re-use feed.(xml,atom) logic

lib/build-feed.ts

@@ -1,8 +1,19 @@
 import { Feed } from "feed";
 import { getAllNotes } from "./parse-notes";
 import * as config from "./config";
+import type { GetServerSidePropsContext, PreviewData } from "next";
+import type { ParsedUrlQuery } from "querystring";
 
-export const buildFeed = () => {
+// handles literally *everything* about building the server-side rss/atom feeds and writing the response.
+// all the page needs to do is `return buildFeed(context, { format: "rss" })` from getServerSideProps.
+
+export const buildFeed = (
+  context: GetServerSidePropsContext<ParsedUrlQuery, PreviewData>,
+  options?: { type: "rss" | "atom" }
+): { props: Record<string, unknown> } => {
+  const { res } = context;
+
+  // https://github.com/jpmonette/feed#example
   const feed = new Feed({
     id: `${config.baseUrl}/`,
     link: `${config.baseUrl}/`,
@@ -22,6 +33,7 @@ export const buildFeed = () => {
     },
   });
 
+  // add notes separately using their frontmatter
   const notes = getAllNotes();
   notes.forEach((note) => {
     feed.addItem({
@@ -40,5 +52,21 @@ export const buildFeed = () => {
     });
   });
 
-  return feed;
+  // cache on edge for one hour
+  res.setHeader("cache-control", "s-maxage=3600, stale-while-revalidate");
+
+  // generates RSS by default
+  if (options?.type === "atom") {
+    res.setHeader("content-type", "application/atom+xml; charset=utf-8");
+    res.write(feed.atom1());
+  } else {
+    res.setHeader("content-type", "application/rss+xml; charset=utf-8");
+    res.write(feed.rss2());
+  }
+
+  res.end();
+
+  return {
+    props: {},
+  };
 };