Knockout: Force a positive integer

knockout-extensions.js

// extension to replace an observable with a writeable computed that forces write to be numeric
// original: https://github.com/tekpub/knockout-cart/blob/master/src/extensions.js
ko.observable.fn.asPositiveInteger = function(defaultForBadValue) {
  var original = this,
      interceptor = ko.computed({
        read: original,
        write: function(newValue) {
            var parsed = parseInt(newValue, 10);
            //if value is bad or negative, then use default
            if (isNaN(parsed) || parsed < 0) {
                 parsed = defaultForBadValue
            } 
            original(parsed);
        }
     });

     //process the initial value
     interceptor(original());

  //return this new writeable computed to "stand in front of" our original observable
  return interceptor;
};