diff --git a/benchmarks/package.json b/benchmarks/package.json
deleted file mode 100644
index 6990891..0000000
--- a/benchmarks/package.json
+++ /dev/null
@@ -1 +0,0 @@
-{"type": "module"}
diff --git a/bind.d.ts b/bind.d.ts
index f89f787..ca4c0d8 100644
--- a/bind.d.ts
+++ b/bind.d.ts
@@ -1,17 +1,3 @@
 import { ArgumentArray } from './index.js';
-
-declare namespace classNames {
-	type Binding = Record<string, string>;
-}
-
-interface ClassNames {
-	(this: classNames.Binding | undefined, ...args: ArgumentArray): string;
-
-	default: ClassNames;
-}
-
-declare const classNames: ClassNames;
-
-export as namespace classNames;
-
-export = classNames;
+export type Binding = Record<string, string>;
+export default function classNames(this: Binding | undefined, ...args: ArgumentArray): string;
diff --git a/bind.js b/bind.js
index 3add1d6..462ac45 100644
--- a/bind.js
+++ b/bind.js
@@ -1,77 +1,54 @@
-/*!
-	Copyright (c) 2018 Jed Watson.
-	Licensed under the MIT License (MIT), see
-	http://jedwatson.github.io/classnames
-*/
-/* global define */
+const hasOwn = {}.hasOwnProperty;
 
-(function () {
-	'use strict';
+export default function classNames () {
+	let classes = '';
 
-	var hasOwn = {}.hasOwnProperty;
+	for (let i = 0; i < arguments.length; i++) {
+		const arg = arguments[i];
+		if (arg) {
+			classes = appendClass(classes, parseValue.call(this, arg));
+		}
+	}
 
-	function classNames () {
-		var classes = '';
+	return classes;
+}
 
-		for (var i = 0; i < arguments.length; i++) {
-			var arg = arguments[i];
-			if (arg) {
-				classes = appendClass(classes, parseValue.call(this, arg));
-			}
-		}
+function parseValue (arg) {
+	if (typeof arg === 'string' || typeof arg === 'number') {
+		return this && this[arg] || arg;
+	}
 
-		return classes;
+	if (typeof arg !== 'object') {
+		return '';
 	}
 
-	function parseValue (arg) {
-		if (typeof arg === 'string' || typeof arg === 'number') {
-			return this && this[arg] || arg;
-		}
+	if (Array.isArray(arg)) {
+		return classNames.apply(this, arg);
+	}
 
-		if (typeof arg !== 'object') {
-			return '';
-		}
+	if (arg.toString !== Object.prototype.toString && !arg.toString.toString().includes('[native code]')) {
+		return arg.toString();
+	}
 
-		if (Array.isArray(arg)) {
-			return classNames.apply(this, arg);
-		}
+	let classes = '';
 
-		if (arg.toString !== Object.prototype.toString && !arg.toString.toString().includes('[native code]')) {
-			return arg.toString();
+	for (const key in arg) {
+		if (hasOwn.call(arg, key) && arg[key]) {
+			classes = appendClass(classes, this && this[key] || key);
 		}
+	}
 
-		var classes = '';
-
-		for (var key in arg) {
-			if (hasOwn.call(arg, key) && arg[key]) {
-				classes = appendClass(classes, this && this[key] || key);
-			}
-		}
+	return classes;
+}
 
-		return classes;
+function appendClass (value, newClass) {
+	if (!newClass) {
+		return value;
 	}
 
-	function appendClass (value, newClass) {
-		if (!newClass) {
-			return value;
-		}
-	
-		if (value) {
-			return value + ' ' + newClass;
-		}
-	
-		return value + newClass;
+	if (value) {
+		return value + ' ' + newClass;
 	}
 
