Fallback to tap prefetch strategy on slow connection (#9092)

Co-authored-by: Sarah Rainsberger <sarah@rainsberger.ca>
Co-authored-by: Bjorn Lu <bjornlu.dev@gmail.com>

3 files changed, 35 insertions, 9 deletions

diff --git a/.changeset/neat-mangos-judge.md b/.changeset/neat-mangos-judge.md
new file mode 100644
index 000000000..f42417d4a
--- /dev/null
+++ b/.changeset/neat-mangos-judge.md
@@ -0,0 +1,5 @@
+---
+'astro': minor
+---
+
+Changes the fallback prefetch behavior on slow connections and when data saver mode is enabled. Instead of disabling prefetch entirely, the `tap` strategy will be used.
diff --git a/.changeset/six-owls-trade.md b/.changeset/six-owls-trade.md
new file mode 100644
index 000000000..cd16cecec
--- /dev/null
+++ b/.changeset/six-owls-trade.md
@@ -0,0 +1,5 @@
+---
+'astro': minor
+---
+
+Adds a `ignoreSlowConnection` option to the `prefetch()` API to prefetch even on data saver mode or slow connection.
diff --git a/packages/astro/src/prefetch/index.ts b/packages/astro/src/prefetch/index.ts
index f47cff060..573efe573 100644
--- a/packages/astro/src/prefetch/index.ts
+++ b/packages/astro/src/prefetch/index.ts
@@ -56,7 +56,7 @@ function initTapStrategy() {
 			event,
 			(e) => {
 				if (elMatchesStrategy(e.target, 'tap')) {
-					prefetch(e.target.href, { with: 'fetch' });
+					prefetch(e.target.href, { with: 'fetch', ignoreSlowConnection: true });
 				}
 			},
 			{ passive: true }
@@ -176,6 +176,10 @@ export interface PrefetchOptions {
 	 * - `'fetch'`: use `fetch()`, has higher loading priority.
 	 */
 	with?: 'link' | 'fetch';
+	/**
+	 * Should prefetch even on data saver mode or slow connection. (default `false`)
+	 */
+	ignoreSlowConnection?: boolean;
 }
 
 /**
@@ -190,7 +194,8 @@ export interface PrefetchOptions {
  * @param opts Additional options for prefetching.
  */
 export function prefetch(url: string, opts?: PrefetchOptions) {
-	if (!canPrefetchUrl(url)) return;
+	const ignoreSlowConnection = opts?.ignoreSlowConnection ?? false;
+	if (!canPrefetchUrl(url, ignoreSlowConnection)) return;
 	prefetchedUrls.add(url);
 
 	const priority = opts?.with ?? 'link';
@@ -211,15 +216,11 @@ export function prefetch(url: string, opts?: PrefetchOptions) {
 	}
 }
 
-function canPrefetchUrl(url: string) {
+function canPrefetchUrl(url: string, ignoreSlowConnection: boolean) {
 	// Skip prefetch if offline
 	if (!navigator.onLine) return false;
-	if ('connection' in navigator) {
-		// Untyped Chrome-only feature: https://developer.mozilla.org/en-US/docs/Web/API/Navigator/connection
-		const conn = navigator.connection as any;
-		// Skip prefetch if using data saver mode or slow connection
-		if (conn.saveData || /(2|3)g/.test(conn.effectiveType)) return false;
-	}
+	// Skip prefetch if using data saver mode or slow connection
+	if (!ignoreSlowConnection && isSlowConnection()) return false;
 	// Else check if URL is within the same origin, not the current page, and not already prefetched
 	try {
 		const urlObj = new URL(url, location.href);
@@ -241,6 +242,12 @@ function elMatchesStrategy(el: EventTarget | null, strategy: string): el is HTML
 	if (attrValue === 'false') {
 		return false;
 	}
+
+	// Fallback to tap strategy if using data saver mode or slow connection
+	if (strategy === 'tap' && (attrValue != null || prefetchAll) && isSlowConnection()) {
+		return true;
+	}
+
 	// If anchor has no dataset but we want to prefetch all, or has dataset but no value,
 	// check against fallback default strategy
 	if ((attrValue == null && prefetchAll) || attrValue === '') {
@@ -254,6 +261,15 @@ function elMatchesStrategy(el: EventTarget | null, strategy: string): el is HTML
 	return false;
 }
 
+function isSlowConnection() {
+	if ('connection' in navigator) {
+		// Untyped Chrome-only feature: https://developer.mozilla.org/en-US/docs/Web/API/Navigator/connection
+		const conn = navigator.connection as any;
+		return conn.saveData || /(2|3)g/.test(conn.effectiveType);
+	}
+	return false;
+}
+
 /**
  * Listen to page loads and handle Astro's View Transition specific events
  */