typescript-eslint · kirkwaiblinger · Jun 30, 2025 · Jun 30, 2025 · Jun 30, 2025 · Jun 30, 2025
diff --git a/packages/typescript-eslint/src/config-helper.ts b/packages/typescript-eslint/src/config-helper.ts
@@ -120,6 +120,7 @@ function configImpl(...configs: unknown[]): ConfigArray {
         extends?: unknown;
         files?: unknown;
         ignores?: unknown;
+        basePath?: unknown;
       };
 
       if (extendsArr == null) {
@@ -163,11 +164,27 @@ function configImpl(...configs: unknown[]): ConfigArray {
         }
         if (extension == null || typeof extension !== 'object') {
           nonObjectExtensions.push(extensionIndex);
+          continue;
+        }
+
+        // https://github.com/eslint/rewrite/blob/82d07fd0e8e06780b552a41f8bcbe2a4f8741d42/packages/config-helpers/src/define-config.js#L448-L450
+        if ('basePath' in extension) {
+          throw new TypeError(
+            `tseslint.config(): Config at index ${configIndex}${nameErrorPhrase} has an 'extends' array that contains a config with a 'basePath' property at index ${extensionIndex}.` +
+              ` 'basePath' in 'extends' is not allowed.`,
+          );
+        }
+
+        if ('extends' in extension) {
+          throw new TypeError(
+            `tseslint.config(): Config at index ${configIndex}${nameErrorPhrase} has an 'extends' array that contains a config with an 'extends' property at index ${extensionIndex}.` +
+              ` Nested 'extends' is not allowed.`,
+          );
         }
       }
       if (nonObjectExtensions.length > 0) {
         const extensionIndices = nonObjectExtensions.join(', ');
-        throw new Error(
+        throw new TypeError(
           `tseslint.config(): Config at index ${configIndex}${nameErrorPhrase} contains non-object` +
             ` extensions at the following indices: ${extensionIndices}.`,
         );
@@ -181,6 +198,7 @@ function configImpl(...configs: unknown[]): ConfigArray {
           files?: unknown;
           ignores?: unknown;
         };
+
         const resolvedConfigName = [name, extension.name]
           .filter(Boolean)
           .join('__');
@@ -195,6 +213,7 @@ function configImpl(...configs: unknown[]): ConfigArray {
             ...extension,
             ...(config.files ? { files: config.files } : {}),
             ...(config.ignores ? { ignores: config.ignores } : {}),
+            ...(config.basePath ? { basePath: config.basePath } : {}),
             ...(resolvedConfigName !== '' ? { name: resolvedConfigName } : {}),
           });
         }
@@ -218,5 +237,7 @@ function configImpl(...configs: unknown[]): ConfigArray {
  * the return value can still be true.
  */
 function isPossiblyGlobalIgnores(config: object): boolean {
-  return Object.keys(config).every(key => ['name', 'ignores'].includes(key));
+  return Object.keys(config).every(key =>
+    ['name', 'ignores', 'basePath'].includes(key),
+  );
 }
diff --git a/packages/typescript-eslint/tests/config-helper.test.ts b/packages/typescript-eslint/tests/config-helper.test.ts
@@ -374,4 +374,61 @@ describe('config helper', () => {
       "tseslint.config(): Config at index 0 has a 'name' property that is not a string.",
     );
   });
+
+  it('basePath works with unextended config', () => {
+    expect(
+      tseslint.config({
+        basePath: 'base/path',
+        rules: { rule1: 'error' },
+      }),
+    ).toStrictEqual([
+      {
+        basePath: 'base/path',
+        rules: { rule1: 'error' },
+      },
+    ]);
+  });
+
+  it('basePath works with extended config', () => {
+    expect(
+      tseslint.config({
+        basePath: 'base/path',
+        extends: [{ rules: { rule1: 'error' } }, { rules: { rule2: 'error' } }],
+      }),
+    ).toStrictEqual([
+      {
+        basePath: 'base/path',
+        rules: { rule1: 'error' },
+      },
+      {
+        basePath: 'base/path',
+        rules: { rule2: 'error' },
+      },
+    ]);
+  });
+
+  it('basePath cannot be used in an extension', () => {
+    expect(() => {
+      tseslint.config({
+        extends: [{ basePath: 'base/path', rules: { rule1: 'error' } }],
+      });
+    }).toThrow(
+      "tseslint.config(): Config at index 0 (anonymous) has an 'extends' array that contains a config with a 'basePath' property at index 0. 'basePath' in 'extends' is not allowed.",
+    );
+  });
+
+  it('should error when trying to use nested extends', () => {
+    expect(() => {
+      tseslint.config({
+        extends: [
+          {
+            extends: [
+              { rules: { rule1: 'error' } },
+              { rules: { rule2: 'error' } },
+            ],
+          },
+        ],
+      });
+    }).toThrow();
+  });
 });
diff --git a/packages/utils/src/ts-eslint/Config.ts b/packages/utils/src/ts-eslint/Config.ts
@@ -255,6 +255,15 @@ export namespace FlatConfig {
   // it's not a json schema so it's nowhere near as nice to read and convert...
   // https://github.com/eslint/eslint/blob/v8.45.0/lib/config/flat-config-schema.js
   export interface Config {
+    /**
+     * The base path for files and ignores.
+     *
+     * Note that this is not permitted inside an `extends` array.
+     *
+     * Since ESLint 9.30.0
+     */
+    basePath?: string;
+
     /**
      * An array of glob patterns indicating the files that the configuration object should apply to.
      * If not specified, the configuration object applies to all files matched by any other configuration object.

diff --git a/packages/utils/tests/ts-eslint/ESLint.test.ts b/packages/utils/tests/ts-eslint/ESLint.test.ts
@@ -1,5 +1,7 @@
 import { FlatESLint, LegacyESLint } from 'eslint/use-at-your-own-risk';
 
+import type { FlatConfig } from '../../src/ts-eslint';
+
 import * as ESLint from '../../src/ts-eslint/ESLint';
 
 describe('ESLint', () => {
@@ -12,6 +14,7 @@ describe('ESLint', () => {
       });
       expect(eslint).toBeInstanceOf(FlatESLint);
     });
+
     it('legacy', () => {
       // eslint-disable-next-line @typescript-eslint/no-deprecated
       const eslint = new ESLint.LegacyESLint({
@@ -27,5 +30,11 @@ describe('ESLint', () => {
       });
       expect(eslint).toBeInstanceOf(LegacyESLint);
     });
+
+    it('config type permits basePath (type test)', () => {
+      const __config: FlatConfig.Config = {
+        basePath: 'some/path',
+      };
+    });
   });
 });