Define Extends

src/extends/eslint.d.ts

@@ -0,0 +1,4 @@
+/**
+ * Eslint extensions.
+ */
+export type EslintExtensions = 'eslint:recommended' | 'eslint:all';

src/extends/index.d.ts

@@ -0,0 +1,14 @@
+import type { LiteralUnion } from '../utility-types';
+import type { EslintExtensions } from './eslint';
+
+/**
+ * All known extensions.
+ */
+export type KnownExtensions = LiteralUnion<EslintExtensions>;
+
+/**
+ * Extending Configuration Files.
+ *
+ * @see [Extends](https://eslint.org/docs/user-guide/configuring/configuration-files#extending-configuration-files)
+ */
+export type Extends = KnownExtensions | Array<KnownExtensions>;

src/index.d.ts

@@ -1,4 +1,5 @@
 import type { Environments } from './env';
+import type { Extends } from './extends';
 import type { Overrides } from './overrides';
 import type { ParserOptions } from './parser-options';
 import type { Rules } from './rules';
@@ -16,7 +17,12 @@ export interface EslintConfig {
    * @see [Environments](https://eslint.org/docs/user-guide/configuring/language-options#specifying-environments)
    */
   env?: Environments;
-  extends?: string[];
+  /**
+   * Extending Configuration Files.
+   *
+   * @see [Extends](https://eslint.org/docs/user-guide/configuring/configuration-files#extending-configuration-files)
+   */
+  extends?: Extends;
   parser?: string;
   parserOptions?: ParserOptions;
   plugins?: string[];

src/utility-types.d.ts

@@ -0,0 +1,6 @@
+/**
+ * A literal type that supports custom further strings but preserves autocompletion in IDEs.
+ *
+ * @see [copied from issue](https://github.com/microsoft/TypeScript/issues/29729#issuecomment-471566609)
+ * */
+export type LiteralUnion<T extends U, U = string> = T | (U & { zz_IGNORE_ME?: never });