../..
 
-  yarn build
+  yarn build --no-lint
 done

test/eslint-plugin-next/no-sync-scripts.unit.test.js

@@ -60,7 +60,7 @@ ruleTester.run('sync-scripts', rule, {
       errors: [
         {
           message:
-            'Synchronous scripts are forbidden. See: https://nextjs.org/docs/messages/no-sync-scripts.',
+            'External synchronous scripts are forbidden. See: https://nextjs.org/docs/messages/no-sync-scripts.',
           type: 'JSXOpeningElement',
         },
       ],
@@ -82,7 +82,7 @@ ruleTester.run('sync-scripts', rule, {
       errors: [
         {
           message:
-            'Synchronous scripts are forbidden. See: https://nextjs.org/docs/messages/no-sync-scripts.',
+            'External synchronous scripts are forbidden. See: https://nextjs.org/docs/messages/no-sync-scripts.',
           type: 'JSXOpeningElement',
         },
       ],

test/integration/create-next-app/index.test.js

@@ -52,6 +52,9 @@ describe('create next app', () => {
         expect(
           fs.existsSync(path.join(cwd, projectName, 'pages/index.js'))
         ).toBeTruthy()
+        expect(
+          fs.existsSync(path.join(cwd, projectName, '.eslintrc'))
+        ).toBeTruthy()
         expect(
           fs.existsSync(path.join(cwd, projectName, 'node_modules/next'))
         ).toBe(true)
@@ -121,6 +124,9 @@ describe('create next app', () => {
       expect(
         fs.existsSync(path.join(cwd, projectName, 'next-env.d.ts'))
       ).toBeTruthy()
+      expect(
+        fs.existsSync(path.join(cwd, projectName, '.eslintrc'))
+      ).toBeTruthy()
       expect(
         fs.existsSync(path.join(cwd, projectName, 'node_modules/next'))
       ).toBe(true)
@@ -138,6 +144,8 @@ describe('create next app', () => {
       ])
       expect(Object.keys(pkgJSON.devDependencies)).toEqual([
         '@types/react',
+        'eslint',
+        'eslint-config-next',
         'typescript',
       ])
     })
@@ -242,7 +250,12 @@ describe('create next app', () => {
         )
         expect(res.exitCode).toBe(0)
 
-        const files = ['package.json', 'pages/index.js', '.gitignore']
+        const files = [
+          'package.json',
+          'pages/index.js',
+          '.gitignore',
+          '.eslintrc',
+        ]
         files.forEach((file) =>
           expect(fs.existsSync(path.join(cwd, projectName, file))).toBeTruthy()
         )
@@ -309,6 +322,7 @@ describe('create next app', () => {
         'pages/index.js',
         '.gitignore',
         'node_modules/next',
+        '.eslintrc',
       ]
       files.forEach((file) =>
         expect(fs.existsSync(path.join(cwd, file))).toBeTruthy()
@@ -327,6 +341,7 @@ describe('create next app', () => {
         'pages/index.js',
         '.gitignore',
         'node_modules/next',
+        '.eslintrc',
       ]
       files.forEach((file) =>
         expect(fs.existsSync(path.join(cwd, projectName, file))).toBeTruthy()
@@ -344,6 +359,7 @@ describe('create next app', () => {
         'package.json',
         'pages/index.js',
         '.gitignore',
+        '.eslintrc',
         'package-lock.json',
         'node_modules/next',
       ]

test/integration/eslint/custom-config/next.config.js

@@ -1 +0,0 @@
-module.exports = { experimental: { eslint: true } }

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

@@ -1,10 +0,0 @@
-{
-  "name": "eslint-custom-config",
-  "version": "1.0.0",
-  "main": "index.js",
-  "license": "MIT",
-  "devDependencies": {
-    "eslint-config-next": "*",
-    "eslint": "7.23.0"
-  }
-}

test/integration/eslint/first-time-setup/next.config.js

@@ -1 +0,0 @@
-module.exports = { experimental: { eslint: true } }

test/integration/eslint/first-time-setup/package.json

@@ -1,5 +0,0 @@
-{
-  "devDependencies": {
-    "eslint-config-next": "*"
-  }
-}

