Update beacon support to handle "gotchas" (close #716)

Author: Paul Boocock

src/js/lib/helpers.js

@@ -348,6 +348,37 @@
 		}
 	};
 
+		/**
+	 * Attempt to get a value from localStorage
+	 *
+	 * @param string key
+	 * @return string The value obtained from localStorage, or
+	 *                undefined if localStorage is inaccessible
+	 */
+	object.attemptGetSessionStorage = function (key) {
+		try {
+			return sessionStorage.getItem(key);
+		} catch(e) {
+			return null;
+		}
+	};
+
+	/**
+	 * Attempt to write a value to localStorage
+	 *
+	 * @param string key
+	 * @param string value
+	 * @return boolean Whether the operation succeeded
+	 */
+	object.attemptWriteSessionStorage = function (key, value) {
+		try {
+			sessionStorage.setItem(key, value);
+			return true;
+		} catch(e) {
+			return false;
+		}
+	};
+
 	/**
 	 * Finds the root domain
 	 */

src/js/out_queue.js

@@ -300,7 +300,7 @@
 				if (batch.length > 0) {
 					var beaconStatus;
 
-					if (useBeacon) {
+					if (useBeacon && helpers.attemptGetSessionStorage('sp_corsPreflight')) {
 						const headers = { type: 'application/json' };
 						const blob = new Blob([encloseInPayloadDataEnvelope(attachStmToEvent(batch))], headers);
 						try {
@@ -319,6 +319,7 @@
 
 					if (!useBeacon || !beaconStatus) {
 						xhr.send(encloseInPayloadDataEnvelope(attachStmToEvent(batch)));
+						helpers.attemptWriteSessionStorage('sp_corsPreflight', true);
 					}
 				}