fix(typescript-eslint): gracefully handle invalid flat config objects in config helper (typescript-eslint#11070)

kirkwaiblinger · phaux · commit 78e7cf48907f · 2025-04-24T14:04:01.000+02:00
redo from main
diff --git a/packages/typescript-eslint/src/config-helper.ts b/packages/typescript-eslint/src/config-helper.ts
@@ -92,70 +92,123 @@ export type ConfigArray = TSESLint.FlatConfig.ConfigArray;
 export function config(
   ...configs: InfiniteDepthConfigWithExtends[]
 ): ConfigArray {
-  const flattened =
-    // @ts-expect-error -- intentionally an infinite type
-    configs.flat(Infinity) as ConfigWithExtends[];
-  return flattened.flatMap((configWithExtends, configIndex) => {
-    const { extends: extendsArr, ...config } = configWithExtends;
-    if (extendsArr == null || extendsArr.length === 0) {
-      return config;
-    }
-    const extendsArrFlattened = extendsArr.flat(
-      Infinity,
-    ) as ConfigWithExtends[];
-
-    const undefinedExtensions = extendsArrFlattened.reduce<number[]>(
-      (acc, extension, extensionIndex) => {
-        const maybeExtension = extension as
-          | TSESLint.FlatConfig.Config
-          | undefined;
-        if (maybeExtension == null) {
-          acc.push(extensionIndex);
+  return configImpl(...configs);
+}
+
+// Implementation of the config function without assuming the runtime type of
+// the input.
+function configImpl(...configs: unknown[]): ConfigArray {
+  const flattened = configs.flat(Infinity);
+  return flattened.flatMap(
+    (
+      configWithExtends,
+      configIndex,
+    ): TSESLint.FlatConfig.Config | TSESLint.FlatConfig.Config[] => {
+      if (
+        configWithExtends == null ||
+        typeof configWithExtends !== 'object' ||
+        !('extends' in configWithExtends)
+      ) {
+        // Unless the object is a config object with extends key, just forward it
+        // along to eslint.
+        return configWithExtends as TSESLint.FlatConfig.Config;
+      }
+
+      const { extends: extendsArr, ..._config } = configWithExtends;
+      const config = _config as {
+        name?: unknown;
+        extends?: unknown;
+        files?: unknown;
+        ignores?: unknown;
+      };
+
+      if (extendsArr == null) {
+        // If the extends value is nullish, just forward along the rest of the
+        // config object to eslint.
+        return config as TSESLint.FlatConfig.Config;
+      }
+
+      const name = ((): string | undefined => {
+        if ('name' in configWithExtends && configWithExtends.name != null) {
+          if (typeof configWithExtends.name !== 'string') {
+            throw new Error(
+              `tseslint.config(): Config at index ${configIndex} has a 'name' property that is not a string.`,
+            );
+          }
+          return configWithExtends.name;
         }
-        return acc;
-      },
-      [],
-    );
-    if (undefinedExtensions.length) {
-      const configName =
-        configWithExtends.name != null
-          ? `, named "${configWithExtends.name}",`
-          : ' (anonymous)';
-      const extensionIndices = undefinedExtensions.join(', ');
-      throw new Error(
-        `Your config at index ${configIndex}${configName} contains undefined` +
-          ` extensions at the following indices: ${extensionIndices}.`,
-      );
-    }
-
-    const configArray = [];
-
-    for (const extension of extendsArrFlattened) {
-      const name = [config.name, extension.name].filter(Boolean).join('__');
-      if (isPossiblyGlobalIgnores(extension)) {
-        // If it's a global ignores, then just pass it along
-        configArray.push({
-          ...extension,
-          ...(name && { name }),
-        });
-      } else {
-        configArray.push({
-          ...extension,
-          ...(config.files && { files: config.files }),
-          ...(config.ignores && { ignores: config.ignores }),
-          ...(name && { name }),
-        });
+        return undefined;
+      })();
+      const nameErrorPhrase =
+        name != null ? `, named "${name}",` : ' (anonymous)';
+
+      if (!Array.isArray(extendsArr)) {
+        throw new TypeError(
+          `tseslint.config(): Config at index ${configIndex}${nameErrorPhrase} has an 'extends' property that is not an array.`,
+        );
       }
-    }
 
