diff --git a/.mocharc.js b/.mocharc.js
index 449f81ecd6..c8ae961f89 100644
--- a/.mocharc.js
+++ b/.mocharc.js
@@ -1,9 +1,12 @@
 const is_unit_test = process.env.UNIT_TEST;
 
 module.exports = {
-	file: is_unit_test ? [] : ['test/test.ts'],
+	file: is_unit_test ? [] : ['test/test.js'],
 	require: [
 		'sucrase/register'
+	],
+	"node-option": [
+		"experimental-modules"
 	]
 };
 
diff --git a/test/css/index.ts b/test/css/index.js
similarity index 100%
rename from test/css/index.ts
rename to test/css/index.js
diff --git a/test/custom-elements/index.ts b/test/custom-elements/index.js
similarity index 97%
rename from test/custom-elements/index.ts
rename to test/custom-elements/index.js
index 534199d035..6e7425b608 100644
--- a/test/custom-elements/index.ts
+++ b/test/custom-elements/index.js
@@ -16,7 +16,7 @@ const page = `
 
 const assert = fs.readFileSync(`${__dirname}/assert.js`, 'utf-8');
 
-describe('custom-elements', function() {
+describe('custom-elements', function () {
 	// Note: Increase the timeout in preparation for restarting Chromium due to SIGSEGV.
 	this.timeout(10000);
 	let svelte;
@@ -81,6 +81,7 @@ describe('custom-elements', function() {
 			const bundle = await rollup({
 				input: `${__dirname}/samples/${dir}/test.js`,
 				plugins: [
+					// @ts-ignore -- TODO: fix this
 					{
 						resolveId(importee) {
 							if (importee === 'svelte/internal' || importee === './internal') {
diff --git a/test/helpers.ts b/test/helpers.js
similarity index 78%
rename from test/helpers.ts
rename to test/helpers.js
index fe78276a25..5c1aaaec52 100644
--- a/test/helpers.ts
+++ b/test/helpers.js
@@ -4,11 +4,15 @@ import glob from 'tiny-glob/sync';
 import * as path from 'path';
 import * as fs from 'fs';
 import * as colors from 'kleur';
-export const assert = (assert$1 as unknown) as typeof assert$1 & { htmlEqual: (actual: string, expected: string, message?: string) => void, htmlEqualWithOptions: (actual: string, expected: string, options: { preserveComments?: boolean, withoutNormalizeHtml?: boolean }, message?: string) => void };
+
+/**
+ * @type {typeof assert$1 & { htmlEqual: (actual: string, expected: string, message?: string) => void, htmlEqualWithOptions: (actual: string, expected: string, options: { preserveComments?: boolean, withoutNormalizeHtml?: boolean }, message?: string) => void }}
+ */
+export const assert = /** @type {any} */ (assert$1);
 
 // for coverage purposes, we need to test source files,
 // but for sanity purposes, we need to test dist files
-export function loadSvelte(test: boolean = false) {
+export function loadSvelte(test = false) {
 	process.env.TEST = test ? 'true' : '';
 
 	const resolved = require.resolve('../compiler.js');
@@ -68,7 +72,7 @@ for (const key of Object.getOwnPropertyNames(global)) {
 }
 
 // implement mock scroll
-window.scrollTo = function(pageXOffset, pageYOffset) {
+window.scrollTo = function (pageXOffset, pageYOffset) {
 	window.pageXOffset = pageXOffset;
 	window.pageYOffset = pageYOffset;
 };
@@ -140,7 +144,14 @@ function cleanChildren(node) {
 	}
 }
 
-export function normalizeHtml(window, html, { removeDataSvelte = false, preserveComments = false }: { removeDataSvelte?: boolean, preserveComments?: boolean }) {
+/**
+ *
+ * @param {Window} window
+ * @param {string} html
+ * @param {{ removeDataSvelte?: boolean, preserveComments?: boolean }} param2
+ * @returns
+ */
+export function normalizeHtml(window, html, { removeDataSvelte = false, preserveComments = false }) {
 	try {
 		const node = window.document.createElement('div');
 		node.innerHTML = html
@@ -155,11 +166,18 @@ export function normalizeHtml(window, html, { removeDataSvelte = false, preserve
 	}
 }
 
-export function normalizeNewline(html: string) {
+/**
+ * @param {string} html
+ * @returns {string}
+ */
+export function normalizeNewline(html) {
 	return html.replace(/\r\n/g, '\n');
 }
 
