typescript-eslint · kirkwaiblinger · Oct 24, 2024 · Oct 19, 2024 · Oct 21, 2024 · Oct 22, 2024
diff --git a/packages/typescript-eslint/src/config-helper.ts b/packages/typescript-eslint/src/config-helper.ts
@@ -85,11 +85,34 @@ export interface ConfigWithExtends extends TSESLint.FlatConfig.Config {
 export function config(
   ...configs: ConfigWithExtends[]
 ): TSESLint.FlatConfig.ConfigArray {
-  return configs.flatMap(configWithExtends => {
+  return configs.flatMap((configWithExtends, configIndex) => {
     const { extends: extendsArr, ...config } = configWithExtends;
     if (extendsArr == null || extendsArr.length === 0) {
       return config;
     }
+    const undefinedExtensions = extendsArr.reduce<number[]>(
+      (acc, extension, extensionIndex) => {
+        const maybeExtension = extension as
+          | TSESLint.FlatConfig.Config
+          | undefined;
+        if (maybeExtension == null) {
+          acc.push(extensionIndex);
+        }
+        return acc;
+      },
+      [],
+    );
+    if (undefinedExtensions.length) {
+      const configName =
+        configWithExtends.name != null
+          ? `, named "${configWithExtends.name}",`
+          : ' (anonymous)';
+      const extensionIndices = undefinedExtensions.join(', ');
+      throw new Error(
+        `Your config at index ${configIndex}${configName} contains undefined` +
+          ` extensions at the following indices: ${extensionIndices}.`,
+      );
+    }
 
     return [
       ...extendsArr.map(extension => {

diff --git a/packages/typescript-eslint/tests/configs.test.ts b/packages/typescript-eslint/tests/configs.test.ts
@@ -1,3 +1,4 @@
+import type { TSESLint } from '@typescript-eslint/utils';
 import type {
   FlatConfig,
   RuleRecommendation,
@@ -368,6 +369,59 @@ describe('config helper', () => {
     ]);
   });
 
+  it('throws error containing config name when some extensions are undefined', () => {
+    const extension: TSESLint.FlatConfig.Config = { rules: { rule1: 'error' } };
+
+    expect(() =>
+      plugin.config(
+        {
+          extends: [extension],
+          files: ['common-file'],
+          ignores: ['common-ignored'],
+          name: 'my-config-1',
+          rules: { rule: 'error' },
+        },
+        {
+          // eslint-disable-next-line @typescript-eslint/no-explicit-any
+          extends: [undefined as any, extension, undefined as any],
+          files: ['common-file'],
+          ignores: ['common-ignored'],
+          name: 'my-config-2',
+          rules: { rule: 'error' },
+        },
+      ),
+    ).toThrow(
+      'Your config at index 1, named "my-config-2", contains undefined ' +
+        'extensions at the following indices: 0, 2',
+    );
+  });
+
+  it('throws error without config name when some extensions are undefined', () => {
+    const extension: TSESLint.FlatConfig.Config = { rules: { rule1: 'error' } };
+
+    expect(() =>
+      plugin.config(
+        {
+          extends: [extension],
+          files: ['common-file'],
+          ignores: ['common-ignored'],
+          name: 'my-config-1',
+          rules: { rule: 'error' },
+        },
+        {
+          // eslint-disable-next-line @typescript-eslint/no-explicit-any
+          extends: [undefined as any, extension, undefined as any],
+          files: ['common-file'],
+          ignores: ['common-ignored'],
+          rules: { rule: 'error' },
+        },
+      ),
+    ).toThrow(
+      'Your config at index 1 (anonymous) contains undefined extensions at ' +
+        'the following indices: 0, 2',
+    );
+  });
+
   it('flattens extended configs with config name', () => {
     expect(
       plugin.config({