为什么.then函数中的location.replace不允许我将数据发送到firestore？

Question

即使在侦听器功能（单击事件）中也有一些功能。在它的内部，我有一个函数，当单击“下订单”按钮时将数据发送到Firestore。我有一个.then函数，可在将数据发送到Firestore后向用户发出“您的订单已下达”警报。这很好用，但是当我在警报功能之后添加另一个.then函数以在警报窗口关闭后切换到另一个页面时，数据永远不会发送到firestore。

var placeOrderButton = document.getElementsByClassName('placeOrderBtn')
 for (var i = 0; i < placeOrderButton.length; i++) {
     var button = placeOrderButton[i]
     button.addEventListener('click', placeOrderClicked)
 }



 function placeOrderClicked(event) {
     var y = year.getFullYear();
     var m = month.getMonth();
     var d = day.getDate();
     var h = hours.getHours();
     var m = minutes.getMinutes();
     var s = seconds.getSeconds();
     var mm = milliseconds.getMilliseconds();

     var orderID = ("" + y + m + d + h + m + s + mm);

     localStorage.setItem('processedOrderID', orderID);


     //getting and sending data to order 
     db.collection('shoppingSession').doc(localStorage.getItem('seshID')).collection('inCart').get().then(function (querySnapshot) {
         querySnapshot.forEach(function (doc) {


             var idName = doc.id;
             var title = doc.data().title;
             var qty = Number(doc.data().qty);
             var price = Number(doc.data().price);
             var itemTotal = qty * price;
             var orderTotal = 90.00;
             var thiss = orderTotal.toFixed(2);

             addingPrices.push(itemTotal);

             //moving data to orders table for cooks
             db.collection('orders').doc(orderID).collection('inCart').add({
                 title: title,
                 price: price,
                 qty: qty
             });

         });

     }).then(function () {
         alert("Processing your order...");


     }).then(function () {
         location.assign("index.html");
     });

     // remember that this inside an onclick event

 }; //end of onlick event