-export function setupHtmlEqual(options: { removeDataSvelte?: boolean } = {}) {
+/**
+ * @param {{ removeDataSvelte?: boolean }} options
+ */
+export function setupHtmlEqual(options = {}) {
 	const window = env();
 
 	// eslint-disable-next-line no-import-assign
@@ -170,8 +188,15 @@ export function setupHtmlEqual(options: { removeDataSvelte?: boolean } = {}) {
 			message
 		);
 	};
-	// eslint-disable-next-line no-import-assign
-	assert.htmlEqualWithOptions = (actual: string, expected: string, { preserveComments, withoutNormalizeHtml }: { preserveComments?: boolean, withoutNormalizeHtml?: boolean }, message?: string) => {
+
+	/**
+	 *
+	 * @param {string} actual
+	 * @param {string} expected
+	 * @param {{ preserveComments?: boolean, withoutNormalizeHtml?: boolean }} param2
+	 * @param {string?} message
+	 */
+	assert.htmlEqualWithOptions = (actual, expected, { preserveComments, withoutNormalizeHtml }, message) => {
 		assert.deepEqual(
 			withoutNormalizeHtml
 				? normalizeNewline(actual).replace(/(\sdata-svelte-h="[^"]+")/g, options.removeDataSvelte ? '' : '$1')
@@ -252,7 +277,8 @@ const original_set_timeout = global.setTimeout;
 export function useFakeTimers() {
 	const callbacks = [];
 
-	global.setTimeout = function(fn) {
+	// @ts-ignore
+	global.setTimeout = function (fn) {
 		callbacks.push(fn);
 	};
 
@@ -289,7 +315,14 @@ export function prettyPrintPuppeteerAssertionError(message) {
 	}
 }
 
-export async function retryAsync<T>(fn: () => Promise<T>, maxAttempts: number = 3, interval: number = 1000): Promise<T> {
+/**
+ *
+ * @param {() => Promise<import ('puppeteer').Browser>} fn
+ * @param {number} maxAttempts
+ * @param {number} interval
+ * @returns {Promise<import ('puppeteer').Browser>}
+ */
+export async function retryAsync(fn, maxAttempts = 3, interval = 1000) {
 	let attempts = 0;
 	while (attempts <= maxAttempts) {
 		try {
@@ -301,8 +334,15 @@ export async function retryAsync<T>(fn: () => Promise<T>, maxAttempts: number =
 	}
 }
 
-// NOTE: Chromium may exit with SIGSEGV, so retry in that case
-export async function executeBrowserTest<T>(browser, launchPuppeteer: () => Promise<T>, additionalAssertion: () => void, onError: (err: Error) => void) {
+/**
+ * NOTE: Chromium may exit with SIGSEGV, so retry in that case
+ * @param {import ('puppeteer').Browser} browser
+ * @param {() => Promise<import ('puppeteer').Browser>} launchPuppeteer
+ * @param {() => void} additionalAssertion
+ * @param {(err: Error) => void} onError
+ * @returns {Promise<import ('puppeteer').Browser>}
+ */
+export async function executeBrowserTest(browser, launchPuppeteer, additionalAssertion, onError) {
 	let count = 0;
 	do {
 		count++;
@@ -310,6 +350,7 @@ export async function executeBrowserTest<T>(browser, launchPuppeteer: () => Prom
 			const page = await browser.newPage();
 
 			page.on('console', (type) => {
+				// @ts-ignore -- TODO: Fix type
 				console[type._type](type._text);
 			});
 
@@ -318,7 +359,10 @@ export async function executeBrowserTest<T>(browser, launchPuppeteer: () => Prom
 				console.error(error);
 			});
 			await page.goto('http://localhost:6789');
-			const result = await page.evaluate(() => test(document.querySelector('main')));
+			const result = await page.evaluate(() => {
+				// @ts-ignore -- It runs in browser context.
+				return test(document.querySelector('main'))
+			});
 			if (result) console.log(result);
 			additionalAssertion();
 			await page.close();
diff --git a/test/hydration/index.ts b/test/hydration/index.js
similarity index 97%
rename from test/hydration/index.ts
rename to test/hydration/index.js
index 6071f25434..0d5bab0034 100644
--- a/test/hydration/index.ts
+++ b/test/hydration/index.js
@@ -19,7 +19,7 @@ describe('hydration', () => {
 	before(() => {
 		const svelte = loadSvelte();
 