-	if (typeof module !== 'undefined' && module.exports) {
-		classNames.default = classNames;
-		module.exports = classNames;
-	} else if (typeof define === 'function' && typeof define.amd === 'object' && define.amd) {
-		// register as 'classnames', consistent with npm package name
-		define('classnames', [], function () {
-			return classNames;
-		});
-	} else {
-		window.classNames = classNames;
-	}
-}());
+	return value + newClass;
+}
diff --git a/dedupe.d.ts b/dedupe.d.ts
index 392248a..b27b8eb 100644
--- a/dedupe.d.ts
+++ b/dedupe.d.ts
@@ -1,5 +1,2 @@
-import classNames = require('./index.js');
-
-export as namespace classNames;
-
-export = classNames;
+import classNames from './index.js';
+export default classNames;
diff --git a/dedupe.js b/dedupe.js
index 3a7264b..7d20bdf 100644
--- a/dedupe.js
+++ b/dedupe.js
@@ -1,115 +1,94 @@
-/*!
-	Copyright (c) 2018 Jed Watson.
-	Licensed under the MIT License (MIT), see
-	http://jedwatson.github.io/classnames
-*/
-/* global define */
-
-(function () {
-	'use strict';
-
-	var classNames = (function () {
-		// don't inherit from Object so we can skip hasOwnProperty check later
-		// http://stackoverflow.com/questions/15518328/creating-js-object-with-object-createnull#answer-21079232
-		function StorageObject() {}
-		StorageObject.prototype = Object.create(null);
-
-		function _parseArray (resultSet, array) {
-			var length = array.length;
-
-			for (var i = 0; i < length; ++i) {
-				_parse(resultSet, array[i]);
-			}
-		}
+const classNames = (function () {
+	// don't inherit from Object so we can skip hasOwnProperty check later
+	// http://stackoverflow.com/questions/15518328/creating-js-object-with-object-createnull#answer-21079232
+	function StorageObject() {}
+	StorageObject.prototype = Object.create(null);
 
-		var hasOwn = {}.hasOwnProperty;
+	function _parseArray (resultSet, array) {
+		const length = array.length;
 
-		function _parseNumber (resultSet, num) {
-			resultSet[num] = true;
+		for (let i = 0; i < length; ++i) {
+			_parse(resultSet, array[i]);
 		}
+	}
 
-		function _parseObject (resultSet, object) {
-			if (object.toString !== Object.prototype.toString && !object.toString.toString().includes('[native code]')) {
-				resultSet[object.toString()] = true;
-				return;
-			}
+	const hasOwn = {}.hasOwnProperty;
 
-			for (var k in object) {
-				if (hasOwn.call(object, k)) {
-					// set value to false instead of deleting it to avoid changing object structure
-					// https://www.smashingmagazine.com/2012/11/writing-fast-memory-efficient-javascript/#de-referencing-misconceptions
-					resultSet[k] = !!object[k];
-				}
+	function _parseNumber (resultSet, num) {
+		resultSet[num] = true;
+	}
+
+	function _parseObject (resultSet, object) {
+		if (object.toString !== Object.prototype.toString && !object.toString.toString().includes('[native code]')) {
+			resultSet[object.toString()] = true;
+			return;
+		}
+
+		for (const k in object) {
+			if (hasOwn.call(object, k)) {
+				// set value to false instead of deleting it to avoid changing object structure
+				// https://www.smashingmagazine.com/2012/11/writing-fast-memory-efficient-javascript/#de-referencing-misconceptions
+				resultSet[k] = !!object[k];
 			}
 		}
+	}
 
-		var SPACE = /\s+/;
-		function _parseString (resultSet, str) {
-			var array = str.split(SPACE);
-			var length = array.length;
+	const SPACE = /\s+/;
+	function _parseString (resultSet, str) {
+		const array = str.split(SPACE);
+		const length = array.length;
 
-			for (var i = 0; i < length; ++i) {
-				resultSet[array[i]] = true;
-			}
+		for (let i = 0; i < length; ++i) {
+			resultSet[array[i]] = true;
 		}
+	}
 
-		function _parse (resultSet, arg) {
-			if (!arg) return;
-			var argType = typeof arg;
+	function _parse (resultSet, arg) {
+		if (!arg) return;
+		const argType = typeof arg;
 
-			// 'foo bar'
-			if (argType === 'string') {
-				_parseString(resultSet, arg);
+		// 'foo bar'
+		if (argType === 'string') {
+			_parseString(resultSet, arg);
 
-			// ['foo', 'bar', ...]
-			} else if (Array.isArray(arg)) {
-				_parseArray(resultSet, arg);
+		// ['foo', 'bar', ...]
+		} else if (Array.isArray(arg)) {
+			_parseArray(resultSet, arg);
 
-			// { 'foo': true, ... }
-			} else if (argType === 'object') {
-				_parseObject(resultSet, arg);
+		// { 'foo': true, ... }
+		} else if (argType === 'object') {
+			_parseObject(resultSet, arg);
 
-			// '130'
-			} else if (argType === 'number') {
-				_parseNumber(resultSet, arg);
-			}
+		// '130'
+		} else if (argType === 'number') {
+			_parseNumber(resultSet, arg);
 		}
+	}
 
