JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://cdnjs.cloudflare.com/ajax/libs/knockout/3.4.0/knockout-min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/moment.js/2.12.0/moment.min.js"></script>
<script src="https://raw.githubusercontent.com/crissdev/knockout.mapping/master/knockout.mapping.js"></script>

JavaScript

var vm = {
	a: ko.observable("demo"),
  b: moment()
};

var js = ko.mapping.toJS(vm);

console.log(vm.b.format('YYYY'));
console.log(js.b.format('YYYY'));