JSFiddle - React, Tailwind, and code Playground

HTML

<div id="view">
    <button data-bind="click: on">On</button>
    <button data-bind="click: off">Off</button>
    
    <div data-bind="visible: visible">
        <span data-bind="foo: ''">foo</span>
        visible
    </div>
    
</div>

JavaScript

ko.bindingHandlers.foo= {
  init: function() {
    alert("init");
  },
  update: function() {
    alert("update");
  }
}

var vm =(function() {
    var self = this;
    self.visible = ko.observable(false);
    
    self.on = function() { self.visible(true); };
    self.off= function() {self.visible(false); };

})();

ko.applyBindings(vm, document.getElementById("view"));