JSFiddle - React, Tailwind, and code Playground

HTML

<script src="http://knockoutjs.com/downloads/knockout-2.2.1.js"></script>
<select data-bind="value: Width">
	<option>10</option>
	<option>100</option>
</select>
<select data-bind="value: Height">
	<option>10</option>
	<option>100</option>
</select>
<div data-bind="text: Area"></div>

JavaScript

function AreaViewModel() {
	var self = this;

	self.Width = ko.observable(10);
	self.Height = ko.observable(10);

	self.Area = ko.computed(function () {
		console.log("In computed function.");

		return self.Width() * self.Height();
    });
}

$(document).ready(function () {
	var viewModel = new AreaViewModel();
	ko.applyBindings(viewModel);
});