From f3088266038543814b3151ab0965c6c9f961596d Mon Sep 17 00:00:00 2001
From: Jake Jarvis <jake@jarv.is>
Date: Thu, 7 Apr 2022 11:03:40 -0400
Subject: [PATCH] Add `.eslintrc.cjs` to list of supported `next lint` config
 files (#35440)
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The logic for finding a user's custom `.eslintrc` config file is missing a check for `.eslintrc.cjs`, which is listed as one of the valid filenames in ESLint's docs:

https://eslint.org/docs/user-guide/configuring/configuration-files#configuration-file-formats

...so this PR makes `next lint`'s list of possible config files consistent with ESLint's.

I stumbled upon this when trying to rename my own config file, since I've pretty much moved completely to ESM by now... but [ESLint doesn't support modules yet](https://github.com/eslint/eslint/issues/14137#issuecomment-785421664), so renaming the file to `.cjs` just keeps things cleaner (and VS Code happier). This is my first PR here so even though I've only changed a couple of lines, please let me know if I'm overlooking something! 😊

## Bug

- [ ] Related issues linked using `fixes #number`
- [x] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`
---
 packages/next/lib/eslint/runLintCheck.ts   | 2 ++
 test/integration/eslint/test/index.test.js | 1 +
 2 files changed, 3 insertions(+)

diff --git a/packages/next/lib/eslint/runLintCheck.ts b/packages/next/lib/eslint/runLintCheck.ts
index 8a50e8fab71e701..5a654d9f9e2538a 100644
--- a/packages/next/lib/eslint/runLintCheck.ts
+++ b/packages/next/lib/eslint/runLintCheck.ts
@@ -265,10 +265,12 @@ export async function runLintCheck(
 ): ReturnType<typeof lint> {
   try {
     // Find user's .eslintrc file
+    // See: https://eslint.org/docs/user-guide/configuring/configuration-files#configuration-file-formats
     const eslintrcFile =
       (await findUp(
         [
           '.eslintrc.js',
+          '.eslintrc.cjs',
           '.eslintrc.yaml',
           '.eslintrc.yml',
           '.eslintrc.json',
diff --git a/test/integration/eslint/test/index.test.js b/test/integration/eslint/test/index.test.js
index b6467f1cf5f7502..31c7be9898851f5 100644
--- a/test/integration/eslint/test/index.test.js
+++ b/test/integration/eslint/test/index.test.js
@@ -348,6 +348,7 @@ describe('ESLint', () => {
         (await findUp(
           [
             '.eslintrc.js',
+            '.eslintrc.cjs',
             '.eslintrc.yaml',
             '.eslintrc.yml',
             '.eslintrc.json',