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:
 
@@ -495,3 +498,23 @@ 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. When ESLint is executed in a runtime that supports TypeScript natively (Deno and Bun), native imports will be used. For other runtimes, the TypeScript support is powered by [`importx`](https://github.com/antfu-collective/importx), which is not automatically installed with ESLint. You need to install it as a dev dependency manually:
+
+```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.
+
+::: tip
+Due to the current limitation of Node.js loaders, top-level await in `eslint.config.ts` is only supported in Node.js v20.0 and later.
+:::

knip.jsonc

@@ -27,6 +27,9 @@
                 "@wdio/cli",
                 "rollup-plugin-node-polyfills",
 
+                // Optional dependencies for loading TypeScript configs
+                "importx",
+
                 // FIXME: not sure why is eslint-config-eslint reported as unused
                 "eslint-config-eslint"
             ]

lib/eslint/eslint.js

@@ -98,7 +98,10 @@ 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();
@@ -271,6 +274,33 @@ 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");
+}
+
+/**
+ * Check if the current runtime supports native TypeScript.
+ * In those cases, we can use `import()` to load TypeScript files.
+ * @returns {boolean} `true` if the runtime supports TypeScript, `false` if it doesn't.
+ */
+function doesRuntimeSupportsTS() {
+
+    /**
+     * We only do cheap early checks here to avoid requiring users to install `importx` in those cases,
+     * full checks are done by `importx` when it's used.
+     *
+     * It's known the Bun and Deno support TypeScript natively.
+     */
+    return Boolean(globalThis.Bun || globalThis.Deno);
+}
+
 /**
  * Load the config array from the given filename.
  * @param {string} filePath The filename to load from.
@@ -314,7 +344,18 @@ async function loadFlatConfigFile(filePath) {
         delete require.cache[filePath];
     }
 
-    const config = (await import(fileURL)).default;
+    let config;
+
+    if (isFileTS(filePath) && !doesRuntimeSupportsTS()) {
+        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",
@@ -157,6 +165,7 @@
     "semver": "^7.5.3",
     "shelljs": "^0.8.5",
     "sinon": "^11.0.0",
+    "importx": "^0.3.10",
     "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();
+                }
             });
         });