Knockout.js (JS) code "accidentally called a method or property" error in IE

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var CartListViewModel = function () {     var self = this;    self.payment = [                 { name: "", value: 1 },                 { name: "", value: 2 },             ];    self.items = ko.observableArray([]);    // {‘id‘:1,‘data‘:[‘aa‘,‘bb‘]}    self.load = function () {                var data = <%=cartListMap %>;                  var items = new Array();         for (var i in data) {             var dataItem = data[i];            var cartItem = new CartItemViewModel()             。。。            items.push(cartItem);         }         self.items(items);     };    self.TotalSum = function(){         var totalsum = 0;         var ilength = this.items().length;         for(var idx = 0; idx < ilength; idx++)         {             var iItem = this.items()[idx];             if (iItem.selected())             {                 totalsum = totalsum + iItem.goodsPrice();             }         }            return totalsum.toFixed(2);                }}

The above section Knockout.js code, where the Totalsum function, return totalsum, (the original code) runs normally in FF, but in IE the error "method or property was called unexpectedly" is always present. Later after the code is added tofixed (2) after normal.