Knockout extenders allow observables to be augmented with additional functionality. An extender is a function that accepts the target observable and any options, and returns the extended observable. For example, a "logChange" extender could subscribe to the target and log value changes to the console. A "numeric" extender rounds values to a specified precision on write to the observable. Extenders make it possible to extend observables with common functionality in a reusable way.

1. Knockout Extender
Bryan Lin
2013/11/01

2. Using extenders to augment
observables
Knockout observables provide the basic features necessary to support
reading/writing values and notifying subscribers when that value changes

3. Using extenders to augment
observables
How to create an extender
Using the ko.extenders object
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"});

4. Using extenders to augment
observables
How to create an extender
 View:
<p><input data-bind="value: myNumberOne" /> (round to whole number)</p>
<p><input data-bind="value: myNumberTwo" /> (round to two decimals)</p>

5. ko.extenders.numeric = function(target, precision) {
//create a writeable computed observable to intercept writes to our observable
var result = ko.computed({
read: target, //always return the original observables value
write: function(newValue) {
var current = target(),
roundingMultiplier = Math.pow(10, precision),
newValueAsNum = isNaN(newValue) ? 0 : parseFloat(+newValue),
valueToWrite = Math.round(newValueAsNum * roundingMultiplier) / roundingMultiplier;
//only write if it changed
if (valueToWrite !== current) {
target(valueToWrite);
} else {
//if the rounded value is the same, but a different value was written, force a
notification for the current field
if (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 observable
return 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));

6. References
http://knockoutjs.com/
http://jsbin.com/ahelar/49/edit