feat: add TypeScript config file support

lib/config-generator.js

@@ -13,10 +13,11 @@ import { spawnSync } from "node:child_process";
 import { writeFile } from "node:fs/promises";
 import enquirer from "enquirer";
 import semverGreaterThanRange from "semver/ranges/gtr.js";
+import semverLessThan from "semver/functions/lt.js";
 import { isPackageTypeModule, installSyncSaveDev, fetchPeerDependencies, findPackageJson } from "./utils/npm-utils.js";
 import { getShorthandName } from "./utils/naming.js";
 import * as log from "./utils/logging.js";
-import { langQuestions, jsQuestions, mdQuestions, installationQuestions } from "./questions.js";
+import { langQuestions, jsQuestions, mdQuestions, installationQuestions, addJitiQuestion } from "./questions.js";
 
 //-----------------------------------------------------------------------------
 // Helpers
@@ -127,6 +128,16 @@ export class ConfigGenerator {
         if (this.answers.languages.includes("md")) {
             Object.assign(this.answers, await enquirer.prompt(mdQuestions));
         }
+
+        if (this.answers.configFileLanguage === "ts") {
+            const nodeVersion = process.versions.node;
+
+            // Node.js v24.3.0 removed the experimental warning from type stripping.
+            if (semverLessThan(nodeVersion, "24.3.0")) {
+                log.info("Jiti is required for Node.js <24.3.0 to read TypeScript configuration files.");
+                Object.assign(this.answers, await enquirer.prompt(addJitiQuestion));
+            }
+        }
     }
 
     /**
@@ -136,7 +147,14 @@ export class ConfigGenerator {
     async calc() {
         const isESMModule = isPackageTypeModule(this.packageJsonPath);
 
-        this.result.configFilename = isESMModule ? "eslint.config.js" : "eslint.config.mjs";
+        let configExt = isESMModule ? "js" : "mjs";
+
+        if (this.answers.configFileLanguage === "ts") {
+            configExt = isESMModule ? "ts" : "mts";
+        }
+
+        this.result.configFilename = `eslint.config.${configExt}`;
+
         this.answers.config = typeof this.answers.config === "string"
             ? { packageName: this.answers.config, type: "flat" }
             : this.answers.config;
@@ -320,6 +338,11 @@ export class ConfigGenerator {
         if (needCompatHelper) {
             this.result.devDependencies.push("@eslint/eslintrc", "@eslint/js");
         }
+
+        if (this.answers.addJiti) {
+            this.result.devDependencies.push("jiti");
+        }
+
         this.result.configContent = `${importContent}
 ${needCompatHelper ? helperContent : ""}
 export default defineConfig([\n${exportContent || "  {}\n"}]);\n`; // defaults to `[{}]` to avoid empty config warning

lib/questions.js

@@ -54,6 +54,8 @@ export const jsQuestions = [
         type: "toggle",
         name: "useTs",
         message: "Does your project use TypeScript?",
+        disabled: "No",
+        enabled: "Yes",
         initial: 0
     },
     {
@@ -66,6 +68,19 @@ export const jsQuestions = [
             { message: "Browser", name: "browser" },
             { message: "Node", name: "node" }
         ]
+    },
+    {
+        type: "select",
+        name: "configFileLanguage",
+        message: "Which language do you want your configuration file be written in?",
+        initial: 0,
+        choices: [
+            { message: "JavaScript", name: "js" },
+            { message: "TypeScript", name: "ts" }
+        ],
+        skip() {
+            return !this.state.answers.useTs;
+        }
     }
 ];
 
