fix: recognize all custom element prop definitions

We didn't account for the `$props` rune being writtin in a way that makes some props unknown, and they would only be visible through the `customElement.props` definition. This changes the iteration to account for that and also adds a note to the documentation that you need to list out the properties explicitly.

fixes #13785

Author: dummdidumm

.changeset/warm-eyes-protect.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: recognize all custom element prop definitions

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

@@ -49,6 +49,8 @@ console.log(el.name);
 el.name = 'everybody';
 ```
 
+Note that you need to list out all properties explicitly, i.e. doing `let props = $props()` without declaring `props` in the [component options](#Component-options) means that Svelte can't know which props to expose as properties on the DOM element.
+
 ## Component lifecycle
 
 Custom elements are created from Svelte components using a wrapper approach. This means the inner Svelte component has no knowledge that it is a custom element. The custom element wrapper takes care of handling its lifecycle appropriately.

packages/svelte/src/compiler/phases/3-transform/client/transform-client.js

@@ -561,17 +561,19 @@ export function client_component(analysis, options) {
 
 	if (analysis.custom_element) {
 		const ce = analysis.custom_element;
+		const ce_props = typeof ce === 'boolean' ? {} : ce.props || {};
 
 		/** @type {ESTree.Property[]} */
 		const props_str = [];
 
-		for (const [name, binding] of properties) {
-			const key = binding.prop_alias ?? name;
-			const prop_def = typeof ce === 'boolean' ? {} : ce.props?.[key] || {};
+		for (const [name, prop_def] of Object.entries(ce_props)) {
+			const binding = analysis.instance.scope.get(name);
+			const key = binding?.prop_alias ?? name;
+
 			if (
 				!prop_def.type &&
-				binding.initial?.type === 'Literal' &&
-				typeof binding.initial.value === 'boolean'
+				binding?.initial?.type === 'Literal' &&
+				typeof binding?.initial.value === 'boolean'
 			) {
 				prop_def.type = 'Boolean';
 			}
@@ -585,9 +587,17 @@ export function client_component(analysis, options) {
 					].filter(Boolean)
 				)
 			);
+
 			props_str.push(b.init(key, value));
 		}
 
+		for (const [name, binding] of properties) {
+			const key = binding.prop_alias ?? name;
+			if (ce_props[key]) continue;
+
+			props_str.push(b.init(key, b.object([])));
+		}
+
 		const slots_str = b.array([...analysis.slot_names.keys()].map((name) => b.literal(name)));
 		const accessors_str = b.array(
 			analysis.exports.map(({ name, alias }) => b.literal(alias ?? name))

packages/svelte/tests/runtime-browser/custom-elements-samples/props-rune-attributes/_config.js

@@ -0,0 +1,21 @@
+import { test } from '../../assert';
+const tick = () => Promise.resolve();
+
+export default test({
+	async test({ assert, target }) {
+		target.innerHTML = '<custom-element foo-bar="1" bar="2" b-az="3"></custom-element>';
+		await tick();
+
+		/** @type {any} */
+		const el = target.querySelector('custom-element');
+
+		assert.htmlEqual(
+			el.shadowRoot.innerHTML,
+			`
+			<p>1</p>
+			<p>2</p>
+			<p>3</p>
+		`
+		);
+	}
+});

packages/svelte/tests/runtime-browser/custom-elements-samples/props-rune-attributes/main.svelte

@@ -0,0 +1,14 @@
+<svelte:options
+	customElement={{
+		tag: "custom-element",
+		props: { foo: { attribute: 'foo-bar' } },
+	}}
+/>
+
+<script>
+	let { bar, 'b-az': baz, ...rest } = $props();
+</script>
+
+<p>{rest.foo}</p>
+<p>{bar}</p>
+<p>{baz}</p>

packages/svelte/tests/runtime-browser/custom-elements-samples/props-rune-attributes/my-widget.svelte

@@ -0,0 +1,14 @@
+<svelte:options
+	customElement={{
+		tag: "my-widget",
+		props: { foo: { attribute: 'foo-bar' } },
+	}}
+/>
+
+<script>
+	let { bar, 'b-az': baz, ...rest } = $props();
+</script>
+
+<p>{rest.foo}</p>
+<p>{bar}</p>
+<p>{baz}</p>