ko.extenders.logChange = function(target, option) {target.subscribe(function(newValue) {console.log(option + ": " + newValue);});return target;
};

this.firstName = ko.observable("Bob").extend({logChange: "first name"});

(round to whole number)

(round to two decimals)

<p><input data-bind="value: myNumberOne" /> (round to whole number)</p>
<p><input data-bind="value: myNumberTwo" /> (round to two decimals)</p>

ko.extenders.numeric = function(target, precision) {//create a writable computed observable to intercept writes to our observablevar result = ko.pureComputed({read: target,  //always return the original observables valuewrite: function(newValue) {var current = target(),roundingMultiplier = Math.pow(10, precision),newValueAsNum = isNaN(newValue) ? 0 : +newValue,valueToWrite = Math.round(newValueAsNum * roundingMultiplier) / roundingMultiplier;//only write if it changedif (valueToWrite !== current) {target(valueToWrite);} else {//if the rounded value is the same, but a different value was written, force a notification for the current fieldif (newValue !== current) {target.notifySubscribers(valueToWrite);}}}}).extend({ notify: 'always' });//initialize with current value to make sure it is rounded appropriately
    result(target());//return the new computed observablereturn result;
};function AppViewModel(one, two) {this.myNumberOne = ko.observable(one).extend({ numeric: 0 });this.myNumberTwo = ko.observable(two).extend({ numeric: 2 });
}ko.applyBindings(new AppViewModel(221.2234, 123.4525));

<p data-bind="css: { error: firstName.hasError }"><input data-bind='value: firstName, valueUpdate: "afterkeydown"' /><span data-bind='visible: firstName.hasError, text: firstName.validationMessage'> </span>
</p>
<p data-bind="css: { error: lastName.hasError }"><input data-bind='value: lastName, valueUpdate: "afterkeydown"' /><span data-bind='visible: lastName.hasError, text: lastName.validationMessage'> </span>
</p>

ko.extenders.required = function(target, overrideMessage) {//add some sub-observables to our observabletarget.hasError = ko.observable();target.validationMessage = ko.observable();//define a function to do validationfunction validate(newValue) {target.hasError(newValue ? false : true);target.validationMessage(newValue ? "" : overrideMessage || "This field is required");}//initial validation
    validate(target());//validate whenever the value changes
    target.subscribe(validate);//return the original observablereturn target;
};function AppViewModel(first, last) {this.firstName = ko.observable(first).extend({ required: "Please enter a first name" });this.lastName = ko.observable(last).extend({ required: "" });
}ko.applyBindings(new AppViewModel("Bob","Smith"));

this.firstName = ko.observable(first).extend({ required: "Please enter a first name", logChange: "first name" });