-		function _classNames () {
-			// don't leak arguments
-			// https://github.com/petkaantonov/bluebird/wiki/Optimization-killers#32-leaking-arguments
-			var len = arguments.length;
-			var args = Array(len);
-			for (var i = 0; i < len; i++) {
-				args[i] = arguments[i];
-			}
+	function _classNames () {
+		// don't leak arguments
+		// https://github.com/petkaantonov/bluebird/wiki/Optimization-killers#32-leaking-arguments
+		const length = arguments.length;
+		const args = Array(length);
+		for (let i = 0; i < length; i++) {
+			args[i] = arguments[i];
+		}
 
-			var classSet = new StorageObject();
-			_parseArray(classSet, args);
+		const classSet = new StorageObject();
+		_parseArray(classSet, args);
 
-			var list = [];
+		const list = [];
 
-			for (var k in classSet) {
-				if (classSet[k]) {
-					list.push(k)
-				}
+		for (const k in classSet) {
+			if (classSet[k]) {
+				list.push(k)
 			}
-
-			return list.join(' ');
 		}
 
-		return _classNames;
-	})();
-
-	if (typeof module !== 'undefined' && module.exports) {
-		classNames.default = classNames;
-		module.exports = classNames;
-	} else if (typeof define === 'function' && typeof define.amd === 'object' && define.amd) {
-		// register as 'classnames', consistent with npm package name
-		define('classnames', [], function () {
-			return classNames;
-		});
-	} else {
-		window.classNames = classNames;
+		return list.join(' ');
 	}
-}());
+
+	return _classNames;
+})();
+
+export default classNames;
diff --git a/index.d.ts b/index.d.ts
index 65efa7d..97f52b4 100644
--- a/index.d.ts
+++ b/index.d.ts
@@ -7,26 +7,12 @@
 //   Sean Kelley <https://github.com/seansfkelley>
 //   Michal Adamczyk <https://github.com/mradamczyk>
 //   Marvin Hagemeister <https://github.com/marvinhagemeister>
-
-declare namespace classNames {
-  type Value = string | number | boolean | undefined | null;
-  type Mapping = Record<string, any>;
-  interface ArgumentArray extends Array<Argument> {}
-  interface ReadonlyArgumentArray extends ReadonlyArray<Argument> {}
-  type Argument = Value | Mapping | ArgumentArray | ReadonlyArgumentArray;
-}
-
-interface ClassNames {
-	(...args: classNames.ArgumentArray): string;
-
-	default: ClassNames;
-}
-
+export type Value = string | number | boolean | undefined | null;
+export type Mapping = Record<string, any>;
+export interface ArgumentArray extends Array<Argument> {}
+export interface ReadonlyArgumentArray extends ReadonlyArray<Argument> {}
+export type Argument = Value | Mapping | ArgumentArray | ReadonlyArgumentArray;
 /**
  * A simple JavaScript utility for conditionally joining classNames together.
  */
-declare const classNames: ClassNames;
-
-export as namespace classNames;
-
-export = classNames;
+export default function classNames(...args: ArgumentArray): string;
diff --git a/index.js b/index.js
index a42ea92..462992c 100644
--- a/index.js
+++ b/index.js
@@ -1,77 +1,54 @@
-/*!
-	Copyright (c) 2018 Jed Watson.
-	Licensed under the MIT License (MIT), see
-	http://jedwatson.github.io/classnames
-*/
-/* global define */
+const hasOwn = {}.hasOwnProperty;
 
-(function () {
-	'use strict';
+export default function classNames () {
+	let classes = '';
 
-	var hasOwn = {}.hasOwnProperty;
+	for (let i = 0; i < arguments.length; i++) {
+		const arg = arguments[i];
+		if (arg) {
+			classes = appendClass(classes, parseValue(arg));
+		}
+	}
 
-	function classNames () {
-		var classes = '';
+	return classes;
+}
 
-		for (var i = 0; i < arguments.length; i++) {
-			var arg = arguments[i];
-			if (arg) {
-				classes = appendClass(classes, parseValue(arg));
-			}
-		}
+function parseValue (arg) {
+	if (typeof arg === 'string' || typeof arg === 'number') {
+		return arg;
+	}
 
-		return classes;
+	if (typeof arg !== 'object') {
+		return '';
 	}
 
-	function parseValue (arg) {
-		if (typeof arg === 'string' || typeof arg === 'number') {
-			return arg;
-		}
+	if (Array.isArray(arg)) {
+		return classNames.apply(null, arg);
+	}
 
