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Observable_Object.subscribe( function (newVal) {
  // ..
});

ko.observable.fn.subscribeOldNew = function (callback, target) {
        var oldValue;
        this.subscribe(function (oldVal) {
            oldValue = oldVal;
        }, null, 'beforeChange');

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