-		require.extensions['.svelte'] = function(module, filename) {
+		require.extensions['.svelte'] = function (module, filename) {
 			const options = Object.assign(
 				{
 					filename,
@@ -135,6 +135,6 @@ describe('hydration', () => {
 	}
 
 	fs.readdirSync(`${__dirname}/samples`).forEach(dir => {
-		runTest(dir, null);
+		runTest(dir);
 	});
 });
diff --git a/test/js/index.ts b/test/js/index.js
similarity index 100%
rename from test/js/index.ts
rename to test/js/index.js
diff --git a/test/motion/index.ts b/test/motion/index.js
similarity index 100%
rename from test/motion/index.ts
rename to test/motion/index.js
diff --git a/test/parser/index.ts b/test/parser/index.js
similarity index 100%
rename from test/parser/index.ts
rename to test/parser/index.js
diff --git a/test/parser/update.ts b/test/parser/update.js
similarity index 100%
rename from test/parser/update.ts
rename to test/parser/update.js
diff --git a/test/preprocess/index.ts b/test/preprocess/index.js
similarity index 100%
rename from test/preprocess/index.ts
rename to test/preprocess/index.js
diff --git a/test/runtime-puppeteer/index.ts b/test/runtime-puppeteer/index.js
similarity index 100%
rename from test/runtime-puppeteer/index.ts
rename to test/runtime-puppeteer/index.js
diff --git a/test/runtime/index.ts b/test/runtime/index.js
similarity index 100%
rename from test/runtime/index.ts
rename to test/runtime/index.js
diff --git a/test/server-side-rendering/index.ts b/test/server-side-rendering/index.js
similarity index 96%
rename from test/server-side-rendering/index.ts
rename to test/server-side-rendering/index.js
index fbbc1a22c5..84decea88c 100644
--- a/test/server-side-rendering/index.ts
+++ b/test/server-side-rendering/index.js
@@ -67,7 +67,7 @@ describe('ssr', () => {
 					format: 'cjs'
 				};
 
-				require('../../register')(compileOptions);
+				require('../../register.js')(compileOptions);
 
 				const Component = require(`${dir}/main.svelte`).default;
 
@@ -174,7 +174,7 @@ describe('ssr', () => {
 				glob('**/*.svelte', { cwd }).forEach(file => {
 					if (file[0] === '_') return;
 
-					const dir  = `${cwd}/_output/ssr`;
+					const dir = `${cwd}/_output/ssr`;
 					const out = `${dir}/${file.replace(/\.svelte$/, '.js')}`;
 
 					if (fs.existsSync(out)) {
@@ -208,12 +208,12 @@ describe('ssr', () => {
 
 					if (config.ssrHtml) {
 						assert.htmlEqualWithOptions(html, config.ssrHtml, {
-							preserveComments: compileOptions.preserveComments, 
+							preserveComments: compileOptions.preserveComments,
 							withoutNormalizeHtml: config.withoutNormalizeHtml
 						});
 					} else if (config.html) {
 						assert.htmlEqualWithOptions(html, config.html, {
-							preserveComments: compileOptions.preserveComments, 
+							preserveComments: compileOptions.preserveComments,
 							withoutNormalizeHtml: config.withoutNormalizeHtml
 						});
 					}
diff --git a/test/setup.js b/test/setup.js
index 74250c10eb..9a2ecb9b3f 100644
--- a/test/setup.js
+++ b/test/setup.js
@@ -2,15 +2,17 @@ const fs = require('fs');
 
 require('source-map-support').install();
 
-process.env.TEST = true;
+process.env.TEST = 'true';
 
-require.extensions['.js'] = function(module, filename) {
+require.extensions['.js'] = function (module, filename) {
 	const exports = [];
 