-    // If the base config could form a global ignores object, then we mustn't include
-    // it in the output. Otherwise, we must add it in order for it to have effect.
-    if (!isPossiblyGlobalIgnores(config)) {
-      configArray.push(config);
-    }
+      const extendsArrFlattened = (extendsArr as unknown[]).flat(Infinity);
+
+      const nonObjectExtensions = [];
+      for (const [extensionIndex, extension] of extendsArrFlattened.entries()) {
+        // special error message to be clear we don't support eslint's stringly typed extends.
+        // https://eslint.org/docs/latest/use/configure/configuration-files#extending-configurations
+        if (typeof extension === 'string') {
+          throw new Error(
+            `tseslint.config(): Config at index ${configIndex}${nameErrorPhrase} has an 'extends' array that contains a string (${JSON.stringify(extension)}) at index ${extensionIndex}.` +
+              " This is a feature of eslint's `defineConfig()` helper and is not supported by typescript-eslint." +
+              ' Please provide a config object instead.',
+          );
+        }
+        if (extension == null || typeof extension !== 'object') {
+          nonObjectExtensions.push(extensionIndex);
+        }
+      }
+      if (nonObjectExtensions.length > 0) {
+        const extensionIndices = nonObjectExtensions.join(', ');
+        throw new Error(
+          `tseslint.config(): Config at index ${configIndex}${nameErrorPhrase} contains non-object` +
+            ` extensions at the following indices: ${extensionIndices}.`,
+        );
+      }
+
+      const configArray = [];
+
+      for (const _extension of extendsArrFlattened) {
+        const extension = _extension as {
+          name?: unknown;
+          files?: unknown;
+          ignores?: unknown;
+        };
+        const resolvedConfigName = [name, extension.name]
+          .filter(Boolean)
+          .join('__');
+        if (isPossiblyGlobalIgnores(extension)) {
+          // If it's a global ignores, then just pass it along
+          configArray.push({
+            ...extension,
+            ...(resolvedConfigName !== '' ? { name: resolvedConfigName } : {}),
+          });
+        } else {
+          configArray.push({
+            ...extension,
+            ...(config.files ? { files: config.files } : {}),
+            ...(config.ignores ? { ignores: config.ignores } : {}),
+            ...(resolvedConfigName !== '' ? { name: resolvedConfigName } : {}),
+          });
+        }
+      }
+
+      // If the base config could form a global ignores object, then we mustn't include
+      // it in the output. Otherwise, we must add it in order for it to have effect.
+      if (!isPossiblyGlobalIgnores(config)) {
+        configArray.push(config);
+      }
 
-    return configArray;
-  });
+      return configArray as ConfigArray;
+    },
+  );
 }
 
 /**
diff --git a/packages/typescript-eslint/tests/config-helper.test.ts b/packages/typescript-eslint/tests/config-helper.test.ts
@@ -1,11 +1,11 @@
 import type { TSESLint } from '@typescript-eslint/utils';
 
-import plugin from '../src/index';
+import tseslint from '../src/index';
 
 describe('config helper', () => {
   it('works without extends', () => {
     expect(
-      plugin.config({
+      tseslint.config({
         files: ['file'],
         ignores: ['ignored'],
         rules: { rule: 'error' },
@@ -21,7 +21,7 @@ describe('config helper', () => {
 
   it('flattens extended configs', () => {
     expect(
-      plugin.config({
+      tseslint.config({
         extends: [{ rules: { rule1: 'error' } }, { rules: { rule2: 'error' } }],
         rules: { rule: 'error' },
       }),
@@ -34,7 +34,7 @@ describe('config helper', () => {
 
   it('flattens extended configs with files and ignores', () => {
     expect(
-      plugin.config({
+      tseslint.config({
         extends: [{ rules: { rule1: 'error' } }, { rules: { rule2: 'error' } }],
         files: ['common-file'],
         ignores: ['common-ignored'],
@@ -63,7 +63,7 @@ describe('config helper', () => {
     const extension: TSESLint.FlatConfig.Config = { rules: { rule1: 'error' } };
 
     expect(() =>
-      plugin.config(
+      tseslint.config(
         {
           extends: [extension],
           files: ['common-file'],
@@ -81,7 +81,7 @@ describe('config helper', () => {
         },
       ),
     ).toThrow(
-      'Your config at index 1, named "my-config-2", contains undefined ' +
+      'tseslint.config(): Config at index 1, named "my-config-2", contains non-object ' +
         'extensions at the following indices: 0, 2',
     );
   });
@@ -90,7 +90,7 @@ describe('config helper', () => {
     const extension: TSESLint.FlatConfig.Config = { rules: { rule1: 'error' } };
 
     expect(() =>
-      plugin.config(
+      tseslint.config(
         {
           extends: [extension],
           files: ['common-file'],
@@ -107,14 +107,14 @@ describe('config helper', () => {
         },
       ),
     ).toThrow(
-      'Your config at index 1 (anonymous) contains undefined extensions at ' +
+      'tseslint.config(): Config at index 1 (anonymous) contains non-object extensions at ' +
         'the following indices: 0, 2',
     );
   });
 