-		if (typeof arg !== 'object') {
-			return '';
-		}
+	if (arg.toString !== Object.prototype.toString && !arg.toString.toString().includes('[native code]')) {
+		return arg.toString();
+	}
 
-		if (Array.isArray(arg)) {
-			return classNames.apply(null, arg);
-		}
+	let classes = '';
 
-		if (arg.toString !== Object.prototype.toString && !arg.toString.toString().includes('[native code]')) {
-			return arg.toString();
+	for (const key in arg) {
+		if (hasOwn.call(arg, key) && arg[key]) {
+			classes = appendClass(classes, key);
 		}
+	}
 
-		var classes = '';
-
-		for (var key in arg) {
-			if (hasOwn.call(arg, key) && arg[key]) {
-				classes = appendClass(classes, key);
-			}
-		}
+	return classes;
+}
 
-		return classes;
+function appendClass (value, newClass) {
+	if (!newClass) {
+		return value;
 	}
 
-	function appendClass (value, newClass) {
-		if (!newClass) {
-			return value;
-		}
-	
-		if (value) {
-			return value + ' ' + newClass;
-		}
-	
-		return value + newClass;
+	if (value) {
+		return value + ' ' + newClass;
 	}
 
-	if (typeof module !== 'undefined' && module.exports) {
-		classNames.default = classNames;
-		module.exports = classNames;
-	} else if (typeof define === 'function' && typeof define.amd === 'object' && define.amd) {
-		// register as 'classnames', consistent with npm package name
-		define('classnames', [], function () {
-			return classNames;
-		});
-	} else {
-		window.classNames = classNames;
-	}
-}());
+	return value + newClass;
+}
diff --git a/package.json b/package.json
index 3f63fc5..d913bb4 100644
--- a/package.json
+++ b/package.json
@@ -8,7 +8,7 @@
     "type": "git",
     "url": "git+https://github.com/JedWatson/classnames.git"
   },
-  "type": "commonjs",
+  "type": "module",
   "main": "./index.js",
   "types": "./index.d.ts",
   "exports": {
@@ -39,7 +39,7 @@
     }
   },
   "scripts": {
-    "test": "node --test ./tests/*.mjs",
+    "test": "node --test ./tests/*.js",
     "bench": "node ./benchmarks/run.js",
     "bench-browser": "rollup --plugin commonjs,json,node-resolve ./benchmarks/runInBrowser.js --file ./benchmarks/runInBrowser.bundle.js && http-server -c-1 ./benchmarks",
     "check-types": "tsd"
diff --git a/tests/bind.mjs b/tests/bind.js
similarity index 100%
rename from tests/bind.mjs
rename to tests/bind.js
diff --git a/tests/bind.test-d.ts b/tests/bind.test-d.ts
index 7c0b07a..71878a4 100644
--- a/tests/bind.test-d.ts
+++ b/tests/bind.test-d.ts
@@ -14,7 +14,6 @@ interface IFoo {
 const ifoo: IFoo = { bar: true };
 
 // bind
-bind.default.bind({foo: 'bar'});
 const bound = bind.bind({foo: 'bar'});
 bind.bind(undefined);
 expectError(bind.bind(Symbol()));
diff --git a/tests/dedupe.mjs b/tests/dedupe.js
similarity index 100%
rename from tests/dedupe.mjs
rename to tests/dedupe.js
diff --git a/tests/dedupe.test-d.ts b/tests/dedupe.test-d.ts
index c2ef547..693562a 100644
--- a/tests/dedupe.test-d.ts
+++ b/tests/dedupe.test-d.ts
@@ -14,7 +14,6 @@ interface IFoo {
 const ifoo: IFoo = { bar: true };
 
 // dedupe
-dedupe.default('foo');
 dedupe('foo');
 dedupe(null);
 dedupe(undefined);
diff --git a/tests/index.mjs b/tests/index.js
similarity index 100%
rename from tests/index.mjs
rename to tests/index.js
diff --git a/tests/index.test-d.ts b/tests/index.test-d.ts
index c7d3138..90b092d 100644
--- a/tests/index.test-d.ts
+++ b/tests/index.test-d.ts
@@ -14,7 +14,6 @@ interface IFoo {
 const ifoo: IFoo = { bar: true };
 
 // default
-classNames.default('foo');
 classNames('foo');
 classNames(null);
 classNames(undefined);