JSFiddle - React, Tailwind, and code Playground

HTML

<div id="result"></div>
<div id="result"></div>

JavaScript

var hoge = ko.observable(100);
var piyo = ko.observable(200);
var count = 0;
var foo = ko.computed(function () {
    count++;
    document.querySelector('#result').innerText = count;
    return hoge() + piyo();
});
hoge(300);