   it('flattens extended configs with config name', () => {
     expect(
-      plugin.config({
+      tseslint.config({
         extends: [{ rules: { rule1: 'error' } }, { rules: { rule2: 'error' } }],
         files: ['common-file'],
         ignores: ['common-ignored'],
@@ -145,7 +145,7 @@ describe('config helper', () => {
 
   it('flattens extended configs with names if base config is unnamed', () => {
     expect(
-      plugin.config({
+      tseslint.config({
         extends: [
           { name: 'extension-1', rules: { rule1: 'error' } },
           { rules: { rule2: 'error' } },
@@ -176,7 +176,7 @@ describe('config helper', () => {
 
   it('merges config items names', () => {
     expect(
-      plugin.config({
+      tseslint.config({
         extends: [
           { name: 'extension-1', rules: { rule1: 'error' } },
           { rules: { rule2: 'error' } },
@@ -210,7 +210,7 @@ describe('config helper', () => {
 
   it('allows nested arrays in the config function', () => {
     expect(
-      plugin.config(
+      tseslint.config(
         { rules: { rule1: 'error' } },
         [{ rules: { rule2: 'error' } }],
         [[{ rules: { rule3: 'error' } }]],
@@ -228,7 +228,7 @@ describe('config helper', () => {
 
   it('allows nested arrays in extends', () => {
     expect(
-      plugin.config({
+      tseslint.config({
         extends: [
           { rules: { rule1: 'error' } },
           [{ rules: { rule2: 'error' } }],
@@ -249,7 +249,7 @@ describe('config helper', () => {
   });
 
   it('does not create global ignores in extends', () => {
-    const configWithIgnores = plugin.config({
+    const configWithIgnores = tseslint.config({
       extends: [{ rules: { rule1: 'error' } }, { rules: { rule2: 'error' } }],
       ignores: ['ignored'],
     });
@@ -265,7 +265,7 @@ describe('config helper', () => {
   });
 
   it('creates noop config in extends', () => {
-    const configWithMetadata = plugin.config({
+    const configWithMetadata = tseslint.config({
       extends: [{ rules: { rule1: 'error' } }, { rules: { rule2: 'error' } }],
       files: ['file'],
       ignores: ['ignored'],
@@ -297,7 +297,7 @@ describe('config helper', () => {
 
   it('does not create global ignores when extending empty configs', () => {
     expect(
-      plugin.config({
+      tseslint.config({
         extends: [{ rules: { rule1: 'error' } }, {}],
         ignores: ['ignored'],
       }),
@@ -310,10 +310,68 @@ describe('config helper', () => {
 
   it('handles name field when global-ignoring in extension', () => {
     expect(
-      plugin.config({
+      tseslint.config({
         extends: [{ ignores: ['files/**/*'], name: 'global-ignore-stuff' }],
         ignores: ['ignored'],
       }),
     ).toEqual([{ ignores: ['files/**/*'], name: 'global-ignore-stuff' }]);
   });
+
+  it('throws error when extends is not an array', () => {
+    expect(() =>
+      tseslint.config({
+        // @ts-expect-error purposely testing invalid values
+        extends: 42,
+      }),
+    ).toThrow(
+      "tseslint.config(): Config at index 0 (anonymous) has an 'extends' property that is not an array.",
+    );
+  });
+
+  it.each([undefined, null, 'not a config object', 42])(
+    'passes invalid arguments through unchanged',
+    config => {
+      expect(
+        tseslint.config(
+          // @ts-expect-error purposely testing invalid values
+          config,
+        ),
+      ).toStrictEqual([config]);
+    },
+  );
+
+  it('gives a special error message for string extends', () => {
+    expect(() =>
+      tseslint.config({
+        // @ts-expect-error purposely testing invalid values
+        extends: ['some-string'],
+      }),
+    ).toThrow(
+      'tseslint.config(): Config at index 0 (anonymous) has an \'extends\' array that contains a string ("some-string") at index 0. ' +
+        "This is a feature of eslint's `defineConfig()` helper and is not supported by typescript-eslint. " +
+        'Please provide a config object instead.',
+    );
+  });
+
+  it('strips nullish extends arrays from the config object', () => {
+    expect(
+      tseslint.config({
+        // @ts-expect-error purposely testing invalid values
+        extends: null,
+        files: ['files'],
+      }),
+    ).toEqual([{ files: ['files'] }]);
+  });
+
+  it('complains when given an object with an invalid name', () => {
+    expect(() =>
+      tseslint.config({
+        extends: [],
+        // @ts-expect-error purposely testing invalid values
+        name: 42,
+      }),
+    ).toThrow(
+      "tseslint.config(): Config at index 0 has a 'name' property that is not a string.",
+    );
+  });
 });