 	let code = fs.readFileSync(filename, 'utf-8')
-		.replace(/^import \* as (\w+) from ['"]([^'"]+)['"];?/gm, 'var $1 = require("$2");')
-		.replace(/^import (\w+) from ['"]([^'"]+)['"];?/gm, 'var {default: $1} = require("$2");')
+		.replace(/^import \* as (\S+) from ['"]([^'"]+)['"];?/gm, 'var $1 = require("$2");')
 		.replace(/^import {([^}]+)} from ['"](.+)['"];?/gm, 'var {$1} = require("$2");')
+		.replace(/^import (\S+),\s+?{([^}]+)} from ['"](.+)['"];?/gm, 'var {default: $1, $2} = require("$3");')
+		.replace(/^import (\S+) from ['"]([./][^'"]+)['"];?/gm, 'var {default: $1} = require("$2");')
+		.replace(/^import (\S+) from ['"]([^'"]+)['"];?/gm, 'var $1 = require("$2");')
 		.replace(/^export default /gm, 'exports.default = ')
 		.replace(/^export (const|let|var|class|function|async\s+function) (\w+)/gm, (match, type, name) => {
 			exports.push(name);
diff --git a/test/sourcemaps/helpers.ts b/test/sourcemaps/helpers.js
similarity index 68%
rename from test/sourcemaps/helpers.ts
rename to test/sourcemaps/helpers.js
index f546566a95..689c3fbf1b 100644
--- a/test/sourcemaps/helpers.ts
+++ b/test/sourcemaps/helpers.js
@@ -2,20 +2,19 @@ import * as assert from 'assert';
 import { getLocator } from 'locate-character';
 import MagicString, { Bundle } from 'magic-string';
 
-type AssertMappedParameters = {
-	code: string;
-	filename?: string;
-	input: string | ReturnType<typeof getLocator>;
-	input_code?: string;
-	preprocessed: any;
-};
+/**
+ * @typedef {{ code: string; filename?: string; input: string | ReturnType<typeof getLocator>; input_code?: string; preprocessed: any; }} AssertMappedParameters
+ */
 
+/**
+ * @param {AssertMappedParameters} param
+ */
 export function assert_mapped(
-	{ code, filename, input, input_code, preprocessed }: AssertMappedParameters
+	{ code, filename, input, input_code, preprocessed }
 ) {
 	const locate_input = typeof input === 'function' ? input : getLocator(input);
 	if (filename === undefined) filename = 'input.svelte';
-	if (input_code === undefined) input_code = code;	
+	if (input_code === undefined) input_code = code;
 
 	const source_loc = locate_input(input_code);
 	assert.notEqual(
@@ -43,14 +42,15 @@ export function assert_mapped(
 	);
 }
 
-type AssertNotMappedParameters = {
-	code: string;
-	filename?: string;
-	preprocessed: any;
-};
+/**
+ * @typedef {{ code: string; filename?: string; preprocessed: any; }} AssertNotMappedParameters
+ */
 
+/**
+ * @param {AssertNotMappedParameters} param
+ */
 export function assert_not_mapped(
-	{ code, filename, preprocessed }: AssertNotMappedParameters
+	{ code, filename, preprocessed }
 ) {
 	if (filename === undefined) filename = 'input.svelte';
 
@@ -73,9 +73,14 @@ export function assert_not_mapped(
 	);
 }
 
+/**
+ * @param {string} code
+ * @param {ReturnType<typeof getLocator>} locate
+ * @param {string} filename
+ */
 export function assert_not_located(
-	code: string,
-	locate: ReturnType<typeof getLocator>,
+	code,
+	locate,
 	filename = 'input.svelte'
 ) {
 	assert.equal(
@@ -85,8 +90,14 @@ export function assert_not_located(
 	);
 }
 
+/**
+ * @param {Array<{ code: string | MagicString, filename: string }>} inputs
+ * @param {string} filename
+ * @param {string} separator
+ * @returns
+ */
 export function magic_string_bundle(
-	inputs: Array<{ code: string | MagicString, filename: string }>,
+	inputs,
 	filename = 'bundle.js',
 	separator = '\n'
 ) {
@@ -107,7 +118,12 @@ export function magic_string_bundle(
 	};
 }
 
-export function magic_string_preprocessor_result(filename: string, src: MagicString) {
+/**
+ * @param {string} filename
+ * @param {MagicString} src
+ * @returns
+ */
+export function magic_string_preprocessor_result(filename, src) {
 	return {
 		code: src.toString(),
 		map: src.generateMap({
@@ -118,7 +134,12 @@ export function magic_string_preprocessor_result(filename: string, src: MagicStr
 	};
 }
 
