If you do not want to rely on the 3.5 kB plugin for jQuery or do not want to construct the HTML string while escapping reserved HTML characters, here is a simple way that works:
function addOptionToSelectBox(selectBox, optionId, optionText, selectIt)
{
var option = document.createElement("option");
option.value = optionId;
option.text = optionText;
selectBox.options[selectBox.options.length] = option;
if (selectIt) {
option.selected = true;
}
}
var selectBox = $('#veryImportantSelectBox')[0];
addOptionToSelectBox(selectBox, "ID1", "Option 1", true);