diff --git a/api/js/etemplate/etemplate2.js b/api/js/etemplate/etemplate2.js
index 85f60ba5fd..0c19c273c2 100644
--- a/api/js/etemplate/etemplate2.js
+++ b/api/js/etemplate/etemplate2.js
@@ -285,9 +285,9 @@ etemplate2.prototype.bind_unload = function()
 	{
 		this.destroy_session = jQuery.proxy(function(ev)
 		{
-			var request = egw.json("EGroupware\\Api\\Etemplate::ajax_destroy_session",
-				[this.etemplate_exec_id], null, null, false);
-			request.sendRequest();
+			// need to use async === "keepalive" to run via beforeunload
+			egw.json("EGroupware\\Api\\Etemplate::ajax_destroy_session",
+				[this.etemplate_exec_id], null, null, "keepalive").sendRequest();
 		}, this);
 
 		if (!window.onbeforeunload)
diff --git a/api/js/jsapi/egw_global.d.ts b/api/js/jsapi/egw_global.d.ts
index f67c817a2c..3f5501aa97 100644
--- a/api/js/jsapi/egw_global.d.ts
+++ b/api/js/jsapi/egw_global.d.ts
@@ -673,13 +673,14 @@ declare class JsonRequest
 {
 	/**
 	 * Sends the assembled request to the server
-	 * @param {boolean} [async=false] Overrides async provided in constructor to give an easy way to make simple async requests
+	 * @param {boolean|"keepalive"} _async true: asynchronious request, false: synchronious request,
+	 * 	"keepalive": async. request with keepalive===true / sendBeacon, to be used in beforeunload event
 	 * @param {string} method ='POST' allow to eg. use a (cachable) 'GET' request instead of POST
 	 * @param {function} error option error callback(_xmlhttp, _err) used instead our default this.error
 	 *
 	 * @return {jqXHR} jQuery jqXHR request object
 	 */
-	sendRequest(async? : boolean, method? : "POST"|"GET", error? : Function)
+	sendRequest(async? : boolean|"keepalive", method? : "POST"|"GET", error? : Function)
 	/**
 	 * Open websocket to push server (and keeps it open)
 	 *
@@ -721,14 +722,14 @@ declare interface IegwWndLocal extends IegwGlobal
 	 * 	which handles the actual request. If the menuaction is a full featured
 	 * 	url, this one will be used instead.
 	 * @param _parameters which should be passed to the menuaction function.
-	 * @param _async specifies whether the request should be asynchronous or
-	 * 	not.
+	 * @param {boolean|"keepalive"} _async true: asynchronious request, false: synchronious request,
+	 * 	"keepalive": async. request with keepalive===true / sendBeacon, to be used in beforeunload event
 	 * @param _callback specifies the callback function which should be
 	 * 	called, once the request has been sucessfully executed.
 	 * @param _context is the context which will be used for the callback function
 	 * @param _sender is a parameter being passed to the _callback function
 	 */
-	json(_menuaction : string, _parameters? : any[], _callback? : Function, _context? : object, _async? : boolean, _sender?) : JsonRequest;
+	json(_menuaction : string, _parameters? : any[], _callback? : Function, _context? : object, _async? : boolean|"keepalive", _sender?) : JsonRequest;
 	/**
 	 * Registers a new handler plugin.
 	 *
diff --git a/api/js/jsapi/egw_json.js b/api/js/jsapi/egw_json.js
index c7866c6d98..4e1b843f26 100644
--- a/api/js/jsapi/egw_json.js
+++ b/api/js/jsapi/egw_json.js
@@ -7,7 +7,6 @@
  * @link http://www.egroupware.org
  * @author Andreas Stöckel (as AT stylite.de)
  * @author Ralf Becker <RalfBecker@outdoor-training.de>
- * @version $Id$
  */
 
 /*egw:uses
@@ -53,7 +52,8 @@ egw.extend('json', egw.MODULE_WND_LOCAL, function(_app, _wnd)
 	 * @param {array} _parameters
 	 * @param {function} _callback
 	 * @param {object} _context
-	 * @param {boolean} _async
+	 * @param {boolean|"keepalive"} _async true: asynchronious request, false: synchronious request,
+	 * 	"keepalive": async. request with keepalive===true / sendBeacon, to be used in boforeunload event
 	 * @param {object} _sender
 	 * @param {egw} _egw
 	 */
@@ -154,11 +154,12 @@ egw.extend('json', egw.MODULE_WND_LOCAL, function(_app, _wnd)
 
 	/**
 	 * Sends the assembled request to the server
-	 * @param {boolean} [async=false] Overrides async provided in constructor to give an easy way to make simple async requests
+	 * @param {boolean|"keepalive"} _async Overrides async provided in constructor: true: asynchronious request,
+	 * 	false: synchronious request, "keepalive": async. request with keepalive===true / sendBeacon, to be used in beforeunload event
 	 * @param {string} method ='POST' allow to eg. use a (cachable) 'GET' request instead of POST
 	 * @param {function} error option error callback(_xmlhttp, _err) used instead our default this.error
 	 *
-	 * @return {jqXHR} jQuery jqXHR request object
+	 * @return {jqXHR|boolean} jQuery jqXHR request object or for async==="keepalive" boolean is returned
 	 */
 	json_request.prototype.sendRequest = function(async, method, error)
 	{
@@ -176,6 +177,15 @@ egw.extend('json', egw.MODULE_WND_LOCAL, function(_app, _wnd)
 			}
 		});
 
+		// send with keepalive===true or sendBeacon to be used in beforeunload event
+		if (this.async === "keepalive" && typeof navigator.sendBeacon !== "undefined")
+		{
+			const data = new FormData();
+			data.append('json_data', request_obj);
+			//(window.opener||window).console.log("navigator.sendBeacon", this.url, request_obj, data.getAll('json_data'));
+			return navigator.sendBeacon(this.url, data);
+		}
+
 		// Send the request via AJAX using the jquery ajax function
 		// we need to use jQuery of window of egw object, as otherwise the one from main window is used!
 		// (causing eg. apply from server with app.$app.method to run in main window instead of popup)
diff --git a/calendar/js/app.js b/calendar/js/app.js
index 8a7b554ecd..ef144ac858 100644
--- a/calendar/js/app.js
+++ b/calendar/js/app.js
@@ -243,7 +243,7 @@ app.classes.calendar = (function(){ "use strict"; return AppJS.extend(
 					{
 						window.onbeforeunload = function () {
 							this.egw.json('calendar.calendar_uiforms.ajax_unlock',
-							[content.data.id, content.data.lock_token],null,true,null,null).sendRequest(true);
+							[content.data.id, content.data.lock_token],null,true,"keepalive",null).sendRequest();
 						};
 					}
 				}