astro/packages/integrations/lit/server.js

import './server-shim.js';
import '@lit-labs/ssr/lib/render-lit-html.js';
import { LitElementRenderer } from '@lit-labs/ssr/lib/lit-element-renderer.js';

function isCustomElementTag(name) {
	return typeof name === 'string' && /-/.test(name);
}

function getCustomElementConstructor(name) {
	if (typeof customElements !== 'undefined' && isCustomElementTag(name)) {
		return customElements.get(name) || null;
	} else if(typeof name === 'function') {
		return name;
	}
	return null;
}

async function isLitElement(Component) {
	const Ctr = getCustomElementConstructor(Component);
	return !!(Ctr && Ctr._$litElement$);
}

async function check(Component, _props, _children) {
	// Lit doesn't support getting a tagName from a Constructor at this time.
	// So this must be a string at the moment.
	return !!(await isLitElement(Component));
}

function* render(Component, attrs, children) {
	let tagName = Component;
	if(typeof tagName !== 'string') {
		tagName = Component[Symbol.for('tagName')];
	}
	const instance = new LitElementRenderer(tagName);

	// LitElementRenderer creates a new element instance, so copy over.
	const Ctr = getCustomElementConstructor(tagName);
	if (attrs) {
		for (let [name, value] of Object.entries(attrs)) {
			// check if this is a reactive property
			if (name in Ctr.prototype) {
				instance.setProperty(name, value);
			} else {
				instance.setAttribute(name, value);
			}
		}
	}

	instance.connectedCallback();

	yield `<${tagName}`;
	yield* instance.renderAttributes();
	yield `>`;
	const shadowContents = instance.renderShadow({});
	if (shadowContents !== undefined) {
		yield '<template shadowroot="open">';
		yield* shadowContents;
		yield '</template>';
	}
	yield children || ''; // don’t print “undefined” as string
	yield `</${tagName}>`;
}

async function renderToStaticMarkup(Component, props, children) {
	let tagName = Component;

	let out = '';
	for (let chunk of render(tagName, props, children)) {
		out += chunk;
	}

	return {
		html: out,
	};
}

export default {
	check,
	renderToStaticMarkup,
};