Rebased, retained existing behavior to be back compat

src/index.js

@@ -25,10 +25,17 @@ function connectedCallback() {
 	(this.hasAttribute('hydrate') ? hydrate : render)(this._vdom, this._root);
 }
 
+function toCamelCase(str) {
+	return str.replace(/-(\w)/g, function(_, c) {
+		return c ? c.toUpperCase() : ''
+	});
+}
+
 function attributeChangedCallback(name, oldValue, newValue) {
 	if (!this._vdom) return;
 	const props = {};
 	props[name] = newValue;
+	props[toCamelCase(name)] = newValue;
 	this._vdom = cloneElement(this._vdom, props);
 	render(this._vdom, this._root);
 }
@@ -46,10 +53,8 @@ function toVdom(element, nodeName) {
 		a = element.attributes,
 		cn = element.childNodes;
 	for (i = a.length; i--; ) {
-	  var propName = a[i].name.replace(/-(\w)/, function(_, c) {
-	    return c ? c.toUpperCase() : ''
-	  });
-	  props[propName] = a[i].value;
+	  props[a[i].name] = a[i].value;
+	  props[toCamelCase(a[i].name)] = a[i].value;
   }
 	for (i = cn.length; i--; ) children[i] = toVdom(cn[i]);
 	return h(nodeName || element.nodeName.toLowerCase(), props, children);

src/index.test.js

@@ -2,28 +2,62 @@ import assert from 'assert';
 import { h } from 'preact';
 import registerElement from './index';
 
+function Clock ({ time }) {
+	return <span>{time}</span>;
+}
+
 registerElement(Clock, 'x-clock', ['time', 'custom-date']);
 
 it('renders ok, updates on attr change', () => {
 	const root = document.createElement('div');
 	const el = document.createElement('x-clock');
 	el.setAttribute('time', '10:28:57 PM');
-	el.setAttribute('custom-date', '11/11/2011');
 
 	root.appendChild(el);
 	document.body.appendChild(root);
 
 	assert.equal(
 		root.innerHTML,
-		'<x-clock time="10:28:57 PM" custom-date="11/11/2011"><span>10:28:57 PM, 11/11/2011</span></x-clock>'
+		'<x-clock time="10:28:57 PM"><span>10:28:57 PM</span></x-clock>'
 	);
 
 	el.setAttribute('time', '11:01:10 AM');
-	el.setAttribute('custom-date', '01/01/2001');
 
 	assert.equal(
 		root.innerHTML,
-		'<x-clock time="11:01:10 AM" custom-date="01/01/2001"><span>11:01:10 AM, 01/01/2001</span></x-clock>'
+		'<x-clock time="11:01:10 AM"><span>11:01:10 AM</span></x-clock>'
+	);
+
+	document.body.removeChild(root);
+});
+
+const kebabName = 'custom-date-long-name';
+const camelName = 'customDateLongName';
+const lowerName = camelName.toLowerCase();
+function PropNameTransform (props) {
+	return <span>{props[kebabName]} {props[lowerName]} {props[camelName]}</span>;
+}
+registerElement(PropNameTransform , 'x-prop-name-transform', [kebabName, camelName]);
+
+it('handles kebab-case attributes with passthrough', () => {
+	const root = document.createElement('div');
+	const el = document.createElement('x-prop-name-transform');
+	el.setAttribute(kebabName, '11/11/2011');
+	el.setAttribute(camelName, 'pretended to be camel');
+
+	root.appendChild(el);
+	document.body.appendChild(root);
+
+	assert.equal(
+		root.innerHTML,
+		`<x-prop-name-transform ${kebabName}="11/11/2011" ${lowerName}="pretended to be camel"><span>11/11/2011 pretended to be camel 11/11/2011</span></x-prop-name-transform>`
+	);
+
+	el.setAttribute(kebabName, '01/01/2001');
+
+	assert.equal(
+		root.innerHTML,
+		`<x-prop-name-transform ${kebabName}="01/01/2001" ${lowerName}="pretended to be camel"><span>01/01/2001 pretended to be camel 01/01/2001</span></x-prop-name-transform>`
 	);
 
 	document.body.removeChild(root);