feat: make <svelte:option> customElement configuration's tag property optional (#12751)

.changeset/four-kids-flow.md

@@ -0,0 +1,5 @@
+---
+"svelte": minor
+---
+
+feat: make custom element `tag` property optional

documentation/docs/02-template-syntax/09-special-elements.md

@@ -185,7 +185,7 @@ The `<svelte:options>` element provides a place to specify per-component compile
 - `accessors={true}` — adds getters and setters for the component's props
 - `accessors={false}` — the default
 - `namespace="..."` — the namespace where this component will be used, most commonly "svg"; use the "foreign" namespace to opt out of case-insensitive attribute names and HTML-specific warnings
-- `customElement="..."` — the name to use when compiling this component as a custom element
+- `customElement={...}` — the [options](/docs/custom-elements-api#component-options) to use when compiling this component as a custom element. If a string is passed, it is used as the `tag` option
 
 ```svelte
 <svelte:options customElement="my-custom-element" />

documentation/docs/05-misc/04-custom-elements.md

@@ -63,7 +63,7 @@ The inner Svelte component is destroyed in the next tick after the `disconnected
 
 When constructing a custom element, you can tailor several aspects by defining `customElement` as an object within `<svelte:options>` since Svelte 4. This object may contain the following properties:
 
-- `tag`: the mandatory `tag` property for the custom element's name
+- `tag: string`: an optional `tag` property for the custom element's name. If set, a custom element with this tag name will be defined with the document's `customElements` registry upon importing this component.
 - `shadow`: an optional property that can be set to `"none"` to forgo shadow root creation. Note that styles are then no longer encapsulated, and you can't use slots
 - `props`: an optional property to modify certain details and behaviors of your component's properties. It offers the following settings:
   - `attribute: string`: To update a custom element's prop, you have two alternatives: either set the property on the custom element's reference as illustrated above or use an HTML attribute. For the latter, the default attribute name is the lowercase property name. Modify this by assigning `attribute: "<desired name>"`.

documentation/tutorial/16-special-elements/09-svelte-options/text.md

@@ -25,6 +25,6 @@ The options that can be set here are:
 - `accessors={true}` — adds getters and setters for the component's props
 - `accessors={false}` — the default
 - `namespace="..."` — the namespace where this component will be used, most commonly `"svg"`
-- `customElement="..."` — the name to use when compiling this component as a custom element
+- `customElement={...}` — the [options](/docs/custom-elements-api#component-options) to use when compiling this component as a custom element. If a string is passed, it is used as the `tag` option
 
 Consult the [API reference](/docs) for more information on these options.

packages/svelte/messages/compile-errors/template.md

@@ -328,7 +328,7 @@ HTML restricts where certain elements can appear. In case of a violation the bro
 
 ## svelte_options_invalid_customelement
 
-> "customElement" must be a string literal defining a valid custom element name or an object of the form { tag: string; shadow?: "open" | "none"; props?: { [key: string]: { attribute?: string; reflect?: boolean; type: .. } } }
+> "customElement" must be a string literal defining a valid custom element name or an object of the form { tag?: string; shadow?: "open" | "none"; props?: { [key: string]: { attribute?: string; reflect?: boolean; type: .. } } }
 
 ## svelte_options_invalid_customelement_props
 

packages/svelte/src/compiler/errors.js

@@ -1285,12 +1285,12 @@ export function svelte_options_invalid_attribute_value(node, list) {
 }
 
 /**
- * "customElement" must be a string literal defining a valid custom element name or an object of the form { tag: string; shadow?: "open" | "none"; props?: { [key: string]: { attribute?: string; reflect?: boolean; type: .. } } }
+ * "customElement" must be a string literal defining a valid custom element name or an object of the form { tag?: string; shadow?: "open" | "none"; props?: { [key: string]: { attribute?: string; reflect?: boolean; type: .. } } }
  * @param {null | number | NodeLike} node
  * @returns {never}
  */