-export function magic_string_replace_all(src: MagicString, search: string, replace: string) {
+/**
+ * @param {MagicString} src
+ * @param {string} search
+ * @param {string} replace
+ */
+export function magic_string_replace_all(src, search, replace) {
 	let idx = src.original.indexOf(search);
 	if (idx == -1) throw new Error('search not found in src');
 	do {
diff --git a/test/sourcemaps/index.ts b/test/sourcemaps/index.js
similarity index 100%
rename from test/sourcemaps/index.ts
rename to test/sourcemaps/index.js
diff --git a/test/stats/index.ts b/test/stats/index.js
similarity index 100%
rename from test/stats/index.ts
rename to test/stats/index.js
diff --git a/test/store/index.ts b/test/store/index.js
similarity index 98%
rename from test/store/index.ts
rename to test/store/index.js
index 920cabdc39..29d233495c 100644
--- a/test/store/index.ts
+++ b/test/store/index.js
@@ -101,6 +101,7 @@ describe('store', () => {
 	describe('readable', () => {
 		it('creates a readable store', () => {
 			let running;
+			/** @type {import('svelte/store').Subscriber<unknown>} */
 			let tick;
 
 			const store = readable(undefined, set => {
@@ -163,11 +164,12 @@ describe('store', () => {
 		});
 	});
 
+	/** @type {any} */
 	const fake_observable = {
 		subscribe(fn) {
 			fn(42);
 			return {
-				unsubscribe: () => {}
+				unsubscribe: () => { }
 			};
 		}
 	};
@@ -290,7 +292,7 @@ describe('store', () => {
 		});
 
 		it('derived dependency does not update and shared ancestor updates', () => {
-			const root = writable({ a: 0, b:0 });
+			const root = writable({ a: 0, b: 0 });
 			const values = [];
 
 			const a = derived(root, $root => {
@@ -451,6 +453,7 @@ describe('store', () => {
 			assert.equal(get(readableStore), 2);
 			assert.equal(get(readableStore), get(writableStore));
 
+			// @ts-ignore
 			assert.throws(() => readableStore.set(3));
 		});
 
diff --git a/test/test.ts b/test/test.js
similarity index 79%
rename from test/test.ts
rename to test/test.js
index 181e9351c1..e33e4124a7 100644
--- a/test/test.ts
+++ b/test/test.js
@@ -1,14 +1,14 @@
 const glob = require('tiny-glob/sync.js');
 
-require('./setup');
+require('./setup.js');
 
 // bind internal to jsdom
-require('./helpers.ts');
+require('./helpers.js');
 require('../internal');
 
 console.clear();
 
-const test_folders = glob('*/index.ts', { cwd: 'test' });
+const test_folders = glob('*/index.js', { cwd: 'test' });
 const solo_folders = test_folders.filter(folder => /\.solo/.test(folder));
 
