feat: support TypeScript config using importx

docs/src/use/configure/configuration-files.md

@@ -21,6 +21,9 @@ The ESLint configuration file may be named any of the following:
 * `eslint.config.js`
 * `eslint.config.mjs`
 * `eslint.config.cjs`
+* `eslint.config.ts` (requires [TypeScript setup](#typescript-support))
+* `eslint.config.mts` (requires [TypeScript setup](#typescript-support))
+* `eslint.config.cts` (requires [TypeScript setup](#typescript-support))
 
 It should be placed in the root directory of your project and export an array of [configuration objects](#configuration-objects). Here's an example:
 
@@ -453,3 +456,19 @@ npx eslint --config some-other-file.js **/*.js
 ```
 
 In this case, ESLint does not search for `eslint.config.js` and instead uses `some-other-file.js`.
+
+## TypeScript Support
+
+::: warning
+Loading TypeScript configuration files is an experimental feature and may change in future releases.
+:::
+
+ESLint supports loading configuration files written in TypeScript optionally. The TypeScript support is powered by [`importx`](https://github.com/antfu/importx), which does not come out-of-the-box with ESLint and you will need to install it as a dev dependency first:
+
+```shell
+npm install --save-dev importx
+```
+
+Note that `importx` only strips off type annotations and does not perform type checking.
+
+When both `eslint.config.js` and `eslint.config.ts` are present in the same directory, JavaScript versions always take precedence unless provided specifically via `--config` option.

knip.jsonc

@@ -36,7 +36,10 @@
  "rollup-plugin-node-polyfills",
 
  // FIXME: not sure why is eslint-config-eslint reported as unused
- "eslint-config-eslint"
+ "eslint-config-eslint",
+
+ // Optional dependencies for loading TypeScript configs
+ "importx"
  ]
  },
  "docs": {

lib/eslint/eslint.js

@@ -97,7 +97,10 @@ const { Retrier } = require("@humanwhocodes/retry");
 const FLAT_CONFIG_FILENAMES = [
  "eslint.config.js",
  "eslint.config.mjs",
- "eslint.config.cjs"
+ "eslint.config.cjs",
+ "eslint.config.ts",
+ "eslint.config.mts",
+ "eslint.config.cts"
 ];
 const debug = require("debug")("eslint:eslint");
 const privateMembers = new WeakMap();
@@ -270,6 +273,17 @@ function findFlatConfigFile(cwd) {
  );
 }
 
+/**
+ * Check if the file is a TypeScript file.
+ * @param {string} filePath The file path to check.
+ * @returns {boolean} `true` if the file is a TypeScript file, `false` if it's not.
+ */
+function isFileTS(filePath) {
+ const fileExtension = path.extname(filePath);
+
+ return fileExtension.endsWith("ts");
+}
+
 /**
  * Load the config array from the given filename.
  * @param {string} filePath The filename to load from.
@@ -313,7 +327,18 @@ async function loadFlatConfigFile(filePath) {
  delete require.cache[filePath];
  }
 
- const config = (await import(fileURL)).default;
+ let config;
+
+ if (isFileTS(filePath)) {
+ config = await import("importx")
+ .then(r => r.import(fileURL, __filename));
+ } else {
+ config = await import(fileURL);
+ }
+
+ if (config.default) {
+ config = await config.default;
+ }
 
  importedConfigFileModificationTime.set(filePath, mtime);
 

package.json

@@ -101,6 +101,14 @@
  "strip-ansi": "^6.0.1",
  "text-table": "^0.2.0"
  },
+ "peerDependencies": {
+ "importx": "*"
+ },
+ "peerDependenciesMeta": {
+ "importx": {
+ "optional": true
+ }
+ },
  "devDependencies": {
  "@babel/core": "^7.4.3",
  "@babel/preset-env": "^7.4.3",
@@ -162,6 +170,7 @@
  "semver": "^7.5.3",
  "shelljs": "^0.8.5",
  "sinon": "^11.0.0",
+ "importx": "^0.3.5",
  "typescript": "^5.3.3",
  "vite-plugin-commonjs": "^0.10.0",
  "webpack": "^5.23.0",

tests/fixtures/config-ts/cts/eslint.config.cts

@@ -0,0 +1,15 @@
