Knockout Js Custom Subscribe

gistfile1.js

ko.subscribable.fn.subscribeChanged = function (callback, target) {
    var oldValue;
    this.subscribe(function (_oldValue) {
        oldValue = _oldValue;
    }, target, 'beforeChange');

    this.subscribe(function (newValue) {
        callback(newValue, oldValue, this);
    }, target);
};

gistfile2.js

function Test(){
  this.title = ko.observable();
  this.title.subscribeChanged(function(newValue, oldValue, target){
     console.log(newValue);
     console.log(oldValue);
  }, this);
}

var test = new Test();
ko.applyBindings(test);
test.title('deneme');