Llenar combo con ES6
HTML
<select id="someSelect">
</select>JavaScript
fetch('https://restcountries.eu/rest/v1/all')
.then((response) => {return response.json()})
.then((countries) => {
var options = document.getElementById('someSelect');
countries.forEach((countrie) => {
options.appendChild(new Option(countrie.name, countrie.name));
});
})