feat: better behavior for 'undefined' return values from 'serialize… (#3723)

* feat: better behavior with 'undefined' return values after 'serialize' func

* build: changeset added

.changeset/tidy-dots-own.md

@@ -0,0 +1,5 @@
+---
+'@astrojs/sitemap': patch
+---
+
+fix: if `serialize` function returns `undefined` for the passed entry, such entry will be excluded from sitemap

packages/integrations/sitemap/README.md

@@ -222,7 +222,9 @@ The `LinkItem` type has two required fields: `url` (the fully-qualified URL for
 
 The `serialize` function should return `SitemapItem`, touched or not.  
 
-The example below shows the ability to add the sitemap specific properties individually.
+To exclude the passed entry from sitemap it should return `undefined`.
+
+The example below shows the ability to exclude certain entries and add the sitemap specific properties individually.
 
 __astro.config.mjs__
 
@@ -234,6 +236,9 @@ export default {
   integrations: [
     sitemap({
       serialize(item) {
+        if (/exclude-from-sitemap/.test(item.url)) {
+          return undefined;
+        }
         if (/your-special-page/.test(item.url)) {
           item.changefreq = 'daily';
           item.lastmod = new Date();

packages/integrations/sitemap/src/index.ts

@@ -38,7 +38,7 @@ export type SitemapOptions =
 			priority?: number;
 
 			// called for each sitemap item just before to save them on disk, sync or async
-			serialize?(item: SitemapItem): SitemapItem | Promise<SitemapItem>;
+			serialize?(item: SitemapItem): SitemapItem | Promise<SitemapItem | undefined> | undefined;
 	  }
 	| undefined;
 
@@ -117,8 +117,14 @@ const createPlugin = (options?: SitemapOptions): AstroIntegration => {
 							const serializedUrls: SitemapItem[] = [];
 							for (const item of urlData) {
 								const serialized = await Promise.resolve(serialize(item));
-								serializedUrls.push(serialized);
+                if (serialized) {
+                  serializedUrls.push(serialized);
+                }
 							}
+              if (serializedUrls.length === 0) {
+                logger.warn('No pages found!');
+                return;
+              }							
 							urlData = serializedUrls;
 						} catch (err) {
 							logger.error(`Error serializing pages\n${(err as any).toString()}`);