diff --git a/index.d.ts b/index.d.ts
new file mode 100644
index 0000000..74949b0
--- /dev/null
+++ b/index.d.ts
@@ -0,0 +1,7 @@
+declare namespace isAsyncFunction {
+    type AsyncFunction = (...args: any[]) => Promise<any>
+}
+
+declare function isAsyncFunction(fn: unknown): fn is isAsyncFunction.AsyncFunction;
+
+export = isAsyncFunction;
\ No newline at end of file
diff --git a/index.js b/index.js
index dcc6ac6..23a06ee 100644
--- a/index.js
+++ b/index.js
@@ -19,8 +19,11 @@ var getAsyncFunc = function () { // eslint-disable-line consistent-return
 	} catch (e) {
 	}
 };
+
+/** @type {import('.').AsyncFunction | false} */
 var AsyncFunction;
 
+/** @type {import('.')} */
 module.exports = function isAsyncFunction(fn) {
 	if (typeof fn !== 'function') {
 		return false;
@@ -37,7 +40,8 @@ module.exports = function isAsyncFunction(fn) {
 	}
 	if (typeof AsyncFunction === 'undefined') {
 		var asyncFunc = getAsyncFunc();
-		AsyncFunction = asyncFunc ? getProto(asyncFunc) : false;
+		// eslint-disable-next-line no-extra-parens
+		AsyncFunction = asyncFunc ? /** @type {import('.').AsyncFunction} */ (getProto(asyncFunc)) : false;
 	}
 	return getProto(fn) === AsyncFunction;
 };
diff --git a/package.json b/package.json
index 0405721..1750232 100644
--- a/package.json
+++ b/package.json
@@ -15,6 +15,7 @@
 		"test:uglified": "node test/uglified",
 		"posttest": "npx npm@\">= 10.2\" audit --production",
 		"lint": "eslint --ext=js,mjs .",
+		"postlint": "tsc && attw -P",
 		"version": "auto-changelog && git add CHANGELOG.md",
 		"postversion": "auto-changelog && git add CHANGELOG.md && git commit --no-edit --amend && git tag -f \"v$(node -e \"console.log(require('./package.json').version)\")\""
 	},
@@ -46,7 +47,13 @@
 		"safe-regex-test": "^1.1.0"
 	},
 	"devDependencies": {
+		"@arethetypeswrong/cli": "^0.17.2",
 		"@ljharb/eslint-config": "^21.1.1",
+		"@ljharb/tsconfig": "^0.2.3",
+		"@types/for-each": "^0.3.3",
+		"@types/make-async-function": "^1.0.2",
+		"@types/make-generator-function": "^2.0.3",
+		"@types/tape": "^5.8.0",
 		"auto-changelog": "^2.5.0",
 		"encoding": "^0.1.13",
 		"eslint": "=8.8.0",
@@ -58,6 +65,7 @@
 		"nyc": "^10.3.2",
 		"safe-publish-latest": "^2.0.0",
 		"tape": "^5.9.0",
+		"typescript": "next",
 		"uglify-register": "^1.0.1"
 	},
 	"testling": {
diff --git a/test/index.js b/test/index.js
index ef6dce5..5de7a6d 100644
--- a/test/index.js
+++ b/test/index.js
@@ -69,6 +69,7 @@ test('returns false for generator functions', function (t) {
 
 test('returns false for non-async function with faked @@toStringTag', { skip: !hasToStringTag || asyncFuncs.length === 0 }, function (t) {
 	var asyncFunc = asyncFuncs[0];
+	/** @type {{ toString(): unknown; valueOf(): unknown; [Symbol.toStringTag]?: unknown }} */
 	var fakeAsyncFunction = {
 		toString: function () { return String(asyncFunc); },
 		valueOf: function () { return asyncFunc; }
diff --git a/test/uglified.js b/test/uglified.js
index fd82b55..e178146 100644
--- a/test/uglified.js
+++ b/test/uglified.js
@@ -1,5 +1,6 @@
 'use strict';
 
+// @ts-expect-error
 require('uglify-register/api').register({
 	exclude: [/\/node_modules\//, /\/test\//],
 	uglify: { mangle: true }
diff --git a/tsconfig.json b/tsconfig.json
new file mode 100644
index 0000000..75e48fd
--- /dev/null
+++ b/tsconfig.json
@@ -0,0 +1,10 @@
+{
+  "extends": "@ljharb/tsconfig",
+  "compilerOptions": {
+    "target": "ES2021",
+    "maxNodeModuleJsDepth": 0,
+  },
+  "exclude": [
+    "coverage",
+  ],
+}