+interface FakeFlatConfigItem {
+ plugins?: Record<string, unknown>;
+ name?: string;
+ rules?: Record<string, unknown>;
+}
+
+const config: FakeFlatConfigItem[] = [
+ {
+ rules: {
+ "no-undef": "error" as string
+ }
+ },
+]
+
+module.exports = config;

tests/fixtures/config-ts/custom/eslint.custom.config.mts

@@ -0,0 +1,15 @@
+interface FakeFlatConfigItem {
+ plugins?: Record<string, unknown>;
+ name?: string;
+ rules?: Record<string, unknown>;
+}
+
+const config: FakeFlatConfigItem[] = [
+ {
+ rules: {
+ "no-undef": "error" as string
+ }
+ },
+]
+
+export default config;

tests/fixtures/config-ts/js-ts-mixed/eslint.config.js

@@ -0,0 +1,5 @@
+module.exports = {
+ rules: {
+ "no-undef": "error"
+ }
+};

tests/fixtures/config-ts/js-ts-mixed/eslint.config.ts

@@ -0,0 +1,5 @@
+module.exports = {
+ rules: {
+ "no-undef": "warn" as string
+ }
+};

tests/fixtures/config-ts/js-ts-mixed/foo.js

@@ -0,0 +1 @@
+foo;

tests/fixtures/config-ts/mts/eslint.config.mts

@@ -0,0 +1,15 @@
+interface FakeFlatConfigItem {
+ plugins?: Record<string, unknown>;
+ name?: string;
+ rules?: Record<string, unknown>;
+}
+
+const config: FakeFlatConfigItem[] = [
+ {
+ rules: {
+ "no-undef": "error" as string
+ }
+ },
+]
+
+export default config;

tests/fixtures/config-ts/top-level-await/eslint.config.mts

@@ -0,0 +1,17 @@
+
+interface FakeFlatConfigItem {
+ plugins?: Record<string, unknown>;
+ name?: string;
+ rules?: Record<string, unknown>;
+}
+
+const config: FakeFlatConfigItem[] = [
+ // Top-level await
+ await Promise.resolve({
+ rules: {
+ "no-undef": "error" as string
+ }
+ }),
+]
+
+export default config;

tests/fixtures/config-ts/ts-const-enum/enum.mts

@@ -0,0 +1,5 @@
+export const enum Level {
+ Error = 2,
+ Warn = 1,
+ Off = 0,
+}

tests/fixtures/config-ts/ts-const-enum/eslint.config.mts

@@ -0,0 +1,9 @@
+import { Level } from "./enum.mts";
+
+export default [
+ {
+ rules: {
+ "no-undef": Level.Error
+ }
+ },
+] as const;

tests/fixtures/config-ts/ts-namespace/eslint.config.mts

@@ -0,0 +1,9 @@
+import { LocalNamespace } from "./namespace.mts";
+
+export default [
+ {
+ rules: {
+ "no-undef": LocalNamespace.Level.Error
+ }
+ },
+] as const;

tests/fixtures/config-ts/ts-namespace/namespace.mts

@@ -0,0 +1,7 @@
+export namespace LocalNamespace {
+ export const enum Level {
+ Error = 2,
+ Warn = 1,
+ Off = 0,
+ }
+}

tests/fixtures/config-ts/ts/eslint.config.ts

@@ -0,0 +1,15 @@
+interface FakeFlatConfigItem {
+ plugins?: Record<string, unknown>;
+ name?: string;
+ rules?: Record<string, unknown>;
+}
+
+const config: FakeFlatConfigItem[] = [
+ {
+ rules: {
+ "no-undef": "error" as string
+ }
+ },
+]
+
+module.exports = config;

tests/lib/cli-engine/lint-result-cache.js

@@ -163,16 +163,21 @@ describe("LintResultCache", () => {
  it("contains node version during hashing", () => {
  const version = "node-=-version";
 
- sandbox.stub(process, "version").value(version);
- const NewLintResultCache = proxyquire("../../../lib/cli-engine/lint-result-cache.js", {
- "./hash": hashStub
- });
- const newLintResultCache = new NewLintResultCache(cacheFileLocation, "metadata");
+ const versionStub = sandbox.stub(process, "version").value(version);
 
- newLintResultCache.getCachedLintResults(filePath, fakeConfig);
+ try {
+ const NewLintResultCache = proxyquire("../../../lib/cli-engine/lint-result-cache.js", {
+ "./hash": hashStub
+ });
+ const newLintResultCache = new NewLintResultCache(cacheFileLocation, "metadata");
 
- assert.ok(hashStub.calledOnce);
- assert.ok(hashStub.calledWithMatch(version));
+ newLintResultCache.getCachedLintResults(filePath, fakeConfig);
+
+ assert.ok(hashStub.calledOnce);
+ assert.ok(hashStub.calledWithMatch(version));
+ } finally {
+ versionStub.restore();
+ }
  });
  });