 if (solo_folders.length) {
diff --git a/test/types/actions.js b/test/types/actions.js
new file mode 100644
index 0000000000..f31fa55cb9
--- /dev/null
+++ b/test/types/actions.js
@@ -0,0 +1,221 @@
+const nullAsAny = /** @type {any} */(null);
+
+// ---------------- Action
+
+/**
+ * @type {import ('$runtime/action').Action<HTMLAnchorElement>}
+ */
+const href = (node) => {
+	node.href = '';
+	// @ts-expect-error
+	node.href = 1;
+};
+href;
+
+/**
+ * @type {import ('$runtime/action').Action<HTMLAnchorElement, boolean>}
+ */
+const required = (node, param) => {
+	node;
+	param;
+};
+required(nullAsAny, true);
+// @ts-expect-error (only in strict mode) boolean missing
+required(nullAsAny);
+// @ts-expect-error no boolean
+required(nullAsAny, 'string');
+
+/**
+ * @type {import ('$runtime/action').Action<HTMLAnchorElement, boolean>}
+ */
+const required1 = (node, param) => {
+	node;
+	param;
+	return {
+		update: (p) => p === true,
+		destroy: () => { }
+	};
+};
+required1;
+
+/**
+ * @type {import ('$runtime/action').Action<HTMLElement, boolean>}
+ */
+const required2 = (node) => {
+	node;
+};
+required2;
+
+/**
+ * @type {import ('$runtime/action').Action<HTMLElement, boolean>}
+ */
+const required3 = (node, param) => {
+	node;
+	param;
+	return {
+		// @ts-expect-error comparison always resolves to false
+		update: (p) => p === 'd',
+		destroy: () => { }
+	};
+};
+required3;
+
+/**
+ * @type {import ('$runtime/action').Action<HTMLElement, boolean | undefined>}
+ */
+const optional = (node, param) => {
+	node;
+	param;
+};
+optional(nullAsAny, true);
+optional(nullAsAny);
+// @ts-expect-error no boolean
+optional(nullAsAny, 'string');
+
+/**
+ * @type {import ('$runtime/action').Action<HTMLElement, boolean | undefined>}
+ */
+const optional1 = (node, param) => {
+	node;
+	param;
+	return {
+		update: (p) => p === true,
+		destroy: () => { }
+	};
+};
+optional1;
+
+/**
+ * @type {import ('$runtime/action').Action<HTMLElement, boolean | undefined>}
+ */
+const optional2 = (node) => {
+	node;
+};
+optional2;
+
+/**
+ * @type {import ('$runtime/action').Action<HTMLElement, boolean | undefined>}
+ */
+const optional3 = (node, param) => {
+	node;
+	param;
+};
+optional3;
+
+/**
+ * @type {import ('$runtime/action').Action<HTMLElement, boolean | undefined>}
+ */
+const optional4 = (node, param) => {
+	node;
+	param;
+	return {
+		// @ts-expect-error comparison always resolves to false
+		update: (p) => p === 'd',
+		destroy: () => { }
+	};
+};
+optional4;
+
+/**
+ * @type {import ('$runtime/action').Action<HTMLElement, never>}
+ */
+const no = (node) => {
+	node;
+};
+// @ts-expect-error second param
+no(nullAsAny, true);
+no(nullAsAny);
+// @ts-expect-error second param
+no(nullAsAny, 'string');
+
+/**
+ * @type {import ('$runtime/action').Action<HTMLElement, never>}
+ */
+const no1 = (node) => {
+	node;
+	return {
+		destroy: () => { }
+	};
+};
+no1;
+
+/**
+ * @type {import ('$runtime/action').Action<HTMLElement, never>}
+ */
+// @ts-expect-error param given
+const no2 = (node, param) => { };
+no2;
+
+/**
+ * @type {import ('$runtime/action').Action<HTMLElement, never>}
+ */
+// @ts-expect-error param given
+const no3 = (node, param) => { };
+no3;
+
+/**
+ * @type {import ('$runtime/action').Action<HTMLElement, never>}
+ */
+const no4 = (_node) => {
+	return {
+		// @ts-expect-error update method given
+		update: () => { },
+		destroy: () => { }
+	};
+};
+no4;
+
+// ---------------- ActionReturn
+
+/**
+ * @type {import ('$runtime/action').ActionReturn<string>}
+ */
+const requiredReturn = {
+	update: (p) => p.toString()
+};
+requiredReturn;
+
+/**
+ * @type {import ('$runtime/action').ActionReturn<boolean | undefined>}
+ */
+const optionalReturn = {
+	update: (p) => {
+		p === true;
+		// @ts-expect-error could be undefined
+		p.toString();
+	}
+};
+optionalReturn;
+
+/**
+ * @type {import ('$runtime/action').ActionReturn}
+ */
+const invalidProperty = {
+	// @ts-expect-error invalid property
+	invalid: () => { }
+};
+invalidProperty;
+
+/**
+ * @typedef {import ('$runtime/action').ActionReturn<never, { a: string; }>['$$_attributes']} Attributes
+ */
+
+/**
+ * @type {Attributes}
+ */
+const attributes = { a: 'a' };
+attributes;
+
+/**
+ * @type {Attributes}
+ */
+// @ts-expect-error wrong type
+const invalidAttributes1 = { a: 1 };
+invalidAttributes1;
+
+/**
+ * @type {Attributes}
+ */
+// @ts-expect-error missing prop
+const invalidAttributes2 = {};
+invalidAttributes2;
diff --git a/test/types/actions.ts b/test/types/actions.ts
