Use chrome.alarms to purge cache where available (#23)

index.ts

@@ -102,8 +102,12 @@ async function deleteWithLogic(logic?: (x: CacheItem<Value>) => boolean): Promis
 	}
 }
 
+let lastRun = 0; // Homemade debouncing due to `chrome.alarms` potentially queueing this function
 async function deleteExpired(): Promise<void> {
-	await deleteWithLogic(cachedItem => Date.now() > cachedItem.maxAge);
+	if (lastRun < Date.now() - 1000) {
+		lastRun = Date.now();
+		await deleteWithLogic(cachedItem => Date.now() > cachedItem.maxAge);
+	}
 }
 
 async function clear(): Promise<void> {
@@ -163,13 +167,24 @@ const cache = {
 };
 
 function init(): void {
+	// Make it available globally for ease of use
+	(window as any).webextStorageCache = cache;
+
 	// Automatically clear cache every day
-	if (isBackgroundPage()) {
+	if (!isBackgroundPage()) {
+		return;
+	}
+
+	if (chrome.alarms) {
+		chrome.alarms.create('webext-storage-cache', {
+			delayInMinutes: 1,
+			periodInMinutes: 60 * 24
+		});
+		chrome.alarms.onAlarm.addListener(deleteExpired);
+	} else {
 		setTimeout(deleteExpired, 60000); // Purge cache on launch, but wait a bit
 		setInterval(deleteExpired, 1000 * 3600 * 24);
 	}
-
-	(window as any).webextStorageCache = cache;
 }
 
 init();

readme.md

@@ -19,14 +19,21 @@ import storageCache from 'webext-storage-cache';
 
 ## Usage
 
-This module requires the `storage` permission:
+This module requires the `storage` permission and it’s suggested to also use `alarms` to safely schedule cache purging:
 
-```json
-// manifest.json
+```json5
+/* manifest.json */
 {
 	"permissions": [
-		"storage"
-	]
+		"storage",
+		"alarms"
+	],
+	"background": {
+		"scripts": [
+			/* Remember to include/import it in the background to enable expired cache purging */
+			"webext-storage-cache.js"
+		]
+	}
 }
 ```
 
@@ -45,7 +52,7 @@ import cache from 'webext-storage-cache';
 })();
 ```
 
-The same code could be also written more effectively with `cache.function`:
+The same code could also be written more effectively with `cache.function`:
 
 ```js
 import cache from 'webext-storage-cache';