Add Flow Type Alias for function which returns an object (session_mutex)

modules/util/session_mutex.js

@@ -1,10 +1,19 @@
+// @flow
 // A per-domain session mutex backed by a cookie and dead man's
 // switch. If the session crashes, the mutex will auto-release
 // after 5 seconds.
 
-export function utilSessionMutex(name) {
-    var mutex = {},
-        intervalID;
+// This is a type alias (https://flow.org/en/docs/types/aliases/) which allows flow to understand the object returned by utilSessionMutex in other files.
+type utilSessionMutexType = {
+    lock: () => boolean,
+    unlock: () => void,
+    locked: () => boolean
+};
+
+// This accepts a string and returns an object that complies with utilSessionMutexType
+export function utilSessionMutex(name: string): utilSessionMutexType {
+    var mutex = {};
+    var intervalID: ?IntervalID; // This indicates a Maybe type - intervalId can be null so we need to use "?IntervalID", not "IntervalID"
 
     function renew() {
         var expires = new Date();
@@ -12,7 +21,7 @@ export function utilSessionMutex(name) {
         document.cookie = name + '=1; expires=' + expires.toUTCString();
     }
 
-    mutex.lock = function() {
+    mutex.lock = function (): boolean {
         if (intervalID) return true;
         var cookie = document.cookie.replace(new RegExp('(?:(?:^|.*;)\\s*' + name + '\\s*\\=\\s*([^;]*).*$)|^.*$'), '$1');
         if (cookie) return false;
@@ -21,14 +30,14 @@ export function utilSessionMutex(name) {
         return true;
     };
 
-    mutex.unlock = function() {
+    mutex.unlock = function (): void {
         if (!intervalID) return;
         document.cookie = name + '=; expires=Thu, 01 Jan 1970 00:00:00 GMT';
         clearInterval(intervalID);
         intervalID = null;
     };
 
-    mutex.locked = function() {
+    mutex.locked = function (): boolean {
         return !!intervalID;
     };