deleted file mode 100644
index 3d56aa3605..0000000000
--- a/test/types/actions.ts
+++ /dev/null
@@ -1,129 +0,0 @@
-import type { Action, ActionReturn } from '$runtime/action';
-
-// ---------------- Action
-
-const href: Action<HTMLAnchorElement> = (node) => {
-	node.href = '';
-	// @ts-expect-error
-	node.href = 1;
-};
-href;
-
-const required: Action<HTMLElement, boolean> = (_node, _param) => {};
-required(null as any, true);
-// @ts-expect-error (only in strict mode) boolean missing
-required(null as any);
-// @ts-expect-error no boolean
-required(null as any, 'string');
-
-const required1: Action<HTMLElement, boolean> = (_node, _param) => {
-	return {
-		update: (p) => p === true,
-		destroy: () => {}
-	};
-};
-required1;
-
-const required2: Action<HTMLElement, boolean> = (_node) => {};
-required2;
-
-const required3: Action<HTMLElement, boolean> = (_node, _param) => {
-	return {
-		// @ts-expect-error comparison always resolves to false
-		update: (p) => p === 'd',
-		destroy: () => {}
-	};
-};
-required3;
-
-const optional: Action<HTMLElement, boolean | undefined> = (_node, _param?) => {};
-optional(null as any, true);
-optional(null as any);
-// @ts-expect-error no boolean
-optional(null as any, 'string');
-
-const optional1: Action<HTMLElement, boolean | undefined> = (_node, _param?) => {
-	return {
-		update: (p) => p === true,
-		destroy: () => {}
-	};
-};
-optional1;
-
-const optional2: Action<HTMLElement, boolean | undefined> = (_node) => {};
-optional2;
-
-const optional3: Action<HTMLElement, boolean | undefined> = (_node, _param) => {};
-optional3;
-
-const optional4: Action<HTMLElement, boolean | undefined> = (_node, _param?) => {
-	return {
-		// @ts-expect-error comparison always resolves to false
-		update: (p) => p === 'd',
-		destroy: () => {}
-	};
-};
-optional4;
-
-const no: Action<HTMLElement, never> = (_node) => {};
-// @ts-expect-error second param
-no(null as any, true);
-no(null as any);
-// @ts-expect-error second param
-no(null as any, 'string');
-
-const no1: Action<HTMLElement, never> = (_node) => {
-	return {
-		destroy: () => {}
-	};
-};
-no1;
-
-// @ts-expect-error param given
-const no2: Action<HTMLElement, never> = (_node, _param?) => {};
-no2;
-
-// @ts-expect-error param given
-const no3: Action<HTMLElement, never> = (_node, _param) => {};
-no3;
-
-// @ts-expect-error update method given
-const no4: Action<HTMLElement, never> = (_node) => {
-	return {
-		update: () => {},
-		destroy: () => {}
-	};
-};
-no4;
-
-// ---------------- ActionReturn
-
-const requiredReturn: ActionReturn<string> = {
-	update: (p) => p.toString()
-};
-requiredReturn;
-
-const optionalReturn: ActionReturn<boolean | undefined> = {
-	update: (p) => {
-		p === true;
-		// @ts-expect-error could be undefined
-		p.toString();
-	}
-};
-optionalReturn;
-
-const invalidProperty: ActionReturn = {
-	// @ts-expect-error invalid property
-	invalid: () => {}
-};
-invalidProperty;
-
-type Attributes = ActionReturn<never, { a: string; }>['$$_attributes'];
-const attributes: Attributes = { a: 'a' };
-attributes;
-// @ts-expect-error wrong type
-const invalidAttributes1: Attributes = { a: 1 };
-invalidAttributes1;
-// @ts-expect-error missing prop
-const invalidAttributes2: Attributes = {};
-invalidAttributes2;
diff --git a/test/types/create-event-dispatcher.ts b/test/types/create-event-dispatcher.js
similarity index 100%
rename from test/types/create-event-dispatcher.ts
rename to test/types/create-event-dispatcher.js
diff --git a/test/types/tsconfig.json b/test/types/tsconfig.json
index 108ed2a2b2..f4e007ba51 100644
--- a/test/types/tsconfig.json
+++ b/test/types/tsconfig.json
@@ -1,7 +1,7 @@
 {
 	"extends": "../../tsconfig.json",
 	"compilerOptions": {
-        "rootDir": "../..",
+		"rootDir": "../..",
 		"baseUrl": "../../",
 		"paths": {
 			"$runtime/*": ["src/runtime/*"]
@@ -11,6 +11,8 @@
 		"noFallthroughCasesInSwitch": true,
 		"noImplicitReturns": true,
 		"strict": true,
+		"allowJs": true,
+		"checkJs": true
 	},
 	"include": ["."]
-}
\ No newline at end of file
+}
diff --git a/test/utils/index.ts b/test/utils/index.js
similarity index 100%
rename from test/utils/index.ts
rename to test/utils/index.js
diff --git a/test/validator/index.ts b/test/validator/index.js
similarity index 100%
rename from test/validator/index.ts
rename to test/validator/index.js
diff --git a/test/vars/index.ts b/test/vars/index.js
similarity index 100%
rename from test/vars/index.ts
rename to test/vars/index.js