ko_code.js

ko.numericObservable = function(initialValue) {
    var _actual = ko.observable(initialValue);

    var result = ko.computed({
      read: function() {
        return _actual();
      },
      write: function(newValue) {
        var parsedValue = parseFloat(newValue);
        _actual(isNaN(parsedValue) ? newValue : parsedValue);
      }
    });

    return result;
};

var vm = {
  firstName: ko.observable("Jose"),
  age: ko.numericObservable(33),
  
  clickHandler: function() {
    console.log(typeof this.age());
  }
};

ko.applyBindings(vm);

console.log(typeof vm.age());