refactor(middleware): improve readability by modularizing logic and adding documentation

middleware.ts

@@ -16,58 +16,125 @@ export const config = {
 
 export default async function middleware(req: NextRequest) {
   const url = req.nextUrl;
+  const hostname = getProcessedHostname(req);
+  const path = getRequestPath(url);
 
-  // Get hostname of request (e.g. demo.vercel.pub, demo.localhost:3000)
-  let hostname = req.headers
-    .get("host")!
-    .replace(".localhost:3000", `.${process.env.NEXT_PUBLIC_ROOT_DOMAIN}`);
-
-  // special case for Vercel preview deployment URLs
-  if (
-    hostname.includes("---") &&
-    hostname.endsWith(`.${process.env.NEXT_PUBLIC_VERCEL_DEPLOYMENT_SUFFIX}`)
-  ) {
-    hostname = `${hostname.split("---")[0]}.${
-      process.env.NEXT_PUBLIC_ROOT_DOMAIN
-    }`;
+  if (isAppDomain(hostname)) {
+    return handleAppDomainRequests(req, path);
   }
 
-  const searchParams = req.nextUrl.searchParams.toString();
-  // Get the pathname of the request (e.g. /, /about, /blog/first-post)
-  const path = `${url.pathname}${
-    searchParams.length > 0 ? `?${searchParams}` : ""
-  }`;
+  if (isVercelPubDomain(hostname)) {
+    return redirectToVercelBlog();
+  }
 
-  // rewrites for app pages
-  if (hostname == `app.${process.env.NEXT_PUBLIC_ROOT_DOMAIN}`) {
-    const session = await getToken({ req });
-    if (!session && path !== "/login") {
-      return NextResponse.redirect(new URL("/login", req.url));
-    } else if (session && path == "/login") {
-      return NextResponse.redirect(new URL("/", req.url));
-    }
-    return NextResponse.rewrite(
-      new URL(`/app${path === "/" ? "" : path}`, req.url),
-    );
+  if (isLocalOrRootDomain(hostname)) {
+    return rewriteToHomeFolder(url, path);
   }
 
-  // special case for `vercel.pub` domain
-  if (hostname === "vercel.pub") {
-    return NextResponse.redirect(
-      "https://vercel.com/blog/platforms-starter-kit",
-    );
+  return rewriteToDynamicRoute(url, hostname, path);
+}
+
+/**
+ * Processes the request hostname, adjusting it for local development and Vercel preview deployments.
+ * It ensures that the hostname is standardized to use the application's root domain environment variable.
+ *
+ * @param {NextRequest} req - The incoming request object.
+ * @returns {string} The processed hostname.
+ */
+function getProcessedHostname(req: NextRequest) {
+  let hostname = req.headers.get("host")!.replace(".localhost:3000", `.${process.env.NEXT_PUBLIC_ROOT_DOMAIN}`);
+  if (hostname.includes("---") && hostname.endsWith(`.${process.env.NEXT_PUBLIC_VERCEL_DEPLOYMENT_SUFFIX}`)) {
+    hostname = `${hostname.split("---")[0]}.${process.env.NEXT_PUBLIC_ROOT_DOMAIN}`;
   }
+  return hostname;
+}
+
+/**
+ * Constructs the full request path including any search parameters.
+ *
+ * @param {URL} url - The URL object of the request.
+ * @returns {string} The full path for the request.
+ */
+function getRequestPath(url: URL) {
+  const searchParams = url.searchParams.toString();
+  return `${url.pathname}${searchParams.length > 0 ? `?${searchParams}` : ""}`;
+}
+
+/**
+ * Checks if the request is targeted at the application's subdomain.
+ *
+ * @param {string} hostname - The hostname from the request.
+ * @returns {boolean} True if the request is for the app domain, false otherwise.
+ */
+function isAppDomain(hostname: string) {
+  return hostname == `app.${process.env.NEXT_PUBLIC_ROOT_DOMAIN}`;
+}
 
-  // rewrite root application to `/home` folder
-  if (
-    hostname === "localhost:3000" ||
-    hostname === process.env.NEXT_PUBLIC_ROOT_DOMAIN
-  ) {
-    return NextResponse.rewrite(
-      new URL(`/home${path === "/" ? "" : path}`, req.url),
-    );
+/**
+ * Handles requests to the app domain, including authentication checks and redirections
+ * based on the session state and requested path.
+ *
+ * @param {NextRequest} req - The incoming request object.
+ * @param {string} path - The path extracted from the request.
+ * @returns {Response} A Next.js response object to redirect or rewrite the request.
+ */
+async function handleAppDomainRequests(req: NextRequest, path: string) {
+  const session = await getToken({ req });
+  if (!session && path !== "/login") {
+    return NextResponse.redirect(new URL("/login", req.url));
+  } else if (session && path == "/login") {
+    return NextResponse.redirect(new URL("/", req.url));
   }
+  return NextResponse.rewrite(new URL(`/app${path === "/" ? "" : path}`, req.url));
+}
+
+/**
+ * Determines if the request's hostname is specifically for a `vercel.pub` domain.
+ *
+ * @param {string} hostname - The hostname from the request.
+ * @returns {boolean} True if the hostname is `vercel.pub`, false otherwise.
+ */
+function isVercelPubDomain(hostname: string) {
+  return hostname === "vercel.pub";
+}
+
+/**
+ * Redirects requests to the `vercel.pub` domain to a specific blog post.
+ *
+ * @returns {Response} A Next.js response object for the redirection.
+ */
+function redirectToVercelBlog() {
+  return NextResponse.redirect("https://vercel.com/blog/platforms-starter-kit");
+}
+
+/**
+ * Checks if the request is coming from localhost or the application's root domain.
+ *
+ * @param {string} hostname - The hostname from the request.
+ * @returns {boolean} True if the hostname is localhost or the root domain, false otherwise.
+ */
+function isLocalOrRootDomain(hostname: string) {
+  return hostname === "localhost:3000" || hostname === process.env.NEXT_PUBLIC_ROOT_DOMAIN;
+}
 
-  // rewrite everything else to `/[domain]/[slug] dynamic route
-  return NextResponse.rewrite(new URL(`/${hostname}${path}`, req.url));
+/**
+ * Rewrites requests from the root or local domain to the '/home' folder, maintaining the path and query.
+ * @param {URL} url - The URL object from the incoming request.
+ * @param {string} path - The path from the request URL.
+ * @returns {NextResponse} A response object rewriting the request to the '/home' folder.
+ */
+function rewriteToHomeFolder(url: URL, path: string) {
+  return NextResponse.rewrite(new URL(`/home${path === "/" ? "" : path}`, url));
 }
+
+/**
+ * Rewrites the request to a dynamic route based on the processed hostname and path. This is particularly useful
+ * for handling requests in a multi-tenant application with custom domains, allowing for flexible routing based on the request's domain.
+ * @param {URL} url - The URL object from the incoming request.
+ * @param {string} hostname - The processed request hostname, adjusted for specific use cases.
+ * @param {string} path - The path from the request URL, including query parameters.
+ * @returns {NextResponse} A response object rewriting the request to a dynamic route.
+ */
+function rewriteToDynamicRoute(url: URL, hostname: string, path: string) {
+  return NextResponse.rewrite(new URL(`/${hostname}${path}`, url));
+}