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
@@ -23,6 +23,13 @@ export type InfiniteDepthConfigWithExtends =
   | InfiniteDepthConfigWithExtends[];
 
 export interface ConfigWithExtends extends TSESLint.FlatConfig.Config {
+  /**
+   * The base path for files and ignores.
+   *
+   * Since ESLint 9.30.0
+   */
+  basePath?: string;
+
   /**
    * Allows you to "extend" a set of configs similar to `extends` from the
    * classic configs.
@@ -120,6 +127,7 @@ function configImpl(...configs: unknown[]): ConfigArray {
         extends?: unknown;
         files?: unknown;
         ignores?: unknown;
+        basePath?: unknown;
       };
 
       if (extendsArr == null) {
@@ -163,11 +171,20 @@ 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 (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 +374,46 @@ 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.",
+    );
+  });
 });