JSFiddle - React, Tailwind, and code Playground

HTML

<select data-bind="options: availableCountries,
                       optionsText: 'countryName',
                       value: selectedCountry,
                       optionsCaption: 'Select...'"></select>

JavaScript

// Constructor for an object with two properties
var Country = function (name, population) {
    this.countryName = name;
    this.countryPopulation = population;
};

var viewModel = {
    availableCountries: ko.observableArray([
    new Country("UK", 65000000),
    new Country("USA", 320000000),
    new Country("Sweden", 29000000)]),
    selectedCountry: ko.observable(), // Nothing selected by default
};

viewModel.selectedCountry.subscribe(function (data) {
        alert(data.countryName)
    });

ko.applyBindings(viewModel);