@@ -99,3 +114,12 @@ export const installationQuestions = [
         }
     }
 ];
+
+export const addJitiQuestion = {
+    type: "toggle",
+    name: "addJiti",
+    message: "Would you like to add Jiti as a devDependency?",
+    disabled: "No",
+    enabled: "Yes",
+    initial: 1
+};

tests/__snapshots__/cjs-configfile-js

@@ -0,0 +1,24 @@
+{
+  "configContent": "import js from "@eslint/js";
+import globals from "globals";
+import tseslint from "typescript-eslint";
+import { defineConfig } from "eslint/config";
+
+
+export default defineConfig([
+  { files: ["**/*.{js,mjs,cjs,ts,mts,cts}"], plugins: { js }, extends: ["js/recommended"], languageOptions: { globals: globals.node } },
+  { files: ["**/*.js"], languageOptions: { sourceType: "commonjs" } },
+  tseslint.configs.recommended,
+]);
+",
+  "configFilename": "eslint.config.mjs",
+  "devDependencies": [
+    "eslint",
+    "@eslint/js",
+    "globals",
+    "typescript-eslint",
+  ],
+  "installFlags": [
+    "-D",
+  ],
+}

tests/__snapshots__/cjs-configfile-ts

@@ -0,0 +1,24 @@
+{
+  "configContent": "import js from "@eslint/js";
+import globals from "globals";
+import tseslint from "typescript-eslint";
+import { defineConfig } from "eslint/config";
+
+
+export default defineConfig([
+  { files: ["**/*.{js,mjs,cjs,ts,mts,cts}"], plugins: { js }, extends: ["js/recommended"], languageOptions: { globals: globals.node } },
+  { files: ["**/*.js"], languageOptions: { sourceType: "commonjs" } },
+  tseslint.configs.recommended,
+]);
+",
+  "configFilename": "eslint.config.mts",
+  "devDependencies": [
+    "eslint",
+    "@eslint/js",
+    "globals",
+    "typescript-eslint",
+  ],
+  "installFlags": [
+    "-D",
+  ],
+}

tests/__snapshots__/cjs-configfile-ts-jiti

@@ -0,0 +1,25 @@
+{
+  "configContent": "import js from "@eslint/js";
+import globals from "globals";
+import tseslint from "typescript-eslint";
+import { defineConfig } from "eslint/config";
+
+
+export default defineConfig([
+  { files: ["**/*.{js,mjs,cjs,ts,mts,cts}"], plugins: { js }, extends: ["js/recommended"], languageOptions: { globals: globals.node } },
+  { files: ["**/*.js"], languageOptions: { sourceType: "commonjs" } },
+  tseslint.configs.recommended,
+]);
+",
+  "configFilename": "eslint.config.mts",
+  "devDependencies": [
+    "eslint",
+    "@eslint/js",
+    "globals",
+    "typescript-eslint",
+    "jiti",
+  ],
+  "installFlags": [
+    "-D",
+  ],
+}

tests/__snapshots__/esm-configfile-js

@@ -0,0 +1,23 @@
+{
+  "configContent": "import js from "@eslint/js";
+import globals from "globals";
+import tseslint from "typescript-eslint";
+import { defineConfig } from "eslint/config";
+
+
+export default defineConfig([
+  { files: ["**/*.{js,mjs,cjs,ts,mts,cts}"], plugins: { js }, extends: ["js/recommended"], languageOptions: { globals: globals.node } },
+  tseslint.configs.recommended,
+]);
+",
+  "configFilename": "eslint.config.js",
+  "devDependencies": [
+    "eslint",
+    "@eslint/js",
+    "globals",
+    "typescript-eslint",
+  ],
+  "installFlags": [
+    "-D",
+  ],
+}

tests/__snapshots__/esm-configfile-ts

@@ -0,0 +1,23 @@
+{
+  "configContent": "import js from "@eslint/js";
+import globals from "globals";
+import tseslint from "typescript-eslint";
+import { defineConfig } from "eslint/config";
+
+
+export default defineConfig([
+  { files: ["**/*.{js,mjs,cjs,ts,mts,cts}"], plugins: { js }, extends: ["js/recommended"], languageOptions: { globals: globals.node } },
+  tseslint.configs.recommended,
+]);
+",
+  "configFilename": "eslint.config.ts",
+  "devDependencies": [
+    "eslint",
+    "@eslint/js",
+    "globals",
+    "typescript-eslint",
+  ],
+  "installFlags": [
+    "-D",
+  ],
+}

