@@ -135,7 +135,25 @@ async function lint(
135135
136136 const mod = await Promise . resolve ( require ( deps . resolved . get ( 'eslint' ) ! ) )
137137
138- const { ESLint } = mod
138+ const useFlatConfig =
139+ // If V9 config was found, use flat config, or else use legacy.
140+ eslintrcFile ?. startsWith ( 'eslint.config.' )
141+
142+ let ESLint
143+ // loadESLint is >= 8.57.0
144+ // PR https://github.com/eslint/eslint/pull/18098
145+ // Release https://github.com/eslint/eslint/releases/tag/v8.57.0
146+ if ( 'loadESLint' in mod ) {
147+ // By default, configType is `flat`. If `useFlatConfig` is false, the return value is `LegacyESLint`.
148+ // https://github.com/eslint/eslint/blob/1def4cdfab1f067c5089df8b36242cdf912b0eb6/lib/types/index.d.ts#L1609-L1613
149+ ESLint = await mod . loadESLint ( {
150+ useFlatConfig,
151+ } )
152+ } else {
153+ // eslint < 8.57.0, use legacy ESLint
154+ ESLint = mod . ESLint
155+ }
156+
139157 let eslintVersion = ESLint ?. version ?? mod . CLIEngine ?. version
140158
141159 if ( ! eslintVersion || semver . lt ( eslintVersion , '7.0.0' ) ) {
@@ -155,6 +173,23 @@ async function lint(
155173 ...eslintOptions ,
156174 }
157175
176+ if ( semver . gte ( eslintVersion , '9.0.0' ) && useFlatConfig ) {
177+ for ( const option of [
178+ 'useEslintrc' ,
179+ 'extensions' ,
180+ 'ignorePath' ,
181+ 'reportUnusedDisableDirectives' ,
182+ 'resolvePluginsRelativeTo' ,
183+ 'rulePaths' ,
184+ 'inlineConfig' ,
185+ 'maxWarnings' ,
186+ ] ) {
187+ if ( option in options ) {
188+ delete options [ option ]
189+ }
190+ }
191+ }
192+
158193 let eslint = new ESLint ( options )
159194
160195 let nextEslintPluginIsEnabled = false
@@ -163,10 +198,20 @@ async function lint(
163198 for ( const configFile of [ eslintrcFile , pkgJsonPath ] ) {
164199 if ( ! configFile ) continue
165200
166- const completeConfig : Config =
201+ const completeConfig : Config | undefined =
167202 await eslint . calculateConfigForFile ( configFile )
203+ if ( ! completeConfig ) continue
204+
205+ const plugins = completeConfig . plugins
206+
207+ const hasNextPlugin =
208+ // in ESLint < 9, `plugins` value is string[]
209+ Array . isArray ( plugins )
210+ ? plugins . includes ( '@next/next' )
211+ : // in ESLint >= 9, `plugins` value is Record<string, unknown>
212+ '@next/next' in plugins
168213
169- if ( completeConfig . plugins ?. includes ( '@next/next' ) ) {
214+ if ( hasNextPlugin ) {
170215 nextEslintPluginIsEnabled = true
171216 for ( const [ name , [ severity ] ] of Object . entries ( completeConfig . rules ) ) {
172217 if ( ! name . startsWith ( '@next/next/' ) ) {
@@ -309,6 +354,17 @@ export async function runLintCheck(
309354 const eslintrcFile =
310355 ( await findUp (
311356 [
357+ // eslint v9
358+ 'eslint.config.js' ,
359+ 'eslint.config.mjs' ,
360+ 'eslint.config.cjs' ,
361+ // TODO(jiwon): Support when it's stable.
362+ // TS extensions are experimental and requires to install another package `jiti`.
363+ // https://eslint.org/docs/latest/use/configure/configuration-files#typescript-configuration-files
364+ // 'eslint.config.ts',
365+ // 'eslint.config.mts',
366+ // 'eslint.config.cts',
367+ // eslint <= v8
312368 '.eslintrc.js' ,
313369 '.eslintrc.cjs' ,
314370 '.eslintrc.yaml' ,
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