fix: support config defined in next.config.js

test/lib/next-modes/next-start.ts

@@ -4,7 +4,7 @@ import { NextInstance } from './base'
 import spawn from 'cross-spawn'
 import { Span } from 'next/dist/trace'
 import stripAnsi from 'strip-ansi'
-import { providesESLintConfiguration } from 'next-test-utils'
+import { shouldDisableLinting } from 'next-test-utils'
 
 export class NextStartInstance extends NextInstance {
   private _buildId: string
@@ -72,7 +72,7 @@ export class NextStartInstance extends NextInstance {
 
     // Disable linting if the directory provides an ESLint configuration. This
     // prevents the project's eslint configuration from affecting the test.
-    if (!providesESLintConfiguration(this.testDir)) {
+    if (shouldDisableLinting(this.testDir)) {
       buildArgs.push('--no-lint')
     }
 

test/lib/next-test-utils.ts

@@ -18,6 +18,7 @@ import fetch from 'node-fetch'
 import qs from 'querystring'
 import treeKill from 'tree-kill'
 import { once } from 'events'
+import vm from 'vm'
 
 import server from 'next/dist/server/next'
 import _pkg from 'next/package.json'
@@ -437,32 +438,61 @@ export function launchApp(
   )
 }
 
+/**
+ * Read a JavaScript file and return the exports.
+ *
+ * @param path The path to the JavaScript file.
+ * @returns The exports of the JavaScript file.
+ */
+function readJS(path: string) {
+  const data = readFileSync(path, 'utf8')
+  const script = new vm.Script(data, { filename: path })
+  const context = { module: { exports: {} } }
+  script.runInNewContext(context)
+  return context.module.exports
+}
+
 /**
  * Check if the directory provides an ESLint configuration.
  *
  * @param dir The directory to check.
  * @returns Whether the directory provides an ESLint configuration.
  */
-export function providesESLintConfiguration(dir: string) {
-  // If some of these files exist, we should not disable linting.
-  if (
-    ['.eslintrc.json', '.eslintrc.js', '.eslintrc'].some((file) =>
-      existsSync(path.join(dir, file))
-    )
-  ) {
-    return true
-  }
+export function shouldDisableLinting(dir: string) {
+  try {
+    // If the project already has a `next.config.js` file that includes an eslint
+    // config, then don't disable linting.
+    if (
+      existsSync(path.join(dir, 'next.config.js')) &&
+      'eslint' in readJS(path.join(dir, 'next.config.js'))
+    ) {
+      return false
+    }
+  } catch {}
 
-  // If the `eslintConfig` field is present in the `package.json` file, we
-  // should not disable linting.
-  if (
-    existsSync(path.join(dir, 'package.json')) &&
-    readJson(path.join(dir, 'package.json')).eslintConfig
-  ) {
-    return true
-  }
+  try {
+    // If some of these files exist, we should not disable linting.
+    if (
+      ['.eslintrc.json', '.eslintrc.js', '.eslintrc'].some((file) =>
+        existsSync(path.join(dir, file))
+      )
+    ) {
+      return false
+    }
+  } catch {}
 
-  return false
+  try {
+    // If the `eslintConfig` field is present in the `package.json` file, we
+    // should not disable linting.
+    if (
+      existsSync(path.join(dir, 'package.json')) &&
+      readJson(path.join(dir, 'package.json')).eslintConfig
+    ) {
+      return false
+    }
+  } catch {}
+
+  return true
 }
 
 export function nextBuild(
@@ -472,7 +502,7 @@ export function nextBuild(
 ) {
   // Disable linting if the directory provides an ESLint configuration. This
   // prevents the project's eslint configuration from affecting the test.
-  if (!providesESLintConfiguration(dir)) args.push('--no-lint')
+  if (shouldDisableLinting(dir)) args.push('--no-lint')
 
   return runNextCommand(['build', dir, ...args], opts)
 }