how to dynamically add options to an existing select in vanilla javascript

I guess something like this would do the job.

var option = document.createElement("option");
option.text = "Text";
option.value = "myvalue";
var select = document.getElementById("daySelect");
select.appendChild(option);

This tutorial shows exactly what you need to do: Add options to an HTML select box with javascript

Basically:

 daySelect = document.getElementById('daySelect');
 daySelect.options[daySelect.options.length] = new Option('Text 1', 'Value1');