tests/__snapshots__/esm-configfile-ts-jiti

@@ -0,0 +1,24 @@
+{
+  "configContent": "import js from "@eslint/js";
+import globals from "globals";
+import tseslint from "typescript-eslint";
+import { defineConfig } from "eslint/config";
+
+
+export default defineConfig([
+  { files: ["**/*.{js,mjs,cjs,ts,mts,cts}"], plugins: { js }, extends: ["js/recommended"], languageOptions: { globals: globals.node } },
+  tseslint.configs.recommended,
+]);
+",
+  "configFilename": "eslint.config.ts",
+  "devDependencies": [
+    "eslint",
+    "@eslint/js",
+    "globals",
+    "typescript-eslint",
+    "jiti",
+  ],
+  "installFlags": [
+    "-D",
+  ],
+}

tests/config-snapshots.spec.js

@@ -36,7 +36,10 @@ describe("generate config for esm projects", () => {
         { name: "esm-markdown-gfm-problems", answers: { languages: ["md"], mdType: "gfm", purpose: "problems" } },
         { name: "esm-javascript-json-problems", answers: { languages: ["javascript", "json"], purpose: "problems", moduleType: "esm", framework: "none", useTs: false, env: ["node"] } },
         { name: "esm-css-syntax", answers: { languages: ["css"], purpose: "syntax" } },
-        { name: "esm-css-problems", answers: { languages: ["css"], purpose: "problems" } }
+        { name: "esm-css-problems", answers: { languages: ["css"], purpose: "problems" } },
+        { name: "esm-configfile-js", answers: { languages: ["javascript"], purpose: "problems", moduleType: "esm", framework: "none", useTs: true, configFileLanguage: "js", env: ["node"] } },
+        { name: "esm-configfile-ts", answers: { languages: ["javascript"], purpose: "problems", moduleType: "esm", framework: "none", useTs: true, configFileLanguage: "ts", addJiti: false, env: ["node"] } },
+        { name: "esm-configfile-ts-jiti", answers: { languages: ["javascript"], purpose: "problems", moduleType: "esm", framework: "none", useTs: true, configFileLanguage: "ts", addJiti: true, env: ["node"] } }
     ];
 
     // generate all possible combinations
@@ -77,7 +80,9 @@ describe("generate config for esm projects", () => {
 
             await generator.calc();
 
-            expect(generator.result.configFilename).toBe("eslint.config.js");
+            const expectedExtension = item.answers.configFileLanguage === "ts" ? "ts" : "js";
+
+            expect(generator.result.configFilename).toBe(`eslint.config.${expectedExtension}`);
             expect(generator.packageJsonPath).toBe(join(esmProjectDir, "./package.json"));
             expect(generator.result.configContent.endsWith("\n")).toBe(true);
             expect(generator.result).toMatchFileSnapshot(`./__snapshots__/${item.name}`);
@@ -129,15 +134,20 @@ describe("generate config for cjs projects", () => {
         answers: {
             config: "eslint-config-standard"
         }
-    }];
+    },
+    { name: "cjs-configfile-js", answers: { languages: ["javascript"], purpose: "problems", moduleType: "commonjs", framework: "none", useTs: true, configFileLanguage: "js", env: ["node"] } },
+    { name: "cjs-configfile-ts", answers: { languages: ["javascript"], purpose: "problems", moduleType: "commonjs", framework: "none", useTs: true, configFileLanguage: "ts", addJiti: false, env: ["node"] } },
+    { name: "cjs-configfile-ts-jiti", answers: { languages: ["javascript"], purpose: "problems", moduleType: "commonjs", framework: "none", useTs: true, configFileLanguage: "ts", addJiti: true, env: ["node"] } }];
 
     inputs.forEach(item => {
         test(`${item.name}`, async () => {
             const generator = new ConfigGenerator({ cwd: cjsProjectDir, answers: item.answers });
 
             await generator.calc();
 
-            expect(generator.result.configFilename).toBe("eslint.config.mjs");
+            const expectedExtension = item.answers.configFileLanguage === "ts" ? "mts" : "mjs";
+
+            expect(generator.result.configFilename).toBe(`eslint.config.${expectedExtension}`);
             expect(generator.packageJsonPath).toBe(join(cjsProjectDir, "./package.json"));
             expect(generator.result.configContent.endsWith("\n")).toBe(true);
             expect(generator.result).toMatchFileSnapshot(`./__snapshots__/${item.name}`);