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

@@ -1,5 +1,5 @@
 import { join } from 'path'
-import { runNextCommand } from 'next-test-utils'
+import { nextBuild, nextLint } from 'next-test-utils'
 import { writeFile, readFile } from 'fs-extra'
 
 import semver from 'next/dist/compiled/semver'
@@ -19,18 +19,20 @@ async function eslintVersion() {
 }
 
 describe('ESLint', () => {
-  it('should populate eslint config automatically for first time setup', async () => {
+  describe('Next Build', () => {
+    test('first time setup', async () => {
       const eslintrc = join(dirFirstTimeSetup, '.eslintrc')
       await writeFile(eslintrc, '')
 
-    const { stdout } = await runNextCommand(['build', dirFirstTimeSetup], {
+      const { stdout, stderr } = await nextBuild(dirFirstTimeSetup, [], {
         stdout: true,
+        stderr: true,
       })
-
+      const output = stdout + stderr
       const eslintrcContent = await readFile(eslintrc, 'utf8')
 
-    expect(stdout).toContain(
-      'We detected ESLint in your project and updated the .eslintrc file for you.'
+      expect(output).toContain(
+        'We detected an empty ESLint configuration file (.eslintrc) and updated it for you to include the base Next.js ESLint configuration.'
       )
       expect(eslintrcContent.trim().replace(/\s/g, '')).toMatch(
         '{"extends":"next"}'
@@ -38,31 +40,66 @@ describe('ESLint', () => {
     })
 
     test('shows warnings and errors', async () => {
-    let output = ''
-
-    const { stdout, stderr } = await runNextCommand(
-      ['build', dirCustomConfig],
-      {
+      const { stdout, stderr } = await nextBuild(dirCustomConfig, [], {
         stdout: true,
         stderr: true,
-      }
-    )
+      })
 
-    output = stdout + stderr
+      const output = stdout + stderr
       const version = await eslintVersion()
 
       if (!version || (version && semver.lt(version, '7.0.0'))) {
         expect(output).toContain(
           'Your project has an older version of ESLint installed'
         )
-      expect(output).toContain(
-        'Please upgrade to v7 or later to run ESLint during the build process'
-      )
+        expect(output).toContain('Please upgrade to v7 or later')
       } else {
-      expect(output).toContain('Failed to compile')
         expect(output).toContain(
           'Error: Comments inside children section of tag should be placed inside braces'
         )
       }
     })
+  })
+
+  describe('Next Lint', () => {
+    test('first time setup', async () => {
+      const eslintrc = join(dirFirstTimeSetup, '.eslintrc')
+      await writeFile(eslintrc, '')
+
+      const { stdout, stderr } = await nextLint(dirFirstTimeSetup, [], {
+        stdout: true,
+        stderr: true,
+      })
+      const output = stdout + stderr
+      const eslintrcContent = await readFile(eslintrc, 'utf8')
+
+      expect(output).toContain(
+        'We detected an empty ESLint configuration file (.eslintrc) and updated it for you to include the base Next.js ESLint configuration.'
+      )
+      expect(eslintrcContent.trim().replace(/\s/g, '')).toMatch(
+        '{"extends":"next"}'
+      )
+    })
+
+    test('shows warnings and errors', async () => {
+      const { stdout, stderr } = await nextLint(dirCustomConfig, [], {
+        stdout: true,
+        stderr: true,
+      })
+
+      const output = stdout + stderr
+      const version = await eslintVersion()
+
+      if (!version || (version && semver.lt(version, '7.0.0'))) {
+        expect(output).toContain(
+          'Your project has an older version of ESLint installed'
+        )
+        expect(output).toContain('Please upgrade to v7 or later')
+      } else {
+        expect(output).toContain(
+          'Error: Comments inside children section of tag should be placed inside braces'
+        )
+      }
+    })
+  })
 })

test/lib/next-test-utils.js

@@ -254,6 +254,10 @@ export function nextExportDefault(dir, opts = {}) {
   return runNextCommand(['export', dir], opts)
 }
 
+export function nextLint(dir, args = [], opts = {}) {
+  return runNextCommand(['lint', dir, ...args], opts)
+}
+
 export function nextStart(dir, port, opts = {}) {
   return runNextCommandDev(['start', '-p', port, dir], undefined, {
     ...opts,