Knockout js – beforeRemove animation while adding items to observable array

Your function “addItem” is executed after 1 second (setInterval 1000 ms) so the “self.data” contains a new element while the animation fadeout is not complete (that’s require 3000 ms). That’s explicate your problem.

To resolve this problem you must make the same interval for “addItem” as the “fadeout”, for your exemple it’s 3000.
The code becomes :

jQuery(function ($) {
    var ViewModel = function (data) {
        var self = this;
        self.data = ko.observableArray(data);
        self.removeLine = function (elem) {
            if (elem.nodeType === 1) $(elem).fadeOut(3000, function () {
                $(elem).remove();
            });
        }

        self.addLine = function (elem) {
            if (elem.nodeType === 1) 
                $(elem).hide().fadeIn(3000);
        }

        self.removeItem = function() {
            self.data.remove(function(item) { return item.test && item.test == 2; });   
        }

        self.addItem = function() {
            self.data.splice(1, 0, { test: 9 }); 
        }

        self.addremove = function () {
            self.removeItem();
            var id = setInterval(function() {
                self.addItem();
                clearInterval(id);
            },3000); // the same interval as the "fadeout"
        }
    }

    var vm = new ViewModel([{ test: 9 }, { test: 2 }, { test: 1 }, { test: 1 }, { test: 1 }, { test: 1 }, { test: 1 }]);

    ko.applyBindings(vm);
});