[Lit] add `client:only` functionality to Lit integration (#6111)

* [Lit] add `client:only` functionality to Lit integration

* add changeset

* update lit changeset to minor

1 files changed, 70 insertions, 19 deletions

diff --git a/packages/integrations/lit/src/client.ts b/packages/integrations/lit/src/client.ts
index fb92ac3df..b63147182 100644
--- a/packages/integrations/lit/src/client.ts
+++ b/packages/integrations/lit/src/client.ts
@@ -1,21 +1,72 @@
-export default (element: HTMLElement) => async (Component: any, props: Record<string, any>) => {
-	// Get the LitElement element instance (may or may not be upgraded).
-	const component = element.children[0] as HTMLElement;
-
-	// If there is no deferral of hydration, then all reactive properties are
-	// already serialzied as reflected attributes, or no reactive props were set
-	if (!component || !component.hasAttribute('defer-hydration')) {
-		return;
-	}
-
-	// Set properties on the LitElement instance for resuming hydration.
-	for (let [name, value] of Object.entries(props)) {
-		// Check if reactive property or class property.
-		if (name in Component.prototype) {
-			(component as any)[name] = value;
+/**
+ * Adds the appropriate slot attribute to each top-level node in the given HTML
+ * string.
+ *
+ * @example
+ * addSlotAttrsToHtmlString('foo', '<div>bar</div><div>baz</div>');
+ * // '<div slot="foo">bar</div><div slot="foo">baz</div>'
+ *
+ * @param slotName Name of slot to apply to HTML string.
+ * @param html Stringified HTML that should be projected into the given slotname.
+ * @returns A stringified HTML string with the slot attribute applied to each top-level node.
+ */
+const addSlotAttrsToHtmlString = (slotName: string, html: string) => {
+	const templ = document.createElement('template');
+	templ.innerHTML = html;
+	Array.from(templ.content.children).forEach((node) => {
+		node.setAttribute('slot', slotName);
+	});
+	return templ.innerHTML;
+};
+
+export default (element: HTMLElement) =>
+	async (
+		Component: any,
+		props: Record<string, any>,
+		{ default: defaultChildren, ...slotted }: { default: string; [slotName: string]: string }
+	) => {
+		// Get the LitElement element instance.
+		let component = element.children[0];
+		// Check if hydration model is client:only
+		const isClientOnly = element.getAttribute('client') === 'only';
+
+		// We need to attach the element and it's children to the DOM since it's not
+		// SSR'd.
+		if (isClientOnly) {
+			component = new Component();
+
+			const otherSlottedChildren = Object.entries(slotted)
+				.map(([slotName, htmlStr]) => addSlotAttrsToHtmlString(slotName, htmlStr))
+				.join('');
+
+			// defaultChildren can actually be undefined, but TS will complain if we
+			// type it as so, make sure we don't render undefined.
+			component.innerHTML = `${defaultChildren ?? ''}${otherSlottedChildren}`;
+			element.appendChild(component);
+
+			// Set props bound to non-reactive properties as attributes.
+			for (let [name, value] of Object.entries(props)) {
+				if (!(name in Component.prototype)) {
+					component.setAttribute(name, value);
+				}
+			}
 		}
-	}
 
-	// Tell LitElement to resume hydration.
-	component.removeAttribute('defer-hydration');
-};
+		// If there is no deferral of hydration, then all reactive properties are
+		// already serialzied as reflected attributes, or no reactive props were set
+		// Alternatively, if hydration is client:only proceed to set props.
+		if (!component || !(component.hasAttribute('defer-hydration') || isClientOnly)) {
+			return;
+		}
+
+		// Set properties on the LitElement instance for resuming hydration.
+		for (let [name, value] of Object.entries(props)) {
+			// Check if reactive property or class property.
+			if (name in Component.prototype) {
+				(component as any)[name] = value;
+			}
+		}
+
+		// Tell LitElement to resume hydration.
+		component.removeAttribute('defer-hydration');
+	};