feat: hyphenated attributes in custom elements

src/compiler/compile/render_dom/index.ts

@@ -561,7 +561,7 @@ export default function dom(
 				computed: false,
 				key: { type: 'Identifier', name: 'observedAttributes' },
 				value: x`function() {
-					return [${props.map(prop => x`"${prop.export_name}"`)}];
+					return [${props.map(prop => x`"${prop.export_name.replace(/[A-Z]/g, c => `-${c.toLowerCase()}`)}"`)}];
 				}` as FunctionExpression
 			});
 		}

src/runtime/internal/Component.ts

@@ -167,7 +167,8 @@ if (typeof HTMLElement === 'function') {
 		}
 
 		attributeChangedCallback(attr, _oldValue, newValue) {
-			this[attr] = newValue;
+			const camelCase = attr.replace(/-./g, c => c[1].toUpperCase());
+			this[camelCase] = newValue;
 		}
 
 		disconnectedCallback() {

src/runtime/internal/dom.ts

@@ -340,15 +340,20 @@ export function set_svg_attributes(node: Element & ElementCSSInlineStyle, attrib
 	}
 }
 
+function kebab_case_to_camel_case(str: string): string {
+	return str.replace(/-./g, c => c[1].toUpperCase());
+}
+
 export function set_custom_element_data_map(node, data_map: Record<string, unknown>) {
 	Object.keys(data_map).forEach((key) => {
 		set_custom_element_data(node, key, data_map[key]);
 	});
 }
 
 export function set_custom_element_data(node, prop, value) {
-	if (prop in node) {
-		node[prop] = typeof node[prop] === 'boolean' && value === '' ? true : value;
+	const camelCaseProp = kebab_case_to_camel_case(prop);
+	if (camelCaseProp in node) {
+		node[camelCaseProp] = typeof node[camelCaseProp] === 'boolean' && value === '' ? true : value;
 	} else {
 		attr(node, prop, value);
 	}
@@ -871,7 +876,8 @@ export class HtmlTagHydration extends HtmlTag {
 export function attribute_to_object(attributes: NamedNodeMap) {
 	const result = {};
 	for (const attribute of attributes) {
-		result[attribute.name] = attribute.value;
+		const name = kebab_case_to_camel_case(attribute.name);
+		result[name] = attribute.value;
 	}
 	return result;
 }

test/custom-elements/samples/$$props/main.svelte

@@ -2,9 +2,11 @@
 
 <script>
 	export let name;
+	export let hyphenatedAttr;
 </script>
 
 <p>name: {name}</p>
+<p>hyphenated attribute: {hyphenatedAttr}</p>
 <p>$$props: {JSON.stringify($$props)}</p>
 <p>$$restProps: {JSON.stringify($$restProps)}</p>
 

test/custom-elements/samples/$$props/test.js

@@ -2,12 +2,13 @@ import * as assert from 'assert';
 import './main.svelte';
 
 export default function (target) {
-	target.innerHTML = '<custom-element name="world" answer="42" test="svelte"></custom-element>';
+	target.innerHTML = '<custom-element name="world" answer="42" test="svelte" hyphenated-attr="galaxy" rest-attr="universe"></custom-element>';
 	const el = target.querySelector('custom-element');
 
 	assert.htmlEqual(el.shadowRoot.innerHTML, `
 		<p>name: world</p>
-		<p>$$props: {"name":"world","answer":"42","test":"svelte"}</p>
-		<p>$$restProps: {"answer":"42","test":"svelte"}</p>
+		<p>hyphenated attribute: galaxy</p>
+		<p>$$props: {"name":"world","answer":"42","test":"svelte","hyphenatedAttr":"galaxy","restAttr":"universe"}</p>
+		<p>$$restProps: {"answer":"42","test":"svelte","restAttr":"universe"}</p>
 	`);
 }

test/custom-elements/samples/props/main.svelte

@@ -5,6 +5,7 @@
 
 	export let items = ['a', 'b', 'c'];
 	export let flagged = false;
+	export let flaggedWithHyphen = false;
 </script>
 
-<my-widget class="foo" {items} flag1={flagged} flag2 />
+<my-widget class="foo" {items} flag1={flagged} flag2 flag-with-hyphen1={flaggedWithHyphen} flag-with-hyphen2 />

test/custom-elements/samples/props/my-widget.svelte

@@ -4,9 +4,13 @@
 	export let items = [];
 	export let flag1 = false;
 	export let flag2 = false;
+	export let flagWithHyphen1 = false;
+	export let flagWithHyphen2 = false;
 </script>
 
 <p>{items.length} items</p>
 <p>{items.join(', ')}</p>
 <p>{flag1 ? 'flagged (dynamic attribute)' : 'not flagged'}</p>
 <p>{flag2 ? 'flagged (static attribute)' : 'not flagged'}</p>
+<p>{flagWithHyphen1 ? 'flagged with hyphen (dynamic attribute)' : 'not flagged'}</p>
+<p>{flagWithHyphen2 ? 'flagged with hyphen (static attribute)' : 'not flagged'}</p>

test/custom-elements/samples/props/test.js

@@ -11,17 +11,21 @@ export default function (target) {
 	const el = target.querySelector('custom-element');
 	const widget = el.shadowRoot.querySelector('my-widget');
 
-	const [p1, p2, p3, p4] = widget.shadowRoot.querySelectorAll('p');
+	const [p1, p2, p3, p4, p5, p6] = widget.shadowRoot.querySelectorAll('p');
 
 	assert.equal(p1.textContent, '3 items');
 	assert.equal(p2.textContent, 'a, b, c');
 	assert.equal(p3.textContent, 'not flagged');
 	assert.equal(p4.textContent, 'flagged (static attribute)');
+	assert.equal(p5.textContent, 'not flagged');
+	assert.equal(p6.textContent, 'flagged with hyphen (static attribute)');
 
 	el.items = ['d', 'e', 'f', 'g', 'h'];
 	el.flagged = true;
+	el.flaggedWithHyphen = true;
 
 	assert.equal(p1.textContent, '5 items');
 	assert.equal(p2.textContent, 'd, e, f, g, h');
 	assert.equal(p3.textContent, 'flagged (dynamic attribute)');
+	assert.equal(p5.textContent, 'flagged with hyphen (dynamic attribute)');
 }