2 files changed, 173 insertions, 0 deletions
diff --git a/packages/astro-rss/src/index.ts b/packages/astro-rss/src/index.ts
new file mode 100644
index 000000000..c9e53f8ce
--- /dev/null
+++ b/packages/astro-rss/src/index.ts
@@ -0,0 +1,154 @@
+import { XMLValidator } from 'fast-xml-parser';
+import { createCanonicalURL, isValidURL } from './util.js';
+
+type GlobResult = Record<string, () => Promise<{ [key: string]: any }>>;
+
+type RSSOptions = {
+	/** (required) Title of the RSS Feed */
+	title: string;
+	/** (required) Description of the RSS Feed */
+	description: string;
+	/**
+	 * List of RSS feed items to render. Accepts either:
+	 * a) list of RSSFeedItems
+	 * b) import.meta.glob result. You can only glob ".md" files within src/pages/ when using this method!
+	 */
+	items: RSSFeedItem[] | GlobResult;
+	/** Specify arbitrary metadata on opening <xml> tag */
+	xmlns?: Record<string, string>;
+	/**
+	 * Specifies a local custom XSL stylesheet. Ex. '/public/custom-feed.xsl'
+	 */
+	stylesheet?: string | boolean;
+	/** Specify custom data in opening of file */
+	customData?: string;
+	/**
+	 * Specify the base URL to use for RSS feed links.
+	 * Defaults to "site" in your project's astro.config
+	 */
+	canonicalUrl?: string;
+};
+
+type RSSFeedItem = {
+	/** Link to item */
+	link: string;
+	/** Title of item */
+	title: string;
+	/** Publication date of item */
+	pubDate: Date;
+	/** Item description */
+	description?: string;
+	/** Append some other XML-valid data to this item */
+	customData?: string;
+};
+
+type GenerateRSSArgs = {
+	site: string;
+	rssOptions: RSSOptions;
+	items: RSSFeedItem[];
+};
+
+function isGlobResult(items: RSSOptions['items']): items is GlobResult {
+	return typeof items === 'object' && !items.length;
+}
+
+function mapGlobResult(items: GlobResult): Promise<RSSFeedItem[]> {
+	return Promise.all(
+		Object.values(items).map(async (getInfo) => {
+			const { url, frontmatter } = await getInfo();
+			if (!Boolean(url)) {
+				throw new Error(
+					`[RSS] When passing an import.meta.glob result directly, you can only glob ".md" files within /pages! Consider mapping the result to an array of RSSFeedItems. See the RSS docs for usage examples: https://docs.astro.build/en/guides/rss/#2-list-of-rss-feed-objects`
+				);
+			}
+			if (!Boolean(frontmatter.title) || !Boolean(frontmatter.pubDate)) {
+				throw new Error(`[RSS] "${url}" is missing a "title" and/or "pubDate" in its frontmatter.`);
+			}
+			return {
+				link: url,
+				title: frontmatter.title,
+				pubDate: frontmatter.pubDate,
+				description: frontmatter.description,
+				customData: frontmatter.customData,
+			};
+		})
+	);
+}
+
+export default async function getRSS(rssOptions: RSSOptions) {
+	const site = rssOptions.canonicalUrl ?? (import.meta as any).env.SITE;
+	if (!site) {
+		throw new Error(
+			`RSS requires a canonical URL. Either add a "site" to your project's astro.config, or supply the canonicalUrl argument.`
+		);
+	}
+	let { items } = rssOptions;
+	if (isGlobResult(items)) {
+		items = await mapGlobResult(items);
+	}
+	return {
+		body: await generateRSS({
+			site,
+			rssOptions,
+			items,
+		}),
+	};
+}
+
+/** Generate RSS 2.0 feed */
+export async function generateRSS({ site, rssOptions, items }: GenerateRSSArgs): Promise<string> {
+	let xml = `<?xml version="1.0" encoding="UTF-8"?>`;
+	if (typeof rssOptions.stylesheet === 'string') {
+		xml += `<?xml-stylesheet href="${rssOptions.stylesheet}" type="text/xsl"?>`;
+	}
+	xml += `<rss version="2.0"`;
+
+	// xmlns
+	if (rssOptions.xmlns) {
+		for (const [k, v] of Object.entries(rssOptions.xmlns)) {
+			xml += ` xmlns:${k}="${v}"`;
+		}
+	}
+	xml += `>`;
+	xml += `<channel>`;
+
+	// title, description, customData
+	xml += `<title><![CDATA[${rssOptions.title}]]></title>`;
+	xml += `<description><![CDATA[${rssOptions.description}]]></description>`;
+	xml += `<link>${createCanonicalURL(site).href}</link>`;
+	if (typeof rssOptions.customData === 'string') xml += rssOptions.customData;
+	// items
+	for (const result of items) {
+		xml += `<item>`;
+		xml += `<title><![CDATA[${result.title}]]></title>`;
+		// If the item's link is already a valid URL, don't mess with it.
+		const itemLink = isValidURL(result.link)
+			? result.link
+			: createCanonicalURL(result.link, site).href;
+		xml += `<link>${itemLink}</link>`;
+		xml += `<guid>${itemLink}</guid>`;
+		if (result.description) xml += `<description><![CDATA[${result.description}]]></description>`;
+		if (result.pubDate) {
+			// note: this should be a Date, but if user provided a string or number, we can work with that, too.
+			if (typeof result.pubDate === 'number' || typeof result.pubDate === 'string') {
+				result.pubDate = new Date(result.pubDate);
+			} else if (result.pubDate instanceof Date === false) {
+				throw new Error('[${filename}] rss.item().pubDate must be a Date');
+			}
+			xml += `<pubDate>${result.pubDate.toUTCString()}</pubDate>`;
+		}
+		if (typeof result.customData === 'string') xml += result.customData;
+		xml += `</item>`;
+	}
+
+	xml += `</channel></rss>`;
+
+	// validate user’s inputs to see if it’s valid XML
+	const isValid = XMLValidator.validate(xml);
+	if (isValid !== true) {
+		// If valid XML, isValid will be `true`. Otherwise, this will be an error object. Throw.
+		throw new Error(isValid as any);
+	}
+
+	return xml;
+}
diff --git a/packages/astro-rss/src/util.ts b/packages/astro-rss/src/util.ts
new file mode 100644
index 000000000..0dad6b239
--- /dev/null
+++ b/packages/astro-rss/src/util.ts
@@ -0,0 +1,19 @@
+import npath from 'path-browserify';
+
+/** Normalize URL to its canonical form */
+export function createCanonicalURL(url: string, base?: string): URL {
+	let pathname = url.replace(/\/index.html$/, ''); // index.html is not canonical
+	pathname = pathname.replace(/\/1\/?$/, ''); // neither is a trailing /1/ (impl. detail of collections)
+	if (!npath.extname(pathname)) pathname = pathname.replace(/(\/+)?$/, '/'); // add trailing slash if there’s no extension
+	pathname = pathname.replace(/\/+/g, '/'); // remove duplicate slashes (URL() won’t)
+	return new URL(pathname, base);
+}
+
+/** Check if a URL is already valid */
+export function isValidURL(url: string): boolean {
+	try {
+		new URL(url);
+		return true;
+	} catch (e) {}
+	return false;
+}