fix: allow using typescript in `customElement.extend` option (#16001)

Closes #15372

---------

Co-authored-by: Dominik G. <dominik.goepel@gmx.de>

.changeset/olive-pandas-trade.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: allow using typescript in `customElement.extend` option

documentation/docs/07-misc/04-custom-elements.md

@@ -114,6 +114,8 @@ When constructing a custom element, you can tailor several aspects by defining `
 ...
 ```
 
+> [!NOTE] While Typescript is supported in the `extend` function, it is subject to limitations: you need to set `lang="ts"` on one of the scripts AND you can only use [erasable syntax](https://www.typescriptlang.org/tsconfig/#erasableSyntaxOnly) in it. They are not processed by script preprocessors.
+
 ## Caveats and limitations
 
 Custom elements can be a useful way to package components for consumption in a non-Svelte app, as they will work with vanilla HTML and JavaScript as well as [most frameworks](https://custom-elements-everywhere.com/). There are, however, some important differences to be aware of:

packages/svelte/src/compiler/index.js

@@ -43,6 +43,11 @@ export function compile(source, options) {
 			instance: parsed.instance && remove_typescript_nodes(parsed.instance),
 			module: parsed.module && remove_typescript_nodes(parsed.module)
 		};
+		if (combined_options.customElementOptions?.extend) {
+			combined_options.customElementOptions.extend = remove_typescript_nodes(
+				combined_options.customElementOptions?.extend
+			);
+		}
 	}
 
 	const analysis = analyze_component(parsed, source, combined_options);

packages/svelte/tests/runtime-browser/custom-elements-samples/extend-with-ts/_config.js

@@ -0,0 +1,19 @@
+import { test } from '../../assert';
+const tick = () => Promise.resolve();
+
+export default test({
+	async test({ assert, target }) {
+		target.innerHTML = '<custom-element name="world"></custom-element>';
+		await tick();
+		/** @type {any} */
+		const el = target.querySelector('custom-element');
+
+		assert.htmlEqual(
+			el.shadowRoot.innerHTML,
+			`
+		<p>name: world</p>
+	`
+		);
+		assert.equal(el.test, `test`);
+	}
+});

packages/svelte/tests/runtime-browser/custom-elements-samples/extend-with-ts/main.svelte

@@ -0,0 +1,14 @@
+<svelte:options customElement={{
+	tag: "custom-element",
+	extend: (customClass)=>{
+		return class extends customClass{
+			public test: string = "test";
+		}
+	},
+}}/>
+
+<script lang="ts">
+	let { name } = $props();
+</script>
+
+<p>name: {name}</p>