 export function svelte_options_invalid_customelement(node) {
-	e(node, "svelte_options_invalid_customelement", "\"customElement\" must be a string literal defining a valid custom element name or an object of the form { tag: string; shadow?: \"open\" | \"none\"; props?: { [key: string]: { attribute?: string; reflect?: boolean; type: .. } } }");
+	e(node, "svelte_options_invalid_customelement", "\"customElement\" must be a string literal defining a valid custom element name or an object of the form { tag?: string; shadow?: \"open\" | \"none\"; props?: { [key: string]: { attribute?: string; reflect?: boolean; type: .. } } }");
 }
 
 /**

packages/svelte/src/compiler/phases/1-parse/read/options.js

@@ -40,7 +40,7 @@ export default function read_options(node) {
 			}
 			case 'customElement': {
 				/** @type {SvelteOptions['customElement']} */
-				const ce = { tag: '' };
+				const ce = {};
 				const { value: v } = attribute;
 				const value = v === true || Array.isArray(v) ? v : [v];
 
@@ -79,8 +79,6 @@ export default function read_options(node) {
 					const tag_value = tag[1]?.value;
 					validate_tag(tag, tag_value);
 					ce.tag = tag_value;
-				} else {
-					e.svelte_options_invalid_customelement(attribute);
 				}
 
 				const props = properties.find(([name]) => name === 'props')?.[1];
@@ -251,8 +249,4 @@ function validate_tag(attribute, tag) {
 	if (tag && !regex_valid_tag_name.test(tag)) {
 		e.svelte_options_invalid_tagname(attribute);
 	}
-	// TODO do we still need this?
-	// if (tag && !component.compile_options.customElement) {
-	// 	component.warn(attribute, compiler_warnings.missing_custom_element_compile_options);
-	// }
 }

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

@@ -593,9 +593,8 @@ export function client_component(analysis, options) {
 			/** @type {any} */ (typeof ce !== 'boolean' ? ce.extend : undefined)
 		);
 
-		// If customElement option is set, we define the custom element directly. Else we still create
-		// the custom element class so that the user may instantiate a custom element themselves later.
-		if (typeof ce !== 'boolean') {
+		// If a tag name is provided, call `customElements.define`, otherwise leave to the user
+		if (typeof ce !== 'boolean' && typeof ce.tag === 'string') {
 			body.push(b.stmt(b.call('customElements.define', b.literal(ce.tag), create_ce)));
 		} else {
 			body.push(b.stmt(create_ce));

packages/svelte/src/compiler/types/template.d.ts

@@ -77,7 +77,7 @@ export interface SvelteOptions {
 	namespace?: Namespace;
 	css?: 'injected';
 	customElement?: {
-		tag: string;
+		tag?: string;
 		shadow?: 'open' | 'none';
 		props?: Record<
 			string,

packages/svelte/tests/runtime-browser/custom-elements-samples/no-tag-ce-options/_config.js

@@ -0,0 +1,18 @@
+import { test } from '../../assert';
+const tick = () => Promise.resolve();
+
+export default test({
+	warnings: [],
+	async test({ assert, target, componentCtor }) {
+		customElements.define('no-tag', componentCtor.element);
+		target.innerHTML = '<no-tag name="world"></no-tag>';
+		await tick();
+
+		/** @type {any} */
+		const el = target.querySelector('no-tag');
+		const h1 = el.querySelector('h1');
+
+		assert.equal(el.shadowRoot, null);
+		assert.equal(h1.textContent, 'Hello world!');
+	}
+});

packages/svelte/tests/runtime-browser/custom-elements-samples/no-tag-ce-options/main.svelte

@@ -0,0 +1,7 @@
+<svelte:options customElement={{ shadow: "none" }} />
+
+<script>
+	export let name;
+</script>
+
+<h1>Hello {name}!</h1>

packages/svelte/tests/validator/samples/tag-non-string/errors.json

@@ -1,7 +1,7 @@
 [
 	{
 		"code": "svelte_options_invalid_customelement",
-		"message": "\"customElement\" must be a string literal defining a valid custom element name or an object of the form { tag: string; shadow?: \"open\" | \"none\"; props?: { [key: string]: { attribute?: string; reflect?: boolean; type: .. } } }",
+		"message": "\"customElement\" must be a string literal defining a valid custom element name or an object of the form { tag?: string; shadow?: \"open\" | \"none\"; props?: { [key: string]: { attribute?: string; reflect?: boolean; type: .. } } }",
 		"start": {
 			"line": 1,
 			"column": 16

packages/svelte/types/index.d.ts

@@ -1535,7 +1535,7 @@ declare module 'svelte/compiler' {
 		namespace?: Namespace;
 		css?: 'injected';
 		customElement?: {
-			tag: string;
+			tag?: string;
 			shadow?: 'open' | 'none';
 			props?: Record<
 				string,