Skip prefetching on 2G connections, or when data saver is enabled

packages/labs/prefetch/@types/network-information.d.ts

@@ -0,0 +1,15 @@
+export { };
+
+declare global {
+	interface NetworkInformation {
+		// http://wicg.github.io/netinfo/#effectiveconnectiontype-enum
+		readonly effectiveType: '2g' | '3g' | '4g' | 'slow-2g';
+		// http://wicg.github.io/netinfo/#savedata-attribute
+		readonly saveData?: boolean;
+	}
+
+	var NetworkInformation: {
+    prototype: NetworkInformation;
+    new(): NetworkInformation;
+	};
+}

packages/labs/prefetch/src/client.ts

@@ -1,3 +1,4 @@
+/// <reference path="../@types/network-information.d.ts" />
 import throttles from 'throttles';
 import requestIdleCallback from './requestIdleCallback.js';
 
@@ -72,6 +73,18 @@ export interface PrefetchOptions {
 }
 
 export default function prefetch({ selector = 'a[href][rel~="prefetch"]', throttle = 1 }: PrefetchOptions) {
+	const conn = navigator.connection;
+
+	if (typeof conn !== 'undefined') {
+    // Don't prefetch if using 2G or if Save-Data is enabled.
+    if (conn.saveData) {
+      return Promise.reject(new Error('Cannot prefetch, Save-Data is enabled'));
+    }
+    if (/2g/.test(conn.effectiveType)) {
+      return Promise.reject(new Error('Cannot prefetch, network conditions are poor'));
+    }
+  }
+
 	const [toAdd, isDone] = throttles(throttle);
 
 	observer =

packages/labs/prefetch/tsconfig.json

@@ -1,6 +1,6 @@
 {
   "extends": "../../../tsconfig.base.json",
-  "include": ["src"],
+  "include": ["src", "@types"],
   "compilerOptions": {
     "allowJs": true,